## [1112.4487] Determining the outcome of cosmic bubble collisions in full General Relativity

Authors: Matthew C. Johnson, Hiranya V. Peiris, Luis Lehner

Date: 19 Dec 2011

Abstract: Cosmic bubble collisions provide an important possible observational window on the dynamics of eternal inflation. In eternal inflation, our observable universe is contained in one of many bubbles formed from an inflating metastable vacuum. The collision between bubbles can leave a detectable imprint on the cosmic microwave background radiation. Although phenomenological models of the observational signature have been proposed, to make the theory fully predictive one must determine the bubble collision spacetime, and thus the cosmological observables, from a scalar field theory giving rise to eternal inflation. Because of the intrinsically non-linear nature of the bubbles and their collision, this requires a numerical treatment incorporating General Relativity. In this paper, we present results from numerical simulations of bubble collisions in full General Relativity. These simulations allow us to accurately determine the outcome of bubble collisions, and examine their effect on the cosmology inside a bubble universe. We confirm the validity of a number of approximations used in previous analytic work, and identify qualitatively new features of bubble collision spacetimes. Both vacuum bubbles and bubbles containing a realistic inflationary cosmology are studied. We identify the constraints on the scalar field potential that must be satisfied in order to obtain collisions that are consistent with our observed cosmology, yet leave detectable signatures.

#### Dec 21, 2011

1112.4487 (/preprints)
2011-12-21, 11:32 

## [1112.4608] Periodic Signals in Binary Microlensing Events

Authors: Xinyi Guo, Ann Esin, Rosanne Di Stefano, Jeffrey Taylor

Date: 20 Dec 2011

Abstract: Gravitational microlensing events provide a potentially powerful tool for the study of stellar populations. In particular, they can be used to discover and study a variety of binary systems. A large number of binary lenses have already been found by the microlensing surveys and a few of these systems show strong evidence of orbital motion on the timescale of the lensing event. We expect that more of such binary lenses will be detected in the future. For binaries whose orbital period is comparable to the event duration, the orbital motion can cause the lensing signal to deviate drastically from that of a static binary lens. The most striking property of such lightcurves is the presence of quasi-periodic features, produced as the source traverses the same regions in the rotating lens plane. These repeated features contain information about the orbital period of the lens. If this period can be extracted, a lot can be learned about the lensing system even without performing a time-consuming detailed lightcurve modeling. However, the relative transverse motion between the source and the lens significantly complicates the problem of period extraction. To resolve this difficulty, we present a modification to the standard Lomb-Scargle periodogram analysis. We test our method for four representative binary lens systems and demonstrate its efficiency in correctly extracting binary orbital periods.

#### Dec 21, 2011

1112.4608 (/preprints)
2011-12-21, 11:32 

## [1112.1565] Searching for Gravitational Waves with a Geostationary Gravitational Wave Interferometer

Authors: J. C. N. de Araujo, O. D. Aguiar, M. E. S. Alves, M. Tinto

Date: 7 Dec 2011

Abstract: We analyze the sensitivities of a geostationary gravitational wave interferometer mission operating in the sub-Hertz band. Our proposed Earth-orbiting detector is expected to meet some of the Laser Interferometer Space Antenna (LISA) mission science goals in the lower part of its accessible frequency band ($10ˆ{-4} - 2 \times 10ˆ{-2}$ Hz), and to outperform them by a large margin in the higher-part of it ($2 \times 10ˆ{-2} - 10$ Hz). Since our proposed interferometer will be more sensitive than LISA to supermassive black holes (SMBHs) of masses smaller than $\sim 10ˆ{6}$ M$_{\odot}$, we will be able to more accurately probe scenarios that account for their formation.

#### Dec 20, 2011

1112.1565 (/preprints)
2011-12-20, 09:16 

## [1112.1542] Updated gravitational-wave upper limits on the internal magnetic field strength of recycled pulsars

Authors: Alpha Mastrano, Andrew Melatos

Date: 7 Dec 2011

Abstract: Recent calculations of the hydromagnetic deformation of a stratified, non-barotropic neutron star are generalized to describe objects with superconducting interiors, whose magnetic permeability \mu is much smaller than the vacuum value \mu_0. It is found that the star remains oblate if the poloidal magnetic field energy is \gtrsim 40% of total magnetic field energy, that the toroidal field is confined to a torus which shrinks as \mu decreases, and that the deformation is much larger (by a factor \sim \mu_0/\mu) than in a non-superconducting object. The results are applied to the latest direct and indirect upper limits on gravitational-wave emission from Laser Interferometer Gravitational Wave Observatory (LIGO) and radio pulse timing (spin-down) observations of 81 millisecond pulsars, to show how one can use these observations to infer the internal field strength. It is found that the indirect spin-down limits already imply astrophysically interesting constraints on the poloidal-toroidal field ratio and diamagnetic shielding factor (by which accretion reduces the observable external magnetic field, e.g. by burial). These constraints will improve following gravitational-wave detections, with implications for accretion-driven magnetic field evolution in recycled pulsars and the hydromagnetic stability of these objects' interiors.

#### Dec 20, 2011

1112.1542 (/preprints)
2011-12-20, 09:16 

## [1112.1637] Search for gravitational waves associated with the InterPlanetary Network short gamma ray bursts

Authors: V. Predoi, for the LIGO Scientific Collaboration, for the Virgo Collaboration, K. Hurley, for IPN

Date: 7 Dec 2011

Abstract: We outline the scientific motivation behind a search for gravitational waves associated with short gamma ray bursts detected by the InterPlanetary Network (IPN) during LIGO's fifth science run and Virgo's first science run. The IPN localisation of short gamma ray bursts is limited to extended error boxes of different shapes and sizes and a search on these error boxes poses a series of challenges for data analysis. We will discuss these challenges and outline the methods to optimise the search over these error boxes.

#### Dec 20, 2011

1112.1637 (/preprints)
2011-12-20, 09:16 

## [1112.1404] Verifying the no-hair property of massive compact objects with intermediate-mass-ratio inspirals in advanced gravitational-wave detectors

Authors: Carl L. Rodriguez, Ilya Mandel, Jonathan R. Gair

Date: 6 Dec 2011

Abstract: The detection of gravitational waves from the inspiral of a neutron star or stellar-mass black hole into an intermediate-mass black hole (IMBH) promises an entirely new look at strong-field gravitational physics. Gravitational waves from these intermediate-mass-ratio inspirals (IMRIs), systems with mass ratios from ~10:1 to ~100:1, may be detectable at rates of up to a few tens per year by Advanced LIGO/Virgo and will encode a signature of the central body's spacetime. Direct observation of the spacetime will allow us to use the "no-hair" theorem of general relativity to determine if the IMBH is a Kerr black hole (or some more exotic object, e.g. a boson star). Using modified post-Newtonian (pN) waveforms, we explore the prospects for constraining the central body's mass-quadrupole moment in the advanced-detector era. We use the Fisher information matrix to estimate the accuracy with which the parameters of the central body can be measured. We find that for favorable mass and spin combinations, the quadrupole moment of a non-Kerr central body can be measured to within a ~15% fractional error or better using 3.5 pN order waveforms; on the other hand, we find the accuracy decreases to ~100% fractional error using 2 pN waveforms, except for a narrow band of values of the best-fit non-Kerr quadrupole moment.

#### Dec 20, 2011

1112.1404 (/preprints)
2011-12-20, 09:15 

## [1112.1898] Accessibility of the Gravitational-Wave Background due to Binary Coalescences to Second and Third Generation Gravitational-Wave Detectors

Authors: Chengjian Wu, Vuk Mandic, Tania Regimbau

Date: 8 Dec 2011

Abstract: Compact binary coalescences, such as binary neutron stars or black holes, are among the most promising candidate sources for the current and future terrestrial gravitational-wave detectors. While such sources are best searched using matched template techniques and chirp template banks, integrating chirp signals from binaries over the entire Universe also leads to a gravitational-wave background (GWB). In this paper we systematically scan the parameter space for the binary coalescence GWB models, taking into account uncertainties in the star formation rate and in the delay time between the formation and coalescence of the binary, and we compare the computed GWB to the sensitivities of the second and third generation gravitational-wave detector networks. We find that second generation detectors are likely to detect the binary coalescence GWB, while the third generation detectors will probe most of the available parameter space. The binary coalescence GWB will, in fact, be a foreground for the third-generation detectors, potentially masking the GWB background due to cosmological sources. Accessing the cosmological GWB with third generation detectors will therefore require identification and subtraction of all inspiral signals from all binaries in the detectors' frequency band.

#### Dec 20, 2011

1112.1898 (/preprints)
2011-12-20, 09:13 

## [1112.2202] OPERA and the GPS

Authors: Llu&#xed;s Bel

Date: 8 Dec 2011

Abstract: I comment about the adequacy of the GPS to model a particularly defined synchronization in a rotating frame of reference described in a general relativistic framework.

#### Dec 20, 2011

1112.2202 (/preprints)
2011-12-20, 09:11 

## [1112.2151] Prospects for Probing the Spacetime of Sgr A* with Pulsars

Authors: K. Liu, N. Wex, M. Kramer, J. M. Cordes, T. J. W. Lazio

Date: 9 Dec 2011

Abstract: The discovery of radio pulsars in compact orbits around Sgr A* would allow an unprecedented and detailed investigation of the spacetime of the supermassive black hole. This paper shows that pulsar timing, including that of a single pulsar, has the potential to provide novel tests of general relativity, in particular its cosmic censorship conjecture and no-hair theorem for rotating black holes. These experiments can be performed by timing observations with 100 micro-second precision, achievable with the Square Kilometre Array for a normal pulsar at frequency above 15 GHz. Based on the standard pulsar timing technique, we develop a method that allows the determination of the mass, spin, and quadrupole moment of Sgr A*, and provides a consistent covariance analysis of the measurement errors. Furthermore, we test this method in detailed mock data simulations. It seems likely that only for orbital periods below ~0.3 yr is there the possibility of having negligible external perturbations. For such orbits we expect a ~10ˆ-3 test of the frame dragging and a ~10ˆ-2 test of the no-hair theorem within 5 years, if Sgr A* is spinning rapidly. Our method is also capable of identifying perturbations caused by distributed mass around Sgr A*, thus providing high confidence in these gravity tests. Our analysis is not affected by uncertainties in our knowledge of the distance to the Galactic center, R0. A combination of pulsar timing with the astrometric results of stellar orbits would greatly improve the measurement precision of R0.

#### Dec 20, 2011

1112.2151 (/preprints)
2011-12-20, 09:10 

## [1112.2782] The Stochastic Gravitational Wave Background from the Single-Degenerate Channel of Type Ia Supernovae

Authors: David Falta, Robert Fisher

Date: 13 Dec 2011

Abstract: We demonstrate that the integrated gravitational wave signal of Type Ia supernovae (SNe Ia) in the single-degenerate channel out to cosmological distances gives rise to a continuous background to spaceborne gravitational wave detectors, including the Big Bang Observer (BBO) and Deci-Hertz Interferometer Gravitational wave Observatory (DECIGO) planned missions. This gravitational wave background from SNe Ia acts as a noise background in the frequency range 0.1 - 10 Hz, which heretofore was thought to be relatively free from astrophysical sources apart from neutron star binaries, and therefore a key window in which to study primordial gravitational waves generated by inflation. While inflationary energy scales of $\gtrsim 10ˆ{16}$ GeV yield inflationary gravitational wave backgrounds in excess of our range of predicted backgrounds, for lower energy scales of $\sim10ˆ{15}$ GeV, the inflationary gravitational wave background becomes comparable to the noise background from SNe Ia.

#### Dec 20, 2011

1112.2782 (/preprints)
2011-12-20, 09:08 

## [1112.3928] Late Inspiral and Merger of Binary Black Holes in Scalar-Tensor Theories of Gravity

Authors: James Healy, Tanja Bode, Roland Haas, Enrique Pazos, Pablo Laguna, Deirdre M. Shoemaker, Nicol&#xe1;s Yunes

Date: 16 Dec 2011

Abstract: Gravitational wave observations will probe non-linear gravitational interactions and thus enable strong tests of Einstein's theory of general relativity. We present a numerical relativity study of the late inspiral and merger of binary black holes in scalar-tensor theories of gravity. We consider black hole binaries in an inhomogeneous scalar field, specifically binaries inside a scalar field bubble, in some cases with a potential. We calculate the emission of dipole radiation. We also show how these configurations trigger detectable differences between gravitational waves in scalar-tensor gravity and the corresponding waves in general relativity. We conclude that, barring an external mechanism to induce dynamics in the scalar field, scalar-tensor gravity binary black holes alone are not capable of awaking a dormant scalar field, and are thus observationally indistinguishable from their general relativistic counterparts.

#### Dec 20, 2011

1112.3928 (/preprints)
2011-12-20, 09:07 

## [1112.3694] Gravitational Waves from Compact Binaries as Probes of the Universe

Authors: Nicolas Yunes

Date: 16 Dec 2011

Abstract: The future detection of gravitational wave forces us to consider the many ways in which astrophysics, gravitational wave theory and fundamental theory will interact. In this paper, I summarize some recent work done to develop such an interface. In particular, I concentrate on how non-vacuum astrophysical environments can modify the gravitational wave signal emitted by compact binary inspirals, and whether signatures from the former are detectable by current and future gravitational wave detectors. I also describe the interface between gravitational wave modeling and fundamental theory, focusing on the status of the parameterized post-Einsteinian framework (a general framework to detect deviations away from General Relativity in future gravitational wave data) and its current data analysis implementation.

#### Dec 20, 2011

1112.3694 (/preprints)
2011-12-20, 09:06 

## [1112.4375] Possible dark energy imprints in gravitational wave spectrum of mixed neutron-dark-energy stars

Authors: Stoytcho S. Yazadjiev, Daniela D. Doneva

Date: 19 Dec 2011

Abstract: In the present paper we study the oscillation spectrum of neutron stars containing both ordinary matter and dark energy in different proportions. Within the model we consider, the equilibrium configurations are numerically constructed and the results show that the properties of the mixed neuron-dark-energy star can differ significantly when the amount of dark energy in the stars is varied. The oscillations of the mixed neuron-dark-energy stars are studied in the Cowling approximation. As a result we find that the frequencies of the fundamental mode and the higher overtones are strongly affected by the dark energy content. This can be used in the future to detect the presence of dark energy in the neutron stars and to constrain the dark-energy models.

#### Dec 20, 2011

1112.4375 (/preprints)
2011-12-20, 09:05 

## [1112.4154] Constraints on perturbative quadratic gravity via neutron stars

Authors: Cemsinan Deliduman, K. Y. Ek&#x15f;i, Vildan Kele&#x15f;

Date: 18 Dec 2011

Abstract: We study the structure of neutron stars in R+\beta Rˆ{\mu \nu} R_{\mu \nu} gravity model with perturbative method. We obtain mass-radius relations for six representative equations of state (EOSs). We find that deviations from the results of general relativity, comparable to the variations due to using different EoSs, are induced for |\beta| ~ 10ˆ11 cmˆ2. Some of the soft EoSs that are excluded within the framework of general relativity can be reconciled for certain values of \beta\ of this order with the 2 solar mass neutron star recently observed. For some of the EoSs we find that a new solution branch, which allows highly massive neutron stars, exists for values of \beta\ greater than a few 10ˆ11 cmˆ2. We find constraints on \beta\ for a variety of EoSs using the recent observational constraints on the mass-radius relation. The associated length scale \sqrt{\beta} ~ 10ˆ6 cm is of the order of the the typical radius of neutron stars, the probe used in this test. This implies that the true value of \beta\ is most likely much smaller than 10ˆ11 cmˆ2.

#### Dec 20, 2011

1112.4154 (/preprints)
2011-12-20, 09:05 

## [1112.3960] Modified Newtonian Dynamics: A Review

Authors: Benoit Famaey, Stacy McGaugh

Date: 16 Dec 2011

Abstract: A wealth of astronomical data indicate the presence of mass discrepancies in the Universe. The motions observed in a variety of classes of extragalactic systems exceed what can be explained by the mass visible in stars and gas. Either (i) there is a vast amount of unseen mass in some novel form - dark matter - or (ii) the data indicate a breakdown of our understanding of gravity on the relevant scales, or (iii) both. Here, we first review a few outstanding challenges for the dark matter interpretation of mass discrepancies in galaxies, purely based on observations and independently of any alternative theoretical framework. We then show that many of these puzzling observations can be summarized by one single scaling relation - Milgrom's law - involving an acceleration constant (or a characteristic surface density) of the order of the square-root of the cosmological constant in natural units. This relation can at present most easily be interpreted as the effect of a single universal force law resulting from a modification of Newtonian dynamics (MOND) on galactic scales. We exhaustively review the current observational successes and problems of this alternative paradigm at all astrophysical scales, and summarize the various theoretical attempts (TeVeS, GEA, BIMOND, and others) made to effectively embed this modification of Newtonian dynamics within a generally covariant theory of gravity.

#### Dec 20, 2011

1112.3960 (/preprints)
2011-12-20, 09:04 

## [1112.3222] Calculating the continued fraction coefficients of a sub-diagonal Pad\'e approximant at arbitrary order

Authors: J&#xe9;r&#xf4;me Carr&#xe9;, Edward K. Porter

Date: 14 Dec 2011

Abstract: The inspiral of two compact objects in gravitational wave astronomy is described by a post-Newtonian expansion in powers of $(v/c)$. In most cases, it is believed that the post-Newtonian expansion is asymptotically divergent. A standard technique for accelerating the convergence of a power series is to re-sum the series by means of a rational polynomial called a Padé approximation. If we liken this approximation to a matrix, the best convergence is achieved by staying close to a diagonal Padé approximation. This broadly presents two subsets of the approximation : a super-diagonal approximation $PˆM_N$ and a sub-diagonal approximation $P_MˆN$, where $M = N+\epsilon$, and $\epsilon$ takes the values of 0 or 1. Left as rational polynomials, the coefficients in both the numerator and denominator need to be re-calculated as the order of the initial power series approximation is increased. However, the sub-diagonal Padé approximant is computationally advantageous as it can be expressed in terms of a Gauss-like continued fraction. Once in this form, each coefficient in the continued fraction is uniquely determined at each order. This means that as we increase the order of approximation of the original power series, we now have only one new additional coefficient to calculate in the continued fraction. While it is possible to provide explicit expressions for the continued fraction coefficients, they rapidly become unwieldy at high orders of approximation. It is also possible to numerically calculate the coefficients by means of ratios of Hankel determinants. However, these determinants can be ill-conditioned and lead to numerical instabilities. In this article, we present a method for calculating the continued fraction coefficients at arbitrary orders of approximation.

#### Dec 16, 2011

1112.3222 (/preprints)
2011-12-16, 22:07 

## [1112.3090] The cross-correlation search for a hot spot of gravitational waves : Numerical study for point spread function

Date: 14 Dec 2011

Abstract: The cross-correlation search for gravitational wave, which is known as 'radiometry', has been previously applied to map of the gravitational wave stochastic background in the sky and also to target on gravitational wave from rotating neutron stars/pulsars. We consider the Virgo cluster where may be appear as ‘hot spot’ spanning few pixels in the sky in radiometry analysis. Our results show that sufficient signal to noise ratio can be accumulated with integration times of the order of a year. We also construct numerical simulation of radiometry analysis, assuming current constructing/upgrading ground-based detectors. Point spread function of the injected sources are confirmed by numerical test. Typical resolution of radiometry analysis is a few square degree which corresponds to several thousand pixels of sky mapping.

#### Dec 16, 2011

1112.3090 (/preprints)
2011-12-16, 22:07 

## [1112.3092] LCGT and the global network of gravitational wave detectors

Authors: Nobuyuki Kanda, the LCGT collaboration

Date: 14 Dec 2011

Abstract: Gravitational wave is a propagation of space-time distortion, which is predicted by Einstein in general relativity. Strong gravitational waves will come from some drastic astronomical objects, e.g. coalescence of neutron star binaries, black holes, supernovae, rotating pulsars and pulsar glitches. Detection of the gravitational waves from these objects will open a new door of \textit{‘gravitational wave astronomy’}. Gravitational wave will be a probe to study the physics and astrophysics. To search these gravitational waves, large-scale laser interferometers will compose a global network of detectors. Advanced LIGO and advanced Virgo are upgrading from currents detectors. One of LIGO detector is considering to move Australia Site. IndIGO or Einstein Telescope are future plans. LCGT (Large-scale Cryogenic Gravitational wave Telescope) is now constructing in Japan with distinctive characters: cryogenic cooling mirror and underground site. We will present a design and a construction status of LCGT, and brief status of current gravitational wave detectors in the world. Network of these gravitational wave detectors will start in late 2016 or 2017, and may discover the gravitational waves. For example, these detectors will reach its search range for coalescence of neutron star binary is over 200 Mpc, and several or more events per year will be expected. Since most of gravitational wave events are from high-energy phenomenon of the astronomical objects, these might have counterpart evidences in electromagnetic radiation (visible light, X/gamma ray), neutrino, high energy particles or others. Thus, the mutual follow-up observations will give us more information of these objects.

#### Dec 16, 2011

1112.3092 (/preprints)
2011-12-16, 22:07 

## [1112.3077] From black holes to their progenitors: A full population study in measuring black hole binary parameters from ringdown signals

Authors: Ioannis Kamaretsos

Date: 14 Dec 2011

Abstract: A perturbed black hole emits gravitational radiation, usually termed the ringdown signal, whose frequency and damping time depends on the mass and spin of the black hole. I investigate the case of a binary black hole merger resulting from two initially non-spinning black holes of various mass ratios, in quasi-circular orbits. The observed ringdown signal will be determined, among other things, by the black hole's spin-axis orientation with respect to Earth, its sky position and polarization angle - parameters which can take any values in a particular observation. I have carried out a statistical analysis of the effect of these variables, focusing on detection and measurement of the multimode ringdown signals using the reformulated European LISA mission, Next Gravitational-Wave Observatory, NGO, the third generation ground-based observatory, Einstein Telescope and the advanced era detector, aLIGO. To the extent possible I have discussed the effect of these results on plausible event rates, as well as astrophysical implications concerning the formation and growth of supermassive and intermediate mass black holes.

#### Dec 16, 2011

1112.3077 (/preprints)
2011-12-16, 22:06 

## [1112.3351] Gravitational Waves from Extreme Mass-Ratio Inspirals as Probes of Scalar-Tensor Theories

Authors: Nicolas Yunes, Paolo Pani, Vitor Cardoso

Date: 14 Dec 2011

Abstract: A stellar-mass compact object spiraling into a supermassive black hole, an extreme-mass-ratio inspiral (EMRI), is one of the targets of future gravitational-wave detectors and it offers a unique opportunity to test General Relativity (GR) in the strong-field. We study whether generic scalar-tensor (ST) theories can be further constrained with EMRIs. We show that in the EMRI limit, all such theories universally reduce to massive or massless Brans-Dicke theory and that black holes do not emit dipolar radiation to all orders in post-Newtonian (PN) theory. For massless theories, we calculate the scalar energy flux in the Teukolsky formalism to all orders in PN theory and fit it to a high-order PN expansion. We derive the PN ST corrections to the Fourier transform of the gravitational wave response and map it to the parameterized post-Einsteinian framework. We use the effective-one-body framework adapted to EMRIs to calculate the ST modifications to the gravitational waveform. We find that such corrections are smaller than those induced in the early inspiral of comparable-mass binaries, leading to projected bounds on the coupling that are worse than current Solar System ones. Brans-Dicke theory modifies the weak-field, with deviations in the energy flux that are largest at small velocities. For massive theories, superradiance can lead to resonances in the scalar energy flux that can lead to floating orbits outside the innermost stable circular orbit and that last until the supermassive black hole loses enough mass and spin-angular momentum. If such floating orbits occur in the frequency band of LISA, they would lead to a large dephasing (~1e6 rads), preventing detection with GR templates. A detection that is consistent with GR would then rule out floating resonances at frequencies lower than the lowest observed frequency, allowing for the strongest constraints yet on massive ST theories.

#### Dec 16, 2011

1112.3351 (/preprints)
2011-12-16, 22:06 

## [1112.1949] What Drives the Growth of Black Holes?

Authors: David M. Alexander (Durham), Ryan C. Hickox (Durham, Dartmouth)

Date: 8 Dec 2011

Abstract: Massive black holes (BHs) are at once exotic and yet ubiquitous, residing in the centers of massive galaxies in the local Universe. Recent years have seen remarkable advances in our understanding of how these BHs form and grow over cosmic time, during which they are revealed as active galactic nuclei (AGN). However, despite decades of research, we still lack a coherent picture of the physical drivers of BH growth, the connection between the growth of BHs and their host galaxies, the role of large-scale environment on the fueling of BHs, and the impact of BH-driven outflows on the growth of galaxies. In this paper we review our progress in addressing these key issues, motivated by the science presented at the "What Drives the Growth of Black Holes?" workshop held at Durham on 26th-29th July 2010, and discuss how these questions may be tackled with current and future facilities.

#### Dec 11, 2011

1112.1949 (/preprints)
2011-12-11, 22:58 

## [1112.1057] Astronomy and astrophysics with gravitational waves in the Advanced Detector Era

Authors: Alan J. Weinstein, for the LIGO Scientific Collaboration, for the Virgo Collaboration

Date: 5 Dec 2011

Abstract: With the advanced gravitational wave detectors coming on line in the next 5 years, we expect to make the first detections of gravitational waves from astrophysical sources, and study the properties of the waves themselves as tests of General Relativity. In addition, these gravitational waves will be powerful tools for the study of their astrophysical sources and source populations. They carry information that is quite complementary to what can be learned from electromagnetic or neutrino observations, probing the central gravitational engines that power the electromagnetic emissions. Preparations are being made to enable near-simultaneous observations of both gravitational wave and electromagnetic observations of transient sources, using low-latency search pipelines and rapid sky localization. We will review the many opportunities for multi-messenger astronomy and astrophysics with gravitational waves enabled by the advanced detectors, and the preparations that are being made to quickly and fully exploit them.

#### Dec 07, 2011

1112.1057 (/preprints)
2011-12-07, 17:59 

## [1112.0567] Lensing of 21-cm Fluctuations by Primordial Gravitational Waves

Authors: Laura Book, Marc Kamionkowski, Fabian Schmidt

Date: 2 Dec 2011

Abstract: Weak-gravitational-lensing distortions to the intensity pattern of 21-cm radiation from the dark ages can be decomposed geometrically into curl and curl-free components. Lensing by primordial gravitational waves induces a curl component, while the contribution from lensing by density fluctuations is strongly suppressed. Angular fluctuations in the 21-cm background extend to very small angular scales, and measurements at different frequencies probe different shells in redshift space. There is thus a huge trove of information with which to reconstruct the curl component of the lensing field, allowing tensor-to-scalar ratios conceivably as small as r ~ 10ˆ{-9} - far smaller than those currently accessible - to be probed.

#### Dec 07, 2011

1112.0567 (/preprints)
2011-12-07, 17:58 

## [1112.0568] General relativistic simulations of black hole-neutron star mergers: Effects of magnetic fields

Authors: Zachariah B. Etienne, Yuk Tung Liu, Vasileios Paschalidis, Stuart L. Shapiro

Date: 2 Dec 2011

Abstract: As a neutron star (NS) is tidally disrupted by a black hole (BH) companion at the end of a BH-NS binary inspiral, its magnetic fields will be stretched and amplified. If sufficiently strong, these magnetic fields may impact the gravitational waveforms, merger evolution and mass of the remnant disk. Formation of highly-collimated magnetic field lines in the disk+spinning BH remnant may launch relativistic jets, providing the engine for a short-hard GRB. We analyze this scenario through fully general relativistic, magnetohydrodynamic (GRMHD) BHNS simulations from inspiral through merger and disk formation. Different initial magnetic field configurations and strengths are chosen for the NS interior for both nonspinning and moderately spinning (a/M=0.75) BHs aligned with the orbital angular momentum. Only strong interior (Bmax~10ˆ17 G) initial magnetic fields in the NS significantly influence merger dynamics, enhancing the remnant disk mass by 100% and 40% in the nonspinning and spinning BH cases, respectively. However, detecting the imprint of even a strong magnetic field may be challenging for Advanced LIGO. Though there is no evidence of mass outflows or magnetic field collimation during the preliminary simulations we have performed, higher resolution, coupled with longer disk evolutions and different initial magnetic field configurations, may be required to definitively assess the possibility of BHNS binaries as short-hard GRB progenitors.

#### Dec 07, 2011

1112.0568 (/preprints)
2011-12-07, 17:58 

## [1112.0569] Observational Evidence for a Correlation Between Jet Power and Black Hole Spin

Authors: Ramesh Narayan, Jeffrey E. McClintock

Date: 2 Dec 2011

Abstract: We show that the 5-GHz radio flux of transient ballistic jets in black hole binaries correlates with the dimensionless black hole spin parameter a* estimated via the continuum-fitting method. The data suggest that jet power scales either as the square of a* or the square of the angular velocity of the horizon. This is the first direct evidence that jets may be powered by black hole spin energy. The observed correlation validates the continuum-fitting method of measuring spin. In addition, for those black holes that have well-sampled radio observations of ballistic jets, the correlation may be used to obtain rough estimates of their spins.

#### Dec 07, 2011

1112.0569 (/preprints)
2011-12-07, 17:58 

## [1112.1081] Recoiling Supermassive Black Holes in Spin-flip Radio Galaxies

Authors: F. K. Liu (PKU), Dong Wang (PKU), Xian Chen (KIAA-PKU)

Date: 5 Dec 2011

Abstract: Numerical relativity simulations predict that coalescence of supermassive black hole (SMBH) binaries not only leads to a spin flip but also to a recoiling of the merger remnant SMBHs. In the literature, X-shaped radio sources are popularly suggested to be candidates for SMBH mergers with spin flip of jet-ejecting SMBHs. Here we investigate the spectral and spatial observational signatures of the recoiling SMBHs in radio sources undergoing black hole spin flip. Our results show that SMBHs in most spin-flip radio sources have mass ratio $q\ga 0.3$ with a minimum possible value $q_{\rm min} \simeq 0.05$. For major mergers, the remnant SMBHs can get a kick velocity as high as $2100 km sˆ{-1}$ in the direction within an angle $\la 40ˆ\circ$ relative to the spin axes of remnant SMBHs, implying that recoiling quasars are biased to be with high Doppler-shifted broad emission lines while recoiling radio galaxies are biased to large apparent spatial off-center displacements. We also calculate the distribution functions of line-of-sight velocity and apparent spatial off-center for spin-flip radio sources with different apparent jet reorientation angles. Our results show that the larger the apparent jet reorientation angle is, the larger the Doppler-shifting recoiling velocity and apparent spatial off-center displacement will be. We investigate the effects of recoiling velocity on the dust torus in spin-flip radio sources and suggest that recoiling of SMBHs would lead to "dust poor" AGNs. Finally, we collect a sample of 19 X-shaped radio objects and for each object give the probability of detecting the predicted signatures of recoiling SMBH.

#### Dec 07, 2011

1112.1081 (/preprints)
2011-12-07, 17:55 

## [1112.0434] Test of the Law of Gravitation at small Accelerations

Authors: H.Meyer (Bergische Universitaet Wuppertal), E.Lohrmann, S.Schubert (Universitaet Hamburg), W.Bartel, A.Glazov, B.Loehr, C.Niebuhr, E.Wuensch (DESY), L.Joensson (University of Lund), G.Kempf (Hamburgische Schiffbau-Versuchsanstalt)

Date: 2 Dec 2011

Abstract: Newton's Law of Gravitation has been tested at small values of the acceleration, down to a=10ˆ{-10} m/sˆ2, the approximate value of MOND's constant a_0. No deviations were found.

#### Dec 05, 2011

1112.0434 (/preprints)
2011-12-05, 17:34 

## [1111.6588] The ELM Survey. III. A Successful Targeted Survey for Extremely Low Mass White Dwarfs

Authors: Warren R. Brown (1), Mukremin Kilic (2), Carlos Allende Prieto (3), Scott J. Kenyon (1) ((1) SAO, (2) OU, (3) IAC)

Date: 28 Nov 2011

Abstract: Extremely low mass (ELM) white dwarfs (WDs) with masses <0.25 Msun are rare objects that result from compact binary evolution. Here, we present a targeted spectroscopic survey of ELM WD candidates selected by color. The survey is 71% complete and has uncovered 18 new ELM WDs. Of the 7 ELM WDs with follow-up observations, 6 are short-period binaries and 4 have merger times less than 5 Gyr. The most intriguing object, J1741+6526, likely has either a pulsar companion or a massive WD companion making the system a possible supernova Type Ia or .Ia progenitor. The overall ELM Survey has now identified 19 double degenerate binaries with <10 Gyr merger times. The significant absence of short orbital period ELM WDs at cool temperatures suggests that common envelope evolution creates ELM WDs directly in short period systems. At least one-third of the merging systems are halo objects, thus ELM WD binaries continue to form and merge in both the disk and the halo.

#### Nov 30, 2011

1111.6588 (/preprints)
2011-11-30, 17:04 

## [1111.6585] The Milky Way has no thick disk

Authors: Jo Bovy (IAS), Hans-Walter Rix (MPIA), David W. Hogg (NYU, MPIA)

Date: 28 Nov 2011

Abstract: Different stellar sub-populations of the Milky Way's stellar disk are known to have different vertical scale heights, their thickness increasing with age. Using SEGUE spectroscopic survey data, we have recently shown that mono-abundance sub-populations, defined in the [\alpha/Fe]-[Fe/H] space, are well described by single exponential spatial-density profiles in both the radial and the vertical direction; therefore any star of a given abundance is clearly associated with a sub-population of scale height h_z. Here, we work out how to determine the stellar surface-mass density contributions at the solar radius R_0 of each such sub-population, accounting for the survey selection function, and for the fraction of the stellar population mass that is reflected in the spectroscopic target stars given populations of different abundances and their presumed age distributions. Taken together, this enables us to derive \Sigma_{R_0}(h_z), the surface-mass contributions of stellar populations with scale height h_z. Surprisingly, we find no hint of a thin-thick disk bi-modality in this mass-weighted scale-height distribution, but a smoothly decreasing function, approximately \Sigma_{R_0}(h_z)\propto \exp(-h_z), from h_z ~ 200 pc to h_z ~ 1 kpc. As h_z is ultimately the structurally defining property of a thin or thick disk, this shows clearly that the Milky Way has a continuous and monotonic distribution of disk thicknesses: there is no 'thick disk' sensibly characterized as a distinct component. We discuss how our result is consistent with evidence for seeming bi-modality in purely geometric disk decompositions, or chemical abundances analyses.

#### Nov 30, 2011

1111.6585 (/preprints)
2011-11-30, 17:03 

## [1111.6638] The stochastic background from cosmic (super)strings: popcorn and (Gaussian) continuous regimes

Authors: Tania Regimbau, Stefanos Giampanis, Xavier Siemens, Vuk Mandic

Date: 28 Nov 2011

Abstract: In the era of the next generation of gravitational wave experiments a stochastic background from cusps of cosmic (super)strings is expected to be probed and, if not detected, to be significantly constrained. A popcorn-like background can be, for part of the parameter space, as pronounced as the (Gaussian) continuous contribution from unresolved sources that overlap in frequency and time. We study both contributions from unresolved cosmic string cusps over a range of frequencies relevant to ground based interferometers, such as LIGO/Virgo second generation (AdLV) and Einstein Telescope (ET) third generation detectors, the space antenna LISA and Pulsar Timing Arrays (PTA). We compute the sensitivity (at $2 \sigma$ level) in the parameter space for AdLV, ET, LISA and PTA. We conclude that the popcorn regime is complementary to the continuous background. Its detection could therefore enhance confidence in a stochastic background detection and possibly help determine fundamental string parameters such as the string tension and the reconnection probability.

#### Nov 30, 2011

1111.6638 (/preprints)
2011-11-30, 17:02 

## [1111.6908] Evolution of inspiral orbits around a Schwarzschild black hole

Authors: Niels Warburton, Sarp Akcay, Leor Barack, Jonathan R. Gair, Norichika Sago

Date: 29 Nov 2011

Abstract: We present results from calculations of the orbital evolution in eccentric binaries of nonrotating black holes with extreme mass-ratios. Our inspiral model is based on the method of osculating geodesics, and is the first to incorporate the full gravitational self-force (GSF) effect, including conservative corrections. The GSF information is encapsulated in an analytic interpolation formula based on numerical GSF data for over a thousand sample geodesic orbits. We assess the importance of including conservative GSF corrections in waveform models for gravitational-wave searches.

#### Nov 30, 2011

1111.6908 (/preprints)
2011-11-30, 17:02 

## [1111.6607] Tidal interaction in compact binaries: a post-Newtonian affine framework

Authors: V. Ferrari, L. Gualtieri, A. Maselli

Date: 28 Nov 2011

Abstract: We develop a semi-analytical approach, based on the post-Newtonian expansion and on the affine approximation, to model the tidal deformation of neutron stars in the coalescence of black hole-neutron star or neutron star-neutron star binaries. Our equations describe, in a unified framework, both the system orbital evolution, and the neutron star deformations. These are driven by the tidal tensor, which we expand at 1/cˆ3 post-Newtonian order, including spin terms. We test the theoretical framework by simulating black hole-neutron star coalescence up to the onset of mass shedding, which we determine by comparing the shape of the star with the Roche lobe. We validate our approach by comparing our results with those of fully relativistic, numerical simulations.

#### Nov 30, 2011

1111.6607 (/preprints)
2011-11-30, 17:01 

## [1111.3742] Origin and Implications of high eccentricities in massive black hole binaries at sub-pc scales

Authors: Constanze Roedig, Alberto Sesana

Date: 16 Nov 2011

Abstract: We outline the eccentricity evolution of sub-parsec massive black hole binaries (MBHBs) forming in galaxy mergers. In both stellar and gaseous environments, MBHBs are expected to grow large orbital eccentricities before they enter the gravitational wave (GW) observational domain. We re--visit the predicted eccentricities detectable by space based laser interferometers (as the proposed ELISA/NGO) for both environments. Close to coalescence, many MBHBs will still maintain detectable eccentricities, spanning a broad range from <10ˆ{-5} up to <~ 0.5. Stellar and gas driven dynamics lead to distinct distributions, with the latter favoring larger eccentricities. At larger binary separations, when emitted GWs will be observed by pulsar timing arrays (PTAs), the expected eccentricities are usually quite large, in the range 0.01-0.7, which poses an important issue for signal modelling and detection algorithms. In this window, large eccentricities also have implications on proposed electromagnetic counterparts to the GW signal, which we briefly review.

#### Nov 29, 2011

1111.3742 (/preprints)
2011-11-29, 04:48 

## [1111.5819] Bayesian model selection for testing the no-hair theorem with black hole ringdowns

Authors: S. Gossan, J. Veitch, B. S. Sathyaprakash

Date: 24 Nov 2011

Abstract: General relativity predicts that a black hole that results from the merger of two compact stars (either black holes or neutron stars) is initially highly deformed but soon settles down to a quiescent state by emitting a superposition of quasi-normal modes (QNMs). The QNMs are damped sinusoids with characteristic frequencies and decay times that depend only on the mass and spin of the black hole and no other parameter - a statement of the no-hair theorem. In this paper we have examined the extent to which QNMs could be used to test the no-hair theorem with future ground- and space-based gravitational-wave detectors. We model departures from general relativity (GR) by introducing extra parameters which change the mode frequencies or decay times from their general relativistic values. With the aid of numerical simulations and Bayesian model selection, we assess the extent to which the presence of such a parameter could be inferred, and its value estimated. We find that it is harder to decipher the departure of decay times from their GR value than it is with the mode frequencies. Einstein Telescope (ET, a third generation ground-based detector) could detect departures of <1% in the frequency of the dominant QNM mode of a 500 Msun black hole, out to a maximum range of 4 Gpc. In contrast, the New Gravitational Observatory (NGO, an ESA space mission to detect gravitational waves) can detect departures of ~ 0.1% in a 10ˆ8 Msun black hole to a luminosity distance of 30 Gpc (z = 3.5).

#### Nov 29, 2011

1111.5819 (/preprints)
2011-11-29, 04:47 

## [1111.5650] Estimating the sensitivity of wide-parameter-space searches for gravitational-wave pulsars

Authors: Karl Wette

Date: 23 Nov 2011

Abstract: This paper presents an in-depth study of how to estimate the sensitivity of searches for gravitational-wave pulsars -- rapidly-rotating neutron stars which emit quasi-sinusoidal gravitational waves. It is particularly concerned with searches over a wide range of possible source parameters, such as searches over the entire sky and broad frequency bands. Traditional approaches to estimating the sensitivity of such searches use either computationally-expensive Monte Carlo simulations, or analytic methods which sacrifice accuracy by making an unphysical assumption about the population of sources being searched for. This paper develops a new, analytic method of estimating search sensitivity which does not rely upon this unphysical assumption. Unlike previous analytic methods, the new method accurately predicts the sensitivity obtained using Monte Carlo simulations, while avoiding their computational expense. The change in estimated sensitivity due to properties of the search template bank, and the geographic configuration of the gravitational wave detector network, are also investigated.

#### Nov 29, 2011

1111.5650 (/preprints)
2011-11-29, 04:46 

## [1111.5661] Stochastic backgrounds in alternative theories of gravity: overlap reduction functions for pulsar timing arrays

Authors: Sydney J. Chamberlin, Xavier Siemens

Date: 24 Nov 2011

Abstract: In the next decade gravitational waves could be detected using a pulsar timing array. In an effort to develop optimal detection strategies for stochastic backgrounds of gravitational waves in generic metric theories of gravity, we investigate the overlap reduction functions for these theories and discuss their features. We show that sensitivity increases for non-transverse gravitational waves and discuss the physical origin of this effect. We calculate the overlap reduction functions for the current NANOGrav Pulsar Timing Array (PTA) and show that the sensitivity to the vector and scalar-longitudinal modes can increase dramatically for pulsar pairs with small angular separations. For example, the J1853-J1857 pulsar pair, with an angular separation of 3.47 degrees, is about 10ˆ4 times more sensitive to the longitudinal component of the stochastic background, if it's present, than the transverse components.

#### Nov 29, 2011

1111.5661 (/preprints)
2011-11-29, 04:46 

## [1111.6125] Imprint of the merger and ring-down on the gravitational wave background from black hole binaries coalescence

Authors: S. Marassi, R. Schneider, G. Corvino, V. Ferrari, S. Portegies Zwart

Date: 25 Nov 2011

Abstract: We compute the gravitational wave background (GWB) generated by a cosmological population of (BH-BH) binaries using hybrid waveforms recently produced by numerical simulations of (BH-BH) coalescence, which include the inspiral, merger and ring-down contributions. A large sample of binary systems is simulated using the population synthesis code SeBa, and we extract fundamental statistical information on (BH-BH) physical parameters (primary and secondary BH masses, orbital separations and eccentricities, formation and merger timescales). We then derive the binary birth and merger rates using the theoretical cosmic star formation history obtained from a numerical study which reproduces the available observational data at redshifts $z < 8$. We evaluate the contributions of the inspiral, merger and ring-down signals to the GWB, and discuss how these depend on the parameters which critically affect the number of coalescing (BH-BH) systems. We find that Advanced LIGO/Virgo have a chance to detect the GWB signal from the inspiral phase with a $(S/N)=10$ only for the most optimistic model, which predicts the highest local merger rate of 0.85 Mpc$ˆ{-3}$ Myr$ˆ{-1}$. Third generation detectors, such as ET, could reveal the GWB from the inspiral phase predicted by any of the considered models. In addition, ET could sample the merger phase of the evolution at least for models which predict local merger rates between $[0.053 - 0.85]$ Mpc$ˆ{-3}$ Myr$ˆ{-1}$, which are more than a factor 2 lower the the upper limit inferred from the analysis of the LIGO S5 run\cite{Abadieetal2011}.

#### Nov 29, 2011

1111.6125 (/preprints)
2011-11-29, 04:43 

## [1106.1616] Measuring neutron-star properties via gravitational waves from binary mergers

Authors: A. Bauswein, H.-Th. Janka (Max Planck Institute for Astrophysics, Garching)

Date: 8 Jun 2011

Abstract: We demonstrate by a large set of merger simulations for symmetric binary neutron stars (NSs) that there is a tight correlation between the frequency peak of the postmerger gravitational-wave (GW) emission and the physical properties of the nuclear equation of state (EoS), e.g. expressed by the radius of the maximum-mass Tolman-Oppenheimer-Volkhoff configuration. Therefore, a single measurement of the peak frequency of the postmerger GW signal will constrain the NS EoS significantly. For plausible optimistic merger-rate estimates a corresponding detection with Advanced LIGO is likely to happen within an operation time of roughly a year.

#### Nov 23, 2011

1106.1616 (/preprints)
2011-11-23, 23:42 

## [1111.5610] The complete non-spinning effective-one-body metric at linear order in the mass ratio

Authors: Enrico Barausse, Alessandra Buonanno, Alexandre Le Tiec

Date: 23 Nov 2011

Abstract: Using the main result of a companion paper, in which the binding energy of a circular-orbit non-spinning compact binary system is computed at leading-order beyond the test-particle approximation, the exact expression of the effective-one-body (EOB) metric component $gˆ\text{eff}_{tt}$ is obtained through first order in the mass ratio. Combining these results with the recent gravitational self-force calculation of the periastron advance for circular orbits in the Schwarzschild geometry, the EOB metric component $gˆ\text{eff}_{rr}$ is also determined at linear order in the mass ratio. These results assume that the mapping between the real and effective Hamiltonians at the second and third post-Newtonian (PN) orders holds at all PN orders. Our findings also confirm the advantage of resumming the PN dynamics around the test-particle limit if the goal is to obtain a flexible model that can smoothly connect the test-mass and equal-mass limits.

#### Nov 23, 2011

1111.5610 (/preprints)
2011-11-23, 23:42 

## [1111.5488] Tail terms in gravitational radiation reaction via effective field theory

Authors: S. Foffa, R. Sturani

Date: 23 Nov 2011

Abstract: Gravitational radiation reaction affects the dynamics of gravitationally bound binary systems via "tail" terms which, at the lowest level, modify the conservative dynamics at fourth post-Newtonian order, as it was first computed by Blanchet and Damour. Here we re-produce this result using effective field theory techniques in the framework of the closed-time-path formalism. This tail term is the lowest order example of a short-distance singularity showing up in the conservative dynamics, and it is correctly taken into account within the effective field theory formalism.

#### Nov 23, 2011

1111.5488 (/preprints)
2011-11-23, 23:41 

## [1111.5609] Gravitational Self-Force Correction to the Binding Energy of Compact Binary Systems

Authors: Alexandre Le Tiec, Enrico Barausse, Alessandra Buonanno

Date: 23 Nov 2011

Abstract: Using the first law of binary black-hole mechanics, we compute the binding energy E and total angular momentum J of two non-spinning compact objects moving on circular orbits with frequency Omega, at leading order beyond the test-particle approximation. By minimizing E(Omega) we recover the exact frequency shift of the Schwarzschild innermost stable circular orbit induced by the conservative piece of the gravitational self-force. Comparing our results for the coordinate invariant relation E(J) to those recently obtained from numerical simulations of comparable-mass non-spinning black-hole binaries, we find a remarkably good agreement, even in the strong-field regime. Our findings confirm that the domain of validity of perturbative calculations may extend well beyond the extreme mass-ratio limit.

#### Nov 23, 2011

1111.5609 (/preprints)
2011-11-23, 23:41 

## [1111.5378] The First Law of Binary Black Hole Mechanics in General Relativity and Post-Newtonian Theory

Authors: Alexandre Le Tiec, Luc Blanchet, Bernard F. Whiting

Date: 23 Nov 2011

Abstract: First laws of black hole mechanics, or thermodynamics, come in a variety of different forms. In this paper, from a purely post-Newtonian (PN) analysis, we obtain a first law for binary systems of point masses moving along an exactly circular orbit. Our calculation is valid through 3PN order and includes, in addition, the contributions of logarithmic terms at 4PN and 5PN orders. This first law of binary point-particle mechanics is then derived from first principles in general relativity, and analogies are drawn with the single and binary black hole cases. Some consequences of the first law are explored for PN spacetimes. As one such consequence, a simple relation between the PN binding energy of the binary system and Detweiler's redshift observable is established. Through it, we are able to determine with high precision the numerical values of some previously unknown high order PN coefficients in the circular-orbit binding energy. Finally, we propose new gauge invariant notions for the energy and angular momentum of a particle in a binary system.

#### Nov 23, 2011

1111.5378 (/preprints)
2011-11-23, 23:40 

## [1111.5274] Towards a generic test of the strong field dynamics of general relativity using compact binary coalescence: Further investigations

Authors: T. G. F. Li, W. Del Pozzo, S. Vitale, C. Van Den Broeck, M. Agathos, J. Veitch, K. Grover, T. Sidery, R. Sturani, A. Vecchio

Date: 22 Nov 2011

Abstract: Coalescences of binary neutron stars and/or black holes are candidate sources for the first direct detection of gravitational waves. These events will also provide us with the very first empirical access to the genuinely strong-field dynamics of General Relativity (GR). We elaborate on a framework based on Bayesian model selection aimed at detecting deviations from GR, subject to the constraints of Advanced Virgo and LIGO detectors, first introduced by Li et al. (2011). The key aspect of the framework is testing the consistency of the post-Newtonian gravitational-wave phase coefficients in the inspiral regime with the predictions made by GR, without relying on any specific alternative theory of gravity. The framework is suitable for low signal-to-noise events through construction of multiple subtests, most of which involving only a limited number of phase coefficients. The framework also naturally allows for the combination of multiple sources to increase the information extracted for GR testing. In our previous work, we conjectured that this framework can detect generic deviations from GR that can in principle not be accomodated by our model waveforms, on condition that the change in phase near frequencies where the detectors are the most sensitive is comparable to that induced by simple shifts in the lower-order phase coefficients of more than a few percent. To further support this claim, we perform additional numerical experiments in Gaussian and stationary noise according to the expected Advanced LIGO/Virgo noise curves, and injecting signals whose phasing differs structurally from the predictions of GR, but with the magnitude of the deviation still being small. We find that even then, a violation of GR can be established with good confidence.

#### Nov 23, 2011

1111.5274 (/preprints)
2011-11-23, 23:39 

## [1111.3243] Importance of including small body spin effects in the modelling of intermediate mass-ratio inspirals. II Accurate parameter extraction of strong sources using higher-order spin effects

Authors: E. A. Huerta, Jonathan R. Gair, Duncan A. Brown

Date: 14 Nov 2011

Abstract: We extend the numerical kludge waveform model introduced in [1] in two ways. We extend the equations of motion for spinning black hole binaries derived by Saijo et al. [2] using spin-orbit and spin-spin couplings taken from perturbative and post-Newtonian (PN) calculations at the highest order available. We also include first-order conservative self-force corrections for spin-orbit and spin-spin couplings, which are derived by comparison to PN results. We generate the inspiral evolution using fluxes that include the most recent calculations of small body spin corrections, spin-spin and spin-orbit couplings and higher-order fits to solutions of the Teukolsky equation. Using a simplified version of this model in [1], we found that small body spin effects may be measured through gravitational wave observations from intermediate-mass ratio inspirals (IMRIs) with mass ratio eta ~ 0.001, when both binary components are rapidly rotating. In this paper we study in detail how the spin of the small/big body affects parameter measurement using a variety of mass and spin combinations for typical IMRIs sources. We find that for IMRI events of a moderately rotating intermediate mass black hole (IMBH) of 10ˆ4 solar masses, and a rapidly rotating central supermassive black hole (SMBH) of 10ˆ6 solar masses, gravitational wave observations made with LISA at a fixed signal-to-noise ratio (SNR) of 1000 will be able to determine the inspiralling IMBH mass, the central SMBH mass, the SMBH spin magnitude, and the IMBH spin magnitude to within fractional errors of ~10ˆ{-3}, 10ˆ{-3}, 10ˆ{-4}, and 9%, respectively. LISA can also determine the location of the source in the sky and the SMBH spin orientation to within ~10ˆ{-4} steradians. We show that by including conservative corrections up to 2.5PN order, systematic errors no longer dominate over statistical errors for IMRIs with typical SNR ~1000.

#### Nov 22, 2011

1111.3243 (/preprints)
2011-11-22, 15:18 

## [1111.3044] Effect of calibration errors on Bayesian parameter estimation for gravitational wave signals from inspiral binary systems in the Advanced Detectors era

Authors: Salvatore Vitale, Walter Del Pozzo, Tjonnie G. F. Li, Chris Van Den Broeck, Ilya Mandel, Ben Aylott, John Veitch

Date: 13 Nov 2011

Abstract: By 2015 the advanced versions of the gravitational-wave detectors Virgo and LIGO will be online. They will collect data in coincidence with enough sensitivity to potentially deliver multiple detections of gravitation waves from inspirals of compact-object binaries. This work is focused on understanding the effects introduced by uncertainties in the calibration of the interferometers. We consider plausible calibration errors based on estimates obtained during LIGO's fifth and Virgo's third science runs, which include frequency-dependent amplitude errors of $\sim 10%$ and frequency-dependent phase errors of $\sim 3$ degrees in each instrument. We quantify the consequences of such errors estimating the parameters of inspiraling binaries. We find that the systematics introduced by calibration errors on the inferred values of the chirp mass and mass ratio are smaller than 20% of the statistical measurement uncertainties in parameter estimation for 90% of signals in our mock catalog. Meanwhile, the calibration-induced systematics in the inferred sky location of the signal are smaller than $\sim 50%$ of the statistical uncertainty. We thus conclude that calibration-induced errors at this level are not a significant detriment to accurate parameter estimation.

#### Nov 22, 2011

1111.3044 (/preprints)
2011-11-22, 15:17 

## [1111.3055] Eccentric black hole-neutron star mergers: effects of black hole spin and equation of state

Authors: William E. East, Frans Pretorius, Branson C. Stephens

Date: 13 Nov 2011

Abstract: There is a high level of interest in black hole-neutron star binaries, not only because their mergers may be detected by gravitational wave observatories in the coming years, but also because of the possibility that they could explain a class of short duration gamma-ray bursts. We study black hole-neutron star mergers that occur with high eccentricity as may arise from dynamical capture in dense stellar regions such as nuclear or globular clusters. We perform general relativistic simulations of binaries with a range of impact parameters, three different initial black hole spins (zero, aligned and anti-aligned with the orbital angular momentum), and neutron stars with three different equations of state. We find a rich diversity across these parameters in the resulting gravitational wave signals and matter dynamics, which should also be reflected in the consequent electromagnetic emission. Before tidal disruption, the gravitational wave emission is significantly larger than perturbative predictions suggest for periapsis distances close to effective innermost stable separations, exhibiting features reflecting the zoom-whirl dynamics of the orbit there. Guided by the simulations, we develop a simple model for the change in orbital parameters of the binary during close encounters. Depending upon the initial parameters of the system, we find that mass transfer during non-merging close encounters can range from essentially zero to a sizable fraction of the initial neutron star mass. The same holds for the amount of material outside the black hole post-merger, and in some cases roughly half of this material is estimated to be unbound. We also see that non-merging close encounters generically excite large oscillations in the neutron star that are qualitatively consistent with f-modes.

#### Nov 22, 2011

1111.3055 (/preprints)
2011-11-22, 15:17 

## [1111.4009] Cosmic Black-Hole Hair Growth and Quasar OJ287

Authors: M. W. Horbatsch, C. P. Burgess

Date: 17 Nov 2011

Abstract: An old result ({\tt astro-ph/9905303}) by Jacobson implies that a black hole with Schwarzschild radius $r_s$ acquires scalar hair, $Q \propto r_sˆ2 \mu$, when the (canonically normalized) scalar field in question is slowly time-dependent far from the black hole, $\partial_t \phi \simeq \mu M_p$ with $\mu r_s \ll 1$ time-independent. Such a time dependence could arise in scalar-tensor theories either from cosmological evolution, or due to the slow motion of the black hole within an asymptotic spatial gradient in the scalar field. Most remarkably, the amount of scalar hair so induced is independent of the strength with which the scalar couples to matter. We argue that Jacobson's Miracle Hair-Growth Formula${}ˆ\copyright$ implies, in particular, that an orbiting pair of black holes can radiate {\em dipole} radiation, provided only that the two black holes have different masses. Quasar OJ 287, situated at redshift $z \simeq 0.306$, has been argued to be a double black-hole binary system of this type, whose orbital decay recently has been indirectly measured and found to agree with the predictions of General Relativity to within 6%. We argue that the absence of observable scalar dipole radiation in this system yields the remarkable bound $|\,\mu| < (16 \, \hbox{days})ˆ{-1}$ on the instantaneous time derivative at this redshift (as opposed to constraining an average field difference, $\Delta \phi$, over cosmological times), provided only that the scalar is light enough to be radiated — i.e. $m \lsim 10ˆ{-23}$ eV — independent of how the scalar couples to matter. This can also be interpreted as constraining (in a more model-dependent way) the binary's motion relative to any spatial variation of the scalar field within its immediate vicinity within its host galaxy.

#### Nov 22, 2011

1111.4009 (/preprints)
2011-11-22, 15:15 

## [1111.4014] Gravitational Waves from Global Second Order Phase Transitions

Authors: John T. Giblin Jr, Larry R. Price, Xavier Siemens, Brian Vlcek

Date: 17 Nov 2011

Abstract: Global second-order phase transitions are expected to produce scale-invariant gravitational wave spectra. In this manuscript we explore the dynamics of a symmetry-breaking phase transition using lattice simulations. We explicitly calculate the stochastic gravitational wave background produced during the transition and subsequent self-ordering phase. We comment on this signal as it compares to the scale-invariant spectrum produced during inflation.

#### Nov 22, 2011

1111.4014 (/preprints)
2011-11-22, 15:15 

## [1111.4516] The Critical Coupling Likelihood Method: A new approach for seamless integration of environmental and operating conditions of gravitational wave detectors into gravitational wave searches

Authors: Cesar A. Costa, Cristina V. Torres

Date: 18 Nov 2011

Abstract: As part of the current LIGO search for gravitational waves (GWs) we find ourselves trying to determine if and when noise is coupling into the instrument. The Critical Coupling Likelihood (CCL) method has been developed to directly fold information about the potential influence of instrument noise sources into GW search efforts. By using the CCL functions of uncoupled (background) and coupled (foreground) instrumental noise sources, CCL should be able to identify undesirable coupled instrumental noise from potential GW candidates. Our preliminary results show that CCL can associate up to ~80% of observed artifacts with SNR>=8, to local noise sources. That reduces the duty cycle of the instrument by less than 15%. An approach like CCL will become increasingly important as we move into the Advanced LIGO era, going from a first GW detection to gravitational wave astronomy.

#### Nov 22, 2011

1111.4516 (/preprints)
2011-11-22, 15:12 

## [1111.3737] Analytical meets numerical relativity - status of complete gravitational waveform models

Authors: Frank Ohme

Date: 16 Nov 2011

Abstract: Models of gravitational waveforms from coalescing black-hole binaries play a crucial role in the efforts to detect and interpret those signatures in the data of large-scale interferometers. Here we summarize recent models that combine information both from analytical approximations and numerical relativity. We briefly lay out and compare the strategies employed to build such complete models and we recapitulate the errors associated with various aspects of the modelling process.

#### Nov 21, 2011

1111.3737 (/preprints)
2011-11-21, 19:05 

## [1111.3484] The 2005 - 2010 multiwavelength campaign of OJ287

Authors: Mauri Valtonen, Aimo Sillanp&#xe4;&#xe4;

Date: 15 Nov 2011

Abstract: The light curve of quasar OJ287 extends from 1891 up today without major gaps. This is partly due to extensive studies of historical plate archives by Rene Hudec and associates, and partly due to several observing campaigns in recent times. Here we summarize the results of the 2005 - 2010 observing campaign, in which several hundred scientists and amateur astronomers took part. The main results are the following: (1) The 2005 October optical outburst came at the expected time, thus confirming the General Relativistic precession in the binary black hole system. At the same time, this result disproved the model of a single black hole system with accretion disk oscillations, as well as several toy models of binaries without relativistic precession. (2) The nature of the radiation of the 2005 October outburst was expected to be bremsstrahlung from hot gas at a temperature of 3 10ˆ5 degrees K. This was confirmed by combined ground based and ultraviolet observations using the XMM-Newton X-ray telescope. (3) A secondary outburst of the same nature was expected at 2007 September 13. Within the accuracy of the observations (about 6 hours), it started at the correct time. Due to the bremsstrahlung nature of the outburst, the radiation was unpolarized, as expected. (4) Further synchrotron outbursts were expected following the two bremsstrahlung outbursts. They came as scheduled between 2007 October and 2009 December. (5) Due to the effect of the secondary on the overall direction of the jet, the parsec scale jet is expected to rotate in the sky by a large angle, which has been confirmed. The OJ287 binary black hole system is currently our best laboratory for testing theories of gravitation. Using OJ287, the correctness of General Relativity has now been demonstrated up to the second Post-Newtonian order, higher than has been possible using binary pulsars.

#### Nov 21, 2011

1111.3484 (/preprints)
2011-11-21, 19:05 

## [1111.3621] Performance of a Chirplet-based analysis for gravitational waves from binary black hole mergers

Authors: Satya Mohapatra, Zachary Nemtzow, Eric Chassande-Mottin, Laura Cadonati

Date: 15 Nov 2011

Abstract: The gravitational wave (GW) signature of a binary black hole (BBH) coalescence is characterized by rapid frequency evolution in the late inspiral and merger phases. For a system with total mass larger than 100 M_sun, ground based GW detectors are sensitive to the merger phase, and the in-band whitened waveform is a short-duration transient lasting about 10-30 ms. For a symmetric mass system with total mass between 10 and 100 M_sun, the detector is sensitive instead to the inspiral phase and the in-band signal has a longer duration, between 30 ms - 3 s. Omega is a search algorithm for GW bursts that, with the assumption of locally stationary frequency evolution, uses sine-Gaussian wavelets as a template bank to decompose interferometer strain data. The local stationarity of sine-Gaussians induces a performance loss for the detection of lower mass BBH signatures, due to the mismatch between template and signal. We present the performance of a modified version of the Omega algorithm, Chirplet Omega, which allows a linear variation of frequency, to target BBH coalescences. The use of Chirplet-like templates enhances the measured signal-to-noise ratio due to less mismatch between template and data, and increases the detectability of lower mass BBH coalescences. We present the results of a performance study of Chirplet Omega in colored Gaussian noise at initial LIGO sensitivity.

#### Nov 17, 2011

1111.3621 (/preprints)
2011-11-17, 08:02 

## [1111.3605] Resonances in Extreme Mass-Ratio Inspirals: Asymptotic and Hyperasymptotic Analysis

Authors: Jonathan R Gair, Nicolas Yunes, Carl M Bender

Date: 15 Nov 2011

Abstract: An expected source of gravitational waves for future detectors in space are the inspirals of small compact objects into much more massive black holes. These sources have the potential to provide a wealth of information about astronomy and fundamental physics. On short timescales the orbit of the small object is approximately geodesic. Generic geodesics for a Kerr black hole spacetime have a complete set of integrals and can be characterized by three frequencies of the motion. Over the course of an inspiral, a typical system will pass through resonances where two of these frequencies become commensurate. The effect of the resonance will be to alter significantly the rate of inspiral for the duration of the resonance. Understanding the impact of these resonances on gravitational wave phasing is important to detect and exploit these signals for astrophysics and fundamental physics. Two differential equations that might describe the passage of an inspiral through such a resonance are investigated. These differ depending on whether it is the phase or the frequency components of a Fourier expansion of the motion that are taken to be continuous through the resonance. Asymptotic and hyperasymptotic analysis are used to find the late-time analytic behavior of the solution for a system that has passed through a resonance. Linearly growing (weak resonances) or linearly decaying (strong resonances) solutions are found depending on the strength of the resonance. In the weak-resonance case, frequency resonances leave an imprint (a resonant memory) on the gravitational frequency evolution. The transition between weak and strong resonances is characterized by a square-root singularity, and as one approaches this transition from above, the solutions to the frequency resonance equation bunch up into families exponentially fast.

#### Nov 17, 2011

1111.3605 (/preprints)
2011-11-17, 08:02 

## [1111.2655] Superconducting Antenna Concept for Gravitational Wave Radiation

Authors: A. Gulian, J. Foreman, V. Nikoghosyan, S. Nussinov, L. Sica, J. Tollaksen

Date: 11 Nov 2011

Abstract: A new concept for detectors of gravitational wave radiation is discussed. Estimates of its sensitivity suggest that these devices will be able to detect gravitational waves with amplitudes as low as h0~10-26. Such sensitivity could be obtained at spatial scales as small as 10 meters. Devices based on this concept require operational temperatures below the critical temperatures of their superconducting components.

#### Nov 14, 2011

1111.2655 (/preprints)
2011-11-14, 08:28 

## [1111.2816] Cosmological distance indicators by coalescing binaries

Authors: Mariafelicia De Laurentis, Salvatore Capozziello, Ivan De Martino, Michelangelo Formisano

Date: 11 Nov 2011

Abstract: Gravitational waves detected from well-localized inspiraling binaries would allow to determine, directly and independently, both binary luminosity and redshift. In this case, such systems could behave as "standard candles" providing an excellent probe of cosmic distances up to z < 0.1 and thus complementing other indicators of cosmological distance ladder.

#### Nov 14, 2011

1111.2816 (/preprints)
2011-11-14, 08:27 

## [1111.0005] Towards improving the prospects for coordinated gravitational-wave and electromagnetic observations

Authors: Ilya Mandel, Luke Z. Kelley, Enrico Ramirez-Ruiz

Date: 31 Oct 2011

Abstract: We discuss two approaches to searches for gravitational-wave (GW) and electromagnetic (EM) counterparts of binary neutron star mergers. The first approach relies on triggering archival searches of GW detector data based on detections of EM transients. We introduce a quantitative approach to evaluate the improvement to GW detector reach due to the extra information gained from the EM transient and the increased confidence in the presence of a signal from a binary merger. We also advocate utilizing other transients in addition to short gamma ray bursts. The second approach involves following up GW candidates with targeted EM observations. We argue for the use of slower but optimal parameter-estimation techniques to localize the source on the sky, and for a more sophisticated use of astrophysical prior information, including galaxy catalogs, to find preferred followup locations.

#### Nov 14, 2011

1111.0005 (/preprints)
2011-11-14, 08:27 

## [1111.2701] The 2.5PN linear momentum flux and associated recoil from inspiralling compact binaries in quasi-circular orbits: Nonspinning case

Authors: Chandra Kant Mishra (RRI & IISc, India), K. G. Arun (CMI, India), Bala R. Iyer (RRI, India)

Date: 11 Nov 2011

Abstract: Anisotropic emission of gravitational-waves (GWs) from inspiralling compact binaries leads to the loss of linear momentum and hence gravitational recoil of the system. The loss rate of linear momentum in the far-zone of the source (a nonspinning binary system of black holes in quasi-circular orbit) is investigated at the 2.5 post-Newtonian (PN) order and used to provide an analytical expression in harmonic coordinates for the 2.5PN accurate recoil velocity of the binary accumulated in the inspiral phase. The maximum recoil velocity of the binary system at the end of its inspiral phase (i.e at the innermost stable circular orbit (ISCO)) estimated by the 2.5PN formula is of the order of 4 km/s which is smaller than the 2PN estimate of 22 km/s and indicates the importance of higher order post-Newtonian (PN) corrections. Going beyond inspiral, we also provide an estimate of the more important contribution to the recoil velocity from the plunge phase. The maximum recoil velocity at the end of the plunge, involving contributions both from inspiral and plunge phase, for a binary with symmetric mass ratio $\nu=0.2$ is of the order of 182 km/s.

#### Nov 14, 2011

1111.2701 (/preprints)
2011-11-14, 08:27 

## [1111.1677] Black hole-neutron star mergers for 10 solar mass black holes

Authors: Francois Foucart, Matthew D. Duez, Lawrence E. Kidder, Mark A. Scheel, Bela Szilagyi, Saul A. Teukolsky

Date: 7 Nov 2011

Abstract: General relativistic simulations of black hole-neutron star mergers have currently been limited to low-mass black holes (less than 7 solar mass), even though population synthesis models indicate that a majority of mergers might involve more massive black holes (10 solar mass or more). We present the first general relativistic simulations of black hole-neutron star mergers with 10 solar mass black holes. For massive black holes, the tidal forces acting on the neutron star are usually too weak to disrupt the star before it reaches the innermost stable circular orbit of the black hole. Varying the spin of the black hole in the range a/M = 0.5-0.9, we find that mergers result in the disruption of the star and the formation of a massive accretion disk only for large spins a/M>0.7-0.9. From these results, we obtain updated constraints on the ability of BHNS mergers to be the progenitors of short gamma-ray bursts as a function of the mass and spin of the black hole. We also discuss the dependence of the gravitational wave signal on the black hole parameters, and provide waveforms and spectra from simulations beginning 7-8 orbits before merger.

#### Nov 14, 2011

1111.1677 (/preprints)
2011-11-14, 08:27 

## [1111.2802] Jets from Tidal Disruptions of Stars by Black Holes

Authors: Julian H. Krolik, Tsvi Piran

Date: 11 Nov 2011

Abstract: Tidal disruption of main sequence stars by black holes has generally been thought to lead to a signal dominated by UV emission. If, however, the black hole spins rapidly and the poloidal magnetic field intensity on the black hole horizon is comparable to the inner accretion disk pressure, a powerful jet may form whose luminosity can easily exceed the thermal UV luminosity. When the jet beam points at Earth, its non-thermal luminosity can dominate the emitted spectrum. The thermal and non-thermal components decay differently with time. In particular, the thermal emission should remain roughly constant for a significant time after the period of maximum accretion, beginning to diminish only after a delay, whereas after the peak accretion rate, the non-thermal jet emission decays, but then reaches a plateau. When the newly-found flare source Swift J2058 is analyzed in terms of this model, it is found to be consistent with an event in which a main sequence solar-type star is disrupted by a black hole of mass at least $\sim 10ˆ7 M_{\odot}$. Swift may have already observed the beginning of the flat phase in the non-thermal emission from this source. Optical photometry over the first $\simeq 40$ d of this flare is also consistent with this picture, but there is a large uncertainty in the bolometric correction. We suggest that future searches for main sequence tidal disruptions use methods sensitive to jet radiation as well as to thermal UV radiation.

#### Nov 14, 2011

1111.2802 (/preprints)
2011-11-14, 08:27 

## [1111.1383] The speed of gravitational wave: could larger than the speed of light

Authors: Xin Li, Zhe Chang

Date: 6 Nov 2011

Abstract: By making use of the weak gravitational field approximation, we obtain the linearized solution of the gravitational vacuum field equation in Finsler spacetime. The plane-waves solution and dispersion relation of gravitational wave in Finsler spacetime is given. It implies that the speed of gravitational wave could larger than the speed of light. It is compatible with the very recent results of OPERA collaboration, which found that the speed of neutrinos is faster than the speed of light. The wave vector of gravitational wave is null in Finsler spacetime, it would not lose energy via gravitational Cherenkov radiation. The results of OPERA collaboration hint that spacetime may be Finslerian.

#### Nov 14, 2011

1111.1383 (/preprints)
2011-11-14, 08:27 

## [1111.2576] A Geostationary Gravitational Wave Interferometer (GEOGRAWI)

Authors: Massimo Tinto, J. C. N. de Araujo, Odylio D. Aguiar, Eduardo da Silva Alves

Date: 10 Nov 2011

Abstract: We propose a Geostationary Gravitational Wave Interferometer (GEOGRAWI) mission concept for making observations in the sub-Hertz band. GEOGRAWI is expected to meet some of LISA's science goals in the lower part of its accessible frequency band ($10ˆ{-4} - 2 \times 10ˆ{-2}$ Hz), and to outperform them by a large margin in the higher-part of it ($2 \times 10ˆ{-2} - 10$ Hz). As a consequence of its Earth-bound orbit, GEOGRAWI is significantly less expensive than the interplanetary LISA mission and could be either an entirely US mission or managed and operated by NASA in partnership with the Brazilian Space Agency.

#### Nov 11, 2011

1111.2576 (/preprints)
2011-11-11, 23:42 

## [1110.4442] Effects of hyperons in binary neutron star mergers

Authors: Yuichiro Sekiguchi, Kenta Kiuchi, Koutarou Kyutoku, Masaru Shibata

Date: 20 Oct 2011

Abstract: Numerical simulations for the merger of binary neutron stars are performed in full general relativity incorporating both nucleonic and hyperonic finite-temperature equations of state (EOS) and neutrino cooling for the first time. It is found that even for the hyperonic EOS, a hypermassive neutron star is first formed after the merger for the typical total mass $\approx$ 2.7M\bigodot, and subsequently collapses to a black hole (BH). It is shown that hyperons play a substantial role in the post-merger dynamics, torus formation around the BH, and emission of gravitational waves (GWs). In particular, the existence of hyperons is imprinted in GWs. Therefore, GW observations will provide a potential opportunity to explore the composition of the neutron star matter.

#### Oct 31, 2011

1110.4442 (/preprints)
2011-10-31, 15:45 

## [1110.4423] Relativistic Binaries in Globular Clusters

Authors: Matthew J. Benacquista, Jonathan M. B. Downing

Date: 20 Oct 2011

Abstract: Galactic globular clusters are old, dense star systems typically containing 10\super{4}--10\super{7} stars. As an old population of stars, globular clusters contain many collapsed and degenerate objects. As a dense population of stars, globular clusters are the scene of many interesting close dynamical interactions between stars. These dynamical interactions can alter the evolution of individual stars and can produce tight binary systems containing one or two compact objects. In this review, we discuss theoretical models of globular cluster evolution and binary evolution, techniques for simulating this evolution that leads to relativistic binaries, and current and possible future observational evidence for this population. Our discussion of globular cluster evolution will focus on the processes that boost the production of hard binary systems and the subsequent interaction of these binaries that can alter the properties of both bodies and can lead to exotic objects. Direct {\it N}-body integrations and Fokker--Planck simulations of the evolution of globular clusters that incorporate tidal interactions and lead to predictions of relativistic binary populations are also discussed. We discuss the current observational evidence for cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries as well as possible future detection of relativistic binaries with gravitational radiation.

#### Oct 31, 2011

1110.4423 (/preprints)
2011-10-31, 15:45 

## [1110.5107] Multiple physical elements to determine the gravitational-wave signatures of core-collapse supernovae

Authors: Kei Kotake

Date: 24 Oct 2011

Abstract: We review recent progress in the theoretical predictions of gravitational waves (GWs) of core-collapse supernovae. Following a brief summary of the methods in the numerical modeling, we summarize multiple physical elements that determine the GW signatures which have been considered to be important in extracting the information of the long-veiled explosion mechanism from the observation of the GWs. We conclude with a summary of the most urgent tasks to make the dream come true.

#### Oct 31, 2011

1110.5107 (/preprints)
2011-10-31, 15:44 

## [1110.5329] Slowly-rotating stars and black holes in dynamical Chern-Simons gravity

Authors: Yacine Ali-Ha&#xef;moud, Yanbei Chen

Date: 24 Oct 2011

Abstract: Chern-Simons (CS) modified gravity is an extension to general relativity (GR) in which the metric is coupled to a scalar field, resulting in modified Einstein field equations. In the dynamical theory, the scalar field is itself sourced by the Pontryagin density of the space-time. In this paper, the coupled system of equations for the metric and the scalar field is solved numerically for slowly-rotating neutron stars described with realistic equations of state and for slowly-rotating black holes. An analytic solution for a constant-density nonrelativistic object is also presented. It is shown that the black hole solution cannot be used to describe the exterior spacetime of a star as was previously assumed. In addition, whereas previous analysis were limited to the small-coupling regime, this paper considers arbitrarily large coupling strengths. It is found that the CS modification leads to two effects on the gravitomagnetic sector of the metric: (i) Near the surface of a star or the horizon of a black hole, the magnitude of the gravitomagnetic potential is decreased and frame-dragging effects are reduced in comparison to GR. (ii) In the case of a star, the angular momentum J, as measured by distant observers, is enhanced in CS gravity as compared to standard GR. For a large coupling strength, the near-zone frame-dragging effects become significantly screened, whereas the far-zone enhancement saturate at a maximum value max(Delta J) ~ (M/R) J. Using measurements of frame-dragging effects around the Earth by Gravity Probe B and the LAGEOS satellites, a weak but robust constraint is set to the characteristic CS lengthscale, xiˆ{¼} <~ 10ˆ8 km.

#### Oct 31, 2011

1110.5329 (/preprints)
2011-10-31, 15:43 

## [1110.5048] TeVeS/MOND is in harmony with gravitational redshifts in galaxy clusters

Authors: Jacob D. Bekenstein, Robert H. Sanders

Date: 23 Oct 2011

Abstract: Wojtak, Hansen and Hjorth have recently claimed to confirm general relativity and to rule out the tensor-vector-scalar (TeVeS) gravitational theory based on an analysis of the gravitational redshifts of galaxies in 7800 clusters. But their ubiquitous modeling of the sources of cluster gravitational fields with Navarro-Frenk-White mass profiles is neither empirically justified out to the necessary radii in clusters, nor germane in the case of TeVeS. Using MONDian isothermal sphere models consistently constructed within MOND (equivalent to TeVeS models), we can fit the determined redshifts no worse than does general relativity with dark halos. Wojtak, Hansen and Hjorth's work is further marred by confusion between the primitive mu-function of TeVeS and the MOND interpolation function.

#### Oct 31, 2011

1110.5048 (/preprints)
2011-10-31, 15:43 

## [1110.6086] Pairing and sinking of binary SMBHs in sub-pc resolution simulations of galaxy mergers

Authors: Damien Chapon, Lucio Mayer, Romain Teyssier

Date: 27 Oct 2011

Abstract: We study the formation of a supermassive black hole (SMBH) binary and the shrinking of the separation of the two holes to sub-pc scales starting from a realistic major merger between two gas-rich spiral galaxies with mass comparable to our Milky Way. The simulations are the first of this kind carried out with an Adaptive Mesh refinement (AMR) code (here RAMSES), and the first capable to resolve separations as small as 0.1 pc. The collision of the two galaxies produces a gravo-turbulent rotating nuclear disk with mass (\sim 10ˆ9 Msun) and size (\sim 60 pc) in excellent agreement with previous SPH simulations with particle splitting that used a similar setup (Mayer et al. 2007) but were limited to separations of a few parsecs. The AMR results confirm that the two black holes sink rapidly as a result of dynamical friction onto the gaseous background, reaching a separation of 1 pc in less than 10ˆ7 yr. We show that the dynamical friction wake is well resolved by our model and we find good agreement with analytical predictions of the drag force as a function of the Mach number. Below 1 pc, black hole pairing slows down significantly, as the relative velocity between the sinking SMBH becomes highly subsonic and the mass contained within their orbit falls below the mass of the binary itself, rendering dynamical friction ineffective. Non-axisymmetric gas torques do not arise to restart sinking in absence of efficient dynamical friction, at variance with previous calculations using idealized equilibrium nuclear disk models. We believe that the rather "hot" EOS we used to model the multiphase turbulent ISM in the nuclear region is playing an important role in preventing efficient SMBH sinking inside the central parsec. We conclude with a discussion of the way forward to address sinking in gaseous backgrounds at sub-pc scales approaching the gravitational wave regime.

#### Oct 31, 2011

1110.6086 (/preprints)
2011-10-31, 15:41 

## [1110.6172] Mapping the Galactic Center with Gravitational Wave measurements using Pulsar Timing

Authors: Bence Kocsis, Alak Ray, Simon Portegies Zwart

Date: 27 Oct 2011

Abstract: We examine the nHz gravitational wave (GW) foreground of stars and black holes (BHs) orbiting SgrA* in the Galactic Center. A cusp of stars and BHs generates a continuous GW spectrum below 40 nHz; individual BHs within 1 mpc to SgrA* stick out in the spectrum at higher GW frequencies. The GWs and gravitational near-field effects can be resolved by timing pulsars within a few pc of this region. Observations with the Square Kilometer Array (SKA) may be especially sensitive to intermediate mass black holes (IMBHs) in this region, if present. A 100ns-10mus timing accuracy is sufficient to detect BHs of mass 1000 Msun with pulsars at distance 0.1-1 pc in a 3 yr observation baseline. Unlike electromagnetic imaging techniques, the prospects for resolving individual objects through GW measurements improve closer to SgrA*, even if the number density of objects increases inwards steeply. Scattering by the interstellar medium will pose the biggest challenge for such observations.

#### Oct 31, 2011

1110.6172 (/preprints)
2011-10-31, 15:40 

## [1110.5950] Post-Newtonian, Quasi-Circular Binary Inspirals in Quadratic Modified Gravity

Authors: Kent Yagi, Leo C. Stein, Nicolas Yunes, Takahiro Tanaka

Date: 27 Oct 2011

Abstract: We consider a general class of quantum gravity-inspired, modified gravity theories, where the Einstein-Hilbert action is extended through the addition of all terms quadratic in the curvature tensor coupled to scalar fields with standard kinetic energy. This class of theories includes Einstein-Dilaton-Gauss-Bonnet and Chern-Simons modified gravity as special cases. We analytically derive and solve the coupled field equations in the post-Newtonian approximation, assuming a comparable-mass, spinning black hole binary source in a quasi-circular, weak-field/slow-motion orbit. We find that a naive subtraction of divergent piece associated with the point-particle approximation is ill-suited to represent compact objects in these theories. Instead, we model them by appropriate effective sources built so that known strong-field solutions are reproduced in the far-field limit. In doing so, we prove that black holes in Einstein-Dilaton-Gauss-Bonnet and Chern-Simons theory can have hair, while neutron stars have no scalar monopole charge, in diametrical opposition to results in scalar-tensor theories. We then employ techniques similar to the direct integration of the relaxed Einstein equations to obtain analytic expressions for the scalar field, metric perturbation, and the associated gravitational wave luminosity measured at infinity. We find that scalar field emission mainly dominates the energy flux budget, sourcing electric-type (even-parity) dipole scalar radiation and magnetic-type (odd-parity) quadrupole scalar radiation, correcting the General Relativistic prediction at relative -1PN and 2PN orders. Such modifications lead to corrections in the emitted gravitational waves that can be mapped to the parameterized post-Einsteinian framework. Such modifications could be strongly constrained with gravitational wave observations.

#### Oct 28, 2011

1110.5950 (/preprints)
2011-10-28, 21:37 

## [1110.4169] Prospects for determination of thermal history after inflation with future gravitational wave detectors

Authors: Sachiko Kuroyanagi, Kazunori Nakayama, Shun Saito

Date: 19 Oct 2011

Abstract: Thermal history of the Universe between inflation and big-bang nucleosynthesis has not yet been revealed observationally. It will be probed by the detection of primordial gravitational waves generated during inflation, which contain information on the reheating temperature as well as the equation of state of the Universe after inflation. Based on Fisher information formalism, we examine how accurately the tensor-to-scalar ratio and reheating temperature after inflation can be simultaneously determined with space-based gravitational wave detectors such as the DECI-hertz Interferometer Gravitational-wave Observatory (DECIGO) and the Big-Bang Observer (BBO). We show that the reheating temperature is best determined if it is around 10ˆ7 GeV for tensor-to-scalar ratio of around 0.1, and explore the detectable parameter space. We also find that equation of state of the early Universe can be also determined accurately enough to distinguish different equation-of-state parameters if the inflationary gravitational waves are successfully detected. Thus future gravitational wave detectors provide a unique and promising opportunity to reveal the thermal history of the Universe around 10ˆ7 GeV.

#### Oct 20, 2011

1110.4169 (/preprints)
2011-10-20, 12:40 

## [1110.3764] Black hole stereotyping: Induced gravito-static polarization

Authors: Barak Kol, Michael Smolkin

Date: 17 Oct 2011

Abstract: We discuss the black hole effective action and define its static subsector. We determine the induced gravito-static polarization constants (electric Love numbers) of static black holes (Schwarzschild) in an arbitrary dimension, namely the induced mass multipole as a result of an external gravitational field. We demonstrate that in 4d these constants vanish thereby settling a disagreement in the literature. Yet in higher dimensions these constants are non-vanishing, thereby disproving (at least in d>4) speculations that black holes have no effective couplings beyond the point particle action. In particular, when l/(d-3) is half integral these constants demonstrate a (classical) renormalization flow consistent with the divergences of the effective field theory. In some other cases the constants are negative indicating a novel non-spherical instability.
The theory of hypergeometric functions plays a central role.

#### Oct 18, 2011

1110.3764 (/preprints)
2011-10-18, 16:25 

## [1110.3759] Neutron star equation of state via gravitational wave observations

Authors: Charalampos Markakis, Jocelyn S. Read, Masaru Shibata, Koji Uryu, Jolien D. E. Creighton, John L. Friedman, Benjamin D. Lackey

Date: 17 Oct 2011

Abstract: Gravitational wave observations can potentially measure properties of neutron star equations of state by measuring departures from the point-particle limit of the gravitational waveform produced in the late inspiral of a neutron star binary. Numerical simulations of inspiraling neutron star binaries computed for equations of state with varying stiffness are compared. As the stars approach their final plunge and merger, the gravitational wave phase accumulates more rapidly if the neutron stars are more compact. This suggests that gravitational wave observations at frequencies around 1 kHz will be able to measure a compactness parameter and place stringent bounds on possible neutron star equations of state. Advanced laser interferometric gravitational wave observatories will be able to tune their frequency band to optimize sensitivity in the required frequency range to make sensitive measures of the late-inspiral phase of the coalescence.

#### Oct 18, 2011

1110.3759 (/preprints)
2011-10-18, 16:25 

## [1110.3169] Searching for electromagnetic counterparts of gravitational wave transients

Authors: M. Branchesi (1,2)on behalf of the LIGO Scientific Collaboration, the Virgo Collaboration, A. Klotz (3), M. Laas-Bourez (4) ((1) Universit&#xe0; degli Studi di Urbino "Carlo Bo", Italy, (2) INFN - Sezione di Firenze, Italy, (3) Universit&#xe8; de Toulouse/IRAP, France, (4) ICRAR/School of Physics, University of Western Australia, Australia)

Date: 14 Oct 2011

Abstract: A pioneering electromagnetic (EM) observation follow-up program of candidate gravitational wave (GW) triggers has been performed, Dec 17 2009 to Jan 8 2010 and Sep 4 to Oct 20 2010, during the recent LIGO/Virgo run. The follow-up program involved ground-based and space EM facilities observing the sky at optical, X-ray and radio wavelengths. The joint GW/EM observation study requires the development of specific image analysis procedures able to discriminate the possible EM counterpart of GW trigger from background events. The paper shows an overview of the EM follow-up program and the developing image analysis procedures as they are applied to data collected with TAROT and Zadko.

#### Oct 17, 2011

1110.3169 (/preprints)
2011-10-17, 15:33 

## [1110.3297] Loosely coherent searches for sets of well-modeled signals

Date: 14 Oct 2011

Abstract: We introduce a high-performance implementation of a loosely coherent statistic sensitive to signals spanning a finite-dimensional manifold in parameter space. Results from full scale simulations on Gaussian noise are discussed, as well as implications for future searches for continuous gravitational waves. We demonstrate an improvement of more than an order of magnitude in analysis speed over previously available algorithms. As searches for continuous gravitational waves are computationally limited, the large speedup results in gain in sensitivity.

#### Oct 17, 2011

1110.3297 (/preprints)
2011-10-17, 15:33 

## [1110.3193] Euclid Definition Study Report

Authors: Editorial Team: R. Laureijs, J. Amiaux, S. Arduini, J.-L. Augu&#xe8;res, J. Brinchmann, R. Cole, M. Cropper, C. Dabin, L. Duvet, A. Ealet, B. Garilli, P. Gondoin, L. Guzzo, J. Hoar, H. Hoekstra, R. Holmes, T. Kitching, T. Maciaszek, Y. Mellier, F. Pasian, W. Percival, J. Rhodes, G. Saavedra Criado, M. Sauvage, R. Scaramella, L. Valenziano, S. Warren

Date: 14 Oct 2011

Abstract: Euclid is a space-based survey mission from the European Space Agency designed to understand the origin of the Universe's accelerating expansion. It will use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the universe and on the cosmic history of structure formation. The mission is optimised for two independent primary cosmological probes: Weak gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO). The Euclid payload consists of a 1.2 m Korsch telescope designed to provide a large field of view. It carries two instruments with a common field-of-view of ~0.54 deg2: the visual imager (VIS) and the near infrared instrument (NISP) which contains a slitless spectrometer and a three bands photometer. The Euclid wide survey will cover 15,000 deg2 of the extragalactic sky and is complemented by two 20 deg2 deep fields. For WL, Euclid measures the shapes of 30-40 resolved galaxies per arcmin2 in one broad visible R+I+Z band (550-920 nm). The photometric redshifts for these galaxies reach a precision of dz/(1+z) < 0.05. They are derived from three additional Euclid NIR bands (Y, J, H in the range 0.92-2.0 micron), complemented by ground based photometry in visible bands derived from public data or through engaged collaborations. The BAO are determined from a spectroscopic survey with a redshift accuracy dz/(1+z) =0.001. The slitless spectrometer, with spectral resolution ~250, predominantly detects Ha emission line galaxies. Euclid is a Medium Class mission of the ESA Cosmic Vision 2015-2025 programme, with a foreseen launch date in 2019. This report (also known as the Euclid Red Book) describes the outcome of the Phase A study.

#### Oct 17, 2011

1110.3193 (/preprints)
2011-10-17, 15:33 

## [1110.3081] Modeling multipolar gravitational-wave emission from small mass-ratio mergers

Authors: Enrico Barausse, Alessandra Buonanno, Scott A. Hughes, Gaurav Khanna, Stephen O&#x27;Sullivan, Yi Pan

Date: 13 Oct 2011

Abstract: Using the effective-one-body (EOB) formalism and a time-domain Teukolsky code, we generate inspiral, merger, and ringdown waveforms in the small mass-ratio limit. We use EOB inspiral and plunge trajectories to build the Teukolsky equation source term, and compute full coalescence waveforms for a range of black hole spins. By comparing EOB waveforms that were recently developed for comparable mass binary black holes to these Teukolsky waveforms, we improve the EOB model for the (2,2), (2,1), (3,3), and (4,4) modes. Our results can be used to quickly and accurately extract useful information about merger waves for binaries with spin, and should be useful for improving analytic models of such binaries. Although in this analysis we only consider equatorial inspirals (orbital angular momentum parallel to spin), there is no issue of principle preventing us from considering inclined binaries. We will extend this analysis to examine misaligned spin-orbit configurations in future work.

#### Oct 17, 2011

1110.3081 (/preprints)
2011-10-17, 15:33 

## [1110.2865] Cosmology with space-based gravitational-wave detectors --- dark energy and primordial gravitational waves ---

Authors: Atsushi Nishizawa, Kent Yagi, Atsushi Taruya, Takahiro Tanaka

Date: 13 Oct 2011

Abstract: Proposed space-based gravitational-wave (GW) detectors such as DECIGO and BBO will detect ~10ˆ6 neutron-star (NS) binaries and determine the luminosity distances to the binaries with high precision. Combining the luminosity distances with cosmologically-induced phase corrections on the GWs, cosmological expansion out to high redshift can be measured without the redshift determinations of host galaxies by electromagnetic observation and be a unique probe for dark energy. On the other hand, such a NS-binary foreground should be subtracted to detect primordial GWs produced during inflation. Thus, the constraining power on dark energy and the detectability of the primordial gravitational waves strongly depend on the detector sensitivity and are in close relation with one another. In this paper, we investigate the constraints on the equation of state of dark energy with future space-based GW detectors with/without identifying the redshifts of host galaxies. We also study the sensitivity to the primordial GWs, properly dealing with the residual of the NS binary foreground. Based on the results, we discuss the detector sensitivity required to achieve the forementioned targeted study of cosmology.

#### Oct 14, 2011

1110.2865 (/preprints)
2011-10-14, 22:16 

## [1110.2785] Multi-physics simulations using a hierarchical interchangeable software interface

Authors: Simon Portegies Zwart, Stephen McMillan, Inti Pelupessy, Arjen van Elteren

Date: 12 Oct 2011

Abstract: We introduce a general-purpose framework for interconnecting scientific simulation programs using a homogeneous, unified software interface. Our framework is intrinsically parallel, and conveniently separates all components in memory. It performs unit conversion between different modules automatically and defines common data structures to communicate across different codes. We use the framework to simulate embedded star clusters. For this purpose we couple solvers for gravitational dynamics, stellar evolution and hydrodynamics to self consistently resolve the dynamical evolution simultaneousy with the internal nuclear evolution of the stars and the hydrodynamic response of the gas. We find, in contrast to earlier studies, that the survival of a young star cluster depends only weakly on the efficiency of star formation. The main reason for this weak dependency is the asymmetric expulsion of the embedding gas from the cluster.

#### Oct 14, 2011

1110.2785 (/preprints)
2011-10-14, 22:15 

## [1110.2965] A geometric approach to the precession of compact binaries

Authors: Michael Boyle, Robert Owen, Harald P. Pfeiffer

Date: 13 Oct 2011

Abstract: We propose a geometrical method to define a preferred reference frame for precessing binary systems. This minimal-rotation frame is aligned with the angular-momentum axis and fixes the rotation about that axis up to a constant angle, resulting in an essentially invariant frame. Gravitational waveforms decomposed in this frame are similarly invariant under rotations of the inertial frame and exhibit relatively smoothly varying phase. By contrast, earlier prescriptions for radiation-aligned frames induce extraneous features in the gravitational-wave phase which depend on the orientation of the inertial frame, which leads to fluctuations in the frequency and may compound to many gravitational-wave cycles. We suggest that the minimal-rotation frame provides a simplified framework for post-Newtonian approximations of precessing systems and describe the construction of analytical/numerical hybrid waveforms for such systems.

#### Oct 14, 2011

1110.2965 (/preprints)
2011-10-14, 22:15 

## [1110.2938] Energy versus Angular Momentum in Black Hole Binaries

Authors: Thibault Damour, Alessandro Nagar, Denis Pollney, Christian Reisswig

Date: 13 Oct 2011

Abstract: Using accurate numerical relativity simulations of (nonspinning) black-hole binaries with mass ratios 1:1, 2:1 and 3:1 we compute the gauge invariant relation between the (reduced) binding energy $E$ and the (reduced) angular momentum $j$ of the system. We show that the relation $E(j)$ is an accurate diagnostic of the dynamics of a black-hole binary in a highly relativistic regime. By comparing the numerical-relativity $Eˆ{\rm NR} (j)$ curve with the predictions of several analytic approximation schemes, we find that, while the usual, non-resummed post-Newtonian-expanded $Eˆ{\rm PN} (j)$ relation exhibits large and growing deviations from $Eˆ{\rm NR} (j)$, the prediction of the effective one-body formalism, based purely on known analytical results (without any calibration to numerical relativity), agrees strikingly well with the numerical-relativity results.

#### Oct 14, 2011

1110.2938 (/preprints)
2011-10-14, 22:15 

## [1110.2720] Constraining Generic Lorentz Violation and the Speed of the Graviton with Gravitational Waves

Authors: Saeed Mirshekari, Nicolas Yunes, Clifford M. Will

Date: 12 Oct 2011

Abstract: Modified gravity theories generically predict a violation of Lorentz invariance, which may lead to a modified dispersion relation for propagating modes of gravitational waves. We construct a parametrized dispersion relation that can reproduce a range of known Lorentz-violating predictions and investigate their impact on the propagation of gravitational waves. A modified dispersion relation forces different wavelengths of the gravitational wave train to travel at slightly different velocities, leading to a modified phase evolution observed at a gravitational-wave detector. We show how such corrections map to the waveform observable and to the parametrized post-Einsteinian framework, proposed to model a range of deviations from General Relativity. Given a gravitational-wave detection, the lack of evidence for such corrections could then be used to place a constraint on Lorentz violation. The constraints we obtain are tightest for dispersion relations that scale with small power of the graviton's momentum and deteriorate for a steeper scaling.

#### Oct 13, 2011

1110.2720 (/preprints)
2011-10-13, 13:50 

## [1110.2229] Accurate gravitational waveforms for binary-black-hole mergers with nearly extremal spins

Authors: Geoffrey Lovelace, Michael Boyle, Mark A. Scheel, Bela Szilagyi

Date: 10 Oct 2011

Abstract: Motivated by the possibility of observing gravitational waves from merging black holes whose spins are nearly extremal (i.e., 1 in dimensionless units), we present numerical waveforms from simulations of merging black holes with the highest spins simulated to date: (1) a 25.5-orbit inspiral, merger, and ringdown of two holes with equal masses and spins of magnitude 0.97 aligned with the orbital angular momentum; and (2) a previously reported 12.5-orbit inspiral, merger, and ringdown of two holes with equal masses and spins of magnitude 0.95 anti-aligned with the orbital angular momentum. First, we consider the horizon mass and spin evolution of the new aligned-spin simulation. During the inspiral, the horizon area and spin evolve in remarkably close agreement with Alvi's analytic predictions, and the remnant hole's final spin agrees reasonably well with several analytic predictions. We also find that the total energy emitted by a real astrophysical system with these parameters — almost all of which is radiated during the time included in this simulation — would be 10.952% of the initial mass at infinite separation. Second, we consider the gravitational waveforms for both simulations. After estimating their uncertainties, we compare the waveforms to several post-Newtonian approximants, finding significant disagreement well before merger, although the phase of the TaylorT4 approximant happens to agree remarkably well with the numerical prediction in the aligned-spin case. We find that the post-Newtonian waveforms have sufficient uncertainty that hybridized waveforms will require far longer numerical simulations (in the absence of improved post-Newtonian waveforms) for accurate parameter estimation of low-mass binary systems.

#### Oct 11, 2011

1110.2229 (/preprints)
2011-10-11, 22:23 

## [1110.1635] Missing Black Holes Unveil The Supernova Explosion Mechanism

Authors: K. Belczynski, G. Wiktorowicz, C. Fryer, D. Holz, V. Kalogera

Date: 7 Oct 2011

Abstract: There exist a wide range of masses and types of stars that form compact object remnants: white dwarfs, neutron stars, or black holes. The stellar mass distribution is smooth, covering the range 0.1-100 Msun. It is expected that the masses of the ensuing compact remnants correlate with the masses of their progenitor stars, and thus it is thought that the remnant masses should be smoothly distributed from the lightest white dwarfs to the heaviest black holes. However, this intuitive prediction is not borne out by observed data. In the rapidly growing population of remnants with determined masses, a striking mass gap has emerged at the boundary between neutron stars and black holes. The heaviest neutron stars reach a maximum of 2 Msun, while the lightest black holes are at least 5 Msun. At first this gap was attributed to a paucity of observations. However, with recent determinations of the masses for more than 20 black holes, the gap has remained intact and become a significant challenge to our understanding of compact object formation. Over a decade after this gap was initially noted, we offer the first insights into the physical processes that bifurcate the formation of remnants into lower mass neutron stars and heavier black holes. Combining the results of full stellar modeling with multidimensional hydrodynamic simulations of supernova explosions, we both explain the existence of the gap, and also put stringent constraints on the inner workings of the supernova explosion mechanism. In particular, we show that core-collapse supernovae are launched within 100-200 ms of the initial stellar collapse. This implies that the explosions are driven by Rayleigh-Taylor instabilities rather than the delayed standing accretion shock instabilities, resolving a major debate in the supernova community.

#### Oct 11, 2011

1110.1635 (/preprints)
2011-10-11, 22:23 

## [1110.2051] The microwave sky after one year of Planck operations

Authors: Aniello Mennella, for the Planck Collaboration

Date: 10 Oct 2011

Abstract: The ESA Planck satellite, launched on May 14th, 2009, is the third generation space mission dedicated to the measurement of the Cosmic Microwave Background (CMB), the first light in the Universe. Planck observes the full sky in nine frequency bands from 30 to 857 GHz and is designed to measure the CMB anisotropies with an unprecedented combination of sensitivity, angular resolution and control of systematic effects. In this presentation we summarise the Planck instruments performance and discuss the main scientific results obtained after one year of operations in the fields of galactic and extragalactic astrophysics.

#### Oct 11, 2011

1110.2051 (/preprints)
2011-10-11, 22:22 

## [1110.2094] Elimination of the spin supplementary condition in the effective field theory approach to the post-Newtonian approximation

Authors: Steven Hergt, Jan Steinhoff, Gerhard Schaefer

Date: 10 Oct 2011

Abstract: The present paper addresses open questions regarding the handling of the spin supplementary condition within the effective field theory approach to the post-Newtonian approximation. In particular it is shown how the spin supplementary condition can be eliminated at the level of the potential and how the dynamics can be cast into a fully reduced Hamiltonian form. Two different methods are used and compared, one based on the well-known Dirac bracket and the other based on an action principle. It is discussed how the latter approach can be used to improve the Feynman rules by formulating them in terms of reduced canonical spin variables.

#### Oct 11, 2011

1110.2094 (/preprints)
2011-10-11, 22:22 

## [1110.1668] On Toroidal Horizons in Binary Black Hole Inspirals

Authors: Michael I. Cohen, Jeffrey D. Kaplan, Mark A. Scheel

Date: 7 Oct 2011

Abstract: We examine the structure of the event horizon for numerical simulations of two black holes that begin in a quasicircular orbit, inspiral, and finally merge. We find that the spatial cross section of the merged event horizon has spherical topology (to the limit of our resolution), despite the expectation that generic binary black hole mergers in the absence of symmetries should result in an event horizon that briefly has a toroidal cross section. Using insight gained from our numerical simulations, we investigate how the choice of time slicing affects both the spatial cross section of the event horizon and the locus of points at which generators of the event horizon cross. To ensure the robustness of our conclusions, our results are checked at multiple numerical resolutions. 3D visualization data for these resolutions are available for public access online. We find that the structure of the horizon generators in our simulations is consistent with expectations, and the lack of toroidal horizons in our simulations is due to our choice of time slicing.

#### Oct 11, 2011

1110.1668 (/preprints)
2011-10-11, 22:22 

## [1110.1099] A test of the (circular) Unruh effect using atoms

Date: 5 Oct 2011

Abstract: We propose a test for the (circular) Unruh effect using certain atoms -- fluorine and oxygen. For these atoms the centripetal acceleration of the outer shell electrons implies an effective Unruh temperature in the range 1000 - 2000 K. This range of Unruh temperatures is large enough to excite a significant fraction of the outer electrons into low lying energy levels above the ground state. Examining these atoms at low background temperatures and finding a larger than expected number of electrons in low lying excited states, beyond what is expected due to the background thermal excitation, would provide experimental evidence for the Unruh effect.

#### Oct 11, 2011

1110.1099 (/preprints)
2011-10-11, 22:22 

## [1109.6660] Observing Lense-Thirring Precession in Tidal Disruption Flares

Authors: Nicholas Stone, Abraham Loeb

Date: 29 Sep 2011

Abstract: When a star is tidally disrupted by a supermassive black hole (SMBH), the streams of liberated gas form an accretion disk after their return to pericenter. We demonstrate that Lense-Thirring precession in the spacetime around a rotating SMBH can produce significant time evolution of the disk angular momentum vector, due to both the periodic precession of the disk and the nonperiodic, differential precession of the bound debris streams. Jet precession and periodic modulation of disk luminosity are possible consequences. The persistence of the jetted X-ray emission in the Swift J164449.3+573451 flare suggests that the jet axis was aligned with the spin axis of the SMBH during this event.

#### Oct 05, 2011

1109.6660 (/preprints)
2011-10-05, 18:54 

## [1110.0746] Towards constraining the central black hole's properties by studying its infrared flares with the GRAVITY instrument

Authors: F. H. Vincent, T. Paumard, G. Perrin, E. Gourgoulhon, F. Eisenhauer, S. Gillessen

Date: 4 Oct 2011

Abstract: The ability of the near future second generation VLTI instrument GRAVITY to constrain the properties of the Galactic center black hole is investigated. The Galactic center infrared flares are used as probes of strong-field gravity, within the framework of the hot spot model according to which the flares are the signature of a blob of gas orbiting close to the black hole's innermost stable circular orbit. Full general relativistic computations are performed, together with realistic observed data simulations, that lead to conclude that GRAVITY could be able to constrain the black hole's inclination parameter.

#### Oct 05, 2011

1110.0746 (/preprints)
2011-10-05, 11:08 

## [1110.0659] Testing Gravitation in the Solar System with Radio Science experiments

Authors: A. Hees, P. Wolf, B. Lamine, M.T. Jaekel, C. Le Poncin-Lafitte, V. Lainey, V. Dehant

Date: 4 Oct 2011

Abstract: The laws of gravitation have been tested for a long time with steadily improving precision, leading at some moment of time to paradigmatic evolutions. Pursuing this continual effort is of great importance for science. In this communication, we focus on Solar System tests of gravity and more precisely on possible tests that can be performed with radio science observations (Range and Doppler). After briefly reviewing the current tests of gravitation at Solar System scales, we give motivations to continue such experiments. In order to obtain signature and estimate the amplitude of anomalous signals that could show up in radio science observables because of modified gravitational laws, we developed a new software that simulates Range/Doppler signals. We present this new tool that simulates radio science observables directly from the space-time metric. We apply this tool to the Cassini mission during its cruise from Jupiter to Saturn and derive constraints on the parameters entering alternative theories of gravity beyond the standard Parametrized Post Newtonian theory.

#### Oct 05, 2011

1110.0659 (/preprints)
2011-10-05, 11:07 

## [1110.0687] Can we constrain the maximum value for the spin parameter of the super-massive objects in galactic nuclei without knowing their actual nature?

Authors: Cosimo Bambi

Date: 4 Oct 2011

Abstract: In 4-dimensional General Relativity, black holes are described by the Kerr solution and are subject to the bound $|a_*| \le 1$, where $a_*$ is the black hole spin parameter. If current black hole candidates are not the black holes predicted in General Relativity, this bound does not hold and $a_*$ might exceed 1. In this letter, I relax the Kerr black hole hypothesis and I find that the value of the spin parameter of the super-massive black hole candidates in galactic nuclei cannot be higher than about 1.2. A higher spin parameter would not be consistent with a radiative efficiency $\eta > 0.15$, as observed at least for the most luminous AGN. While a rigorous proof is lacking, I conjecture that the bound $|a_*| \lesssim 1.2$ is independent of the exact nature of these objects.

#### Oct 05, 2011

1110.0687 (/preprints)
2011-10-05, 11:07 

## [1110.0530] Towards a generic test of the strong field dynamics of general relativity using compact binary coalescence

Authors: T. G. F. Li, W. Del Pozzo, S. Vitale, C. Van Den Broeck, M. Agathos, J. Veitch, K. Grover, T. Sidery, R. Sturani, A. Vecchio

Date: 3 Oct 2011

Abstract: Coalescences of binary neutron stars and/or black holes are amongst the most likely gravitational-wave signals to be observed in ground based interferometric detectors. Apart from the astrophysical importance of their detection, they will also provide us with our very first empirical access to the genuinely strong-field dynamics of General Relativity (GR). We present a Bayesian data analysis method aimed at detecting deviations from GR, through measuring the consistency of the gravitational-wave phase coefficients in the inspiral regime with the predictions made by GR, without relying on any specific alternative theory of gravity. Sources in the Advanced LIGO and Virgo detectors are likely to have low a signal-to-noise ratio (SNR). Therefore, here we introduce a framework in which individual sources are analysed for deviations in a limited number of the first few phase coefficients, which are the most easily determined in a low-SNR scenario. We also show that by combining the results of multiple observations one can achieve a more powerful test than for any individual source. In order to explore this problem, we perform a range of numerical experiments in which simulated gravitational waves modeled in the restricted post-Newtonian, stationary phase approximation are added to Gaussian and stationary noise that follows the expected Advanced LIGO/Virgo noise curves.

#### Oct 05, 2011

1110.0530 (/preprints)
2011-10-05, 11:06 

## [1110.0410] The Electromagnetic Christodoulou Memory Effect and its Application to Neutron Star Binary Mergers

Authors: Lydia Bieri, PoNing Chen, Shing-Tung Yau

Date: 3 Oct 2011

Abstract: Gravitational waves are predicted by the general theory of relativity. It has been shown that gravitational waves have a nonlinear memory, displacing test masses permanently. This is called the Christodoulou memory. We proved that the electromagnetic field contributes at highest order to the nonlinear memory effect of gravitational waves, enlarging the permanent displacement of test masses. In experiments like LISA or LIGO which measure distances of test masses, the Christodoulou memory will manifest itself as a permanent displacement of these objects. It has been suggested to detect the Christodoulou memory effect using radio telescopes investigating small changes in pulsar's pulse arrival times. The latter experiments are based on present-day technology and measure changes in frequency. In the present paper, we study the electromagnetic Christodoulou memory effect and compute it for binary neutron star mergers. These are typical sources of gravitational radiation. During these processes, not only mass and momenta are radiated away in form of gravitational waves, but also very strong magnetic fields are produced and radiated away. Moreover, a large portion of the energy is carried away by neutrinos. We give constraints on the conditions, where the energy transported by electromagnetic radiation is of similar or slightly higher order than the energy radiated in gravitational waves or in form of neutrinos. We find that for coalescing neutron stars, large magnetic fields magnify the Christodoulou memory as long as the gaseous environment is sufficiently rarefied. Thus the observed effect on test masses of a laser interferometer gravitational wave detector will be enlarged by the contribution of the electromagnetic field. Therefore, the present results are important for the planned experiments.

#### Oct 04, 2011

1110.0410 (/preprints)
2011-10-04, 17:40 

## [1110.0017] Empirical Constraints on the Coevolution of Supermassive Black Holes and their Host Spheroids

Authors: Gongjie Li, Charlie Conroy, Abraham Loeb

Date: 30 Sep 2011

Abstract: We investigate the evolution of the MBH-{\sigma} relation by examining the relationship between the intrinsic scatter in the MBH-{\sigma} relation and galaxy bolometric nuclear luminosity, the latter being a probe of the accretion rate of the black hole (BH). Our sample is composed of galaxies with classical bulges when possible, of which 38 have dynamically measured BHs masses, and 17 have BHs masses measured by reverberation mapping. In order to obtain the bolometric nuclear luminosity for galaxies with low nuclear luminosity, we convert the X-ray nuclear luminosity measured by Chandra to bolometric luminosity. We find that the scatter in the MBH-{\sigma} relation is uncorrelated with nuclear luminosity over seven orders of magnitude in luminosity, with the high luminosity end approaching the Eddington luminosity. This suggests that at the present epoch galaxies evolve along the MBH-{\sigma} relation. This conclusion is consistent with the standard paradigm that BHs grow contemporaneously with their host stellar spheroids.

#### Oct 04, 2011

1110.0017 (/preprints)
2011-10-04, 17:40 

## [1110.0342] Measuring the absolute non-gravitational acceleration of a spacecraft: goals, devices, methods, performances

Authors: Benjamin Lenoir, Bruno Christophe, Serge Reynaud

Date: 3 Oct 2011

Abstract: Space provides unique opportunities to test gravitation. By using an interplanetary spacecraft as a test mass, it is possible to test General Relativity at the Solar System distance scale. This requires to compute accurately the trajectory of the spacecraft, a process which relies on radio tracking and is limited by the uncertainty on the spacecraft non-gravitational acceleration.
The Gravity Advanced Package (GAP) is designed to measure the non-gravitational acceleration without bias. It is composed of an electrostatic accelerometer supplemented by a rotating stage. This article presents the instrument and its performances, and describes the method to make unbiased measurements. Finally, it addresses briefly the improvement brought by the instrument as far as orbit reconstruction is concerned.

#### Oct 04, 2011

1110.0342 (/preprints)
2011-10-04, 17:39 

## [1110.0109] Relativistic Two-Body Coulomb-Breit Hamiltonian in an External Weak Gravitational Field

Authors: J. A. Caicedo, L. F. Urrutia

Date: 1 Oct 2011

Abstract: A construction of the Coulomb-Breit Hamiltonian for a pair of fermions, considered as a quantum two-body system, immersed in an arbitrary background gravitational field described by Einstein's General Relativity is presented. Working with Fermi normal coordinates for a freely falling observer in a spacetime region where there are no background sources and ignoring the gravitational back-reaction of the system, the effective Coulomb-Breit Hamiltonian is obtained starting from the S-matrix element corresponding to the one-photon exchange between the charged fermionic currents. The contributions due to retardation are considered up to order (v/c)ˆ2 and they are subsequently written as effective operators in the relativistic quantum mechanical Hilbert space of the system. The final Hamiltonian includes effects linear in the curvature and up to order (v/c)ˆ2.

#### Oct 04, 2011

1110.0109 (/preprints)
2011-10-04, 17:39 

## [1109.6848] Perturbative, Post-Newtonian, and General Relativistic Dynamics of Black Hole Binaries

Authors: Alexandre Le Tiec

Date: 30 Sep 2011

Abstract: The orbital motion of inspiralling and coalescing black hole binaries can be investigated using a variety of approximation schemes and numerical methods within general relativity: post-Newtonian expansions, black hole perturbation theory, numerical relativity, and the effective-one-body formalism. We review two recent comparisons of the predictions from these various techniques. Both comparisons rely on the calculation of a coordinate invariant relation, in the case of non-spinning binary black holes on quasi-circular orbits. All methods are shown to agree very well in their common domain of validity.

#### Oct 03, 2011

1109.6848 (/preprints)
2011-10-03, 12:24 

## [1005.1614] Faster Than Light?

Authors: Robert Geroch

Date: 10 May 2010

Abstract: It is argued that special relativity remains a viable physical theory even when there is permitted signals traveling faster than light.

#### Oct 01, 2011

1005.1614 (/preprints)
2011-10-01, 21:14 

## [1109.4839] Nonlinear Decay of r modes in Rapidly Rotating Neutron Stars

Authors: Wolfgang Kastaun

Date: 22 Sep 2011

Abstract: We investigate the dynamics of r modes at amplitudes in the nonlinear regime for rapidly but uniformly rotating neutron stars with a polytropic equation of state. For this, we perform three-dimensional relativistic hydrodynamical simulations, making the simplifying assumption of a fixed spacetime. We find that for initial dimensionless amplitudes around three, r modes decay on timescales around ten oscillation periods, while at amplitudes of order unity, they are stable over the evolution timescale. Together with the decay, a strong differential rotation develops, conserving the total angular momentum, with angular velocities in the range 0.5..1.2 of the initial one. By comparing two models, we found that increasing rotation slows down the r-mode decay. We present r-mode eigenfunctions and frequencies, and compare them to known analytic results for slowly rotating Newtonian stars. As a diagnostic tool, we discuss conserved energy and angular momentum for the case of a fixed axisymmetric background metric and introduce a measure for the energy of non-axisymmetric fluid oscillation modes.

#### Sep 30, 2011

1109.4839 (/preprints)
2011-09-30, 11:56 

## [1109.5128] Eccentric binary neutron star mergers

Authors: Roman Gold, Sebastiano Bernuzzi, Marcus Thierfelder, Bernd Bruegmann, Frans Pretorius

Date: 23 Sep 2011

Abstract: Neutron star binaries offer a rich phenomenology in terms of gravitational waves and merger remnants. However, most general relativistic studies have been performed for nearly circular binaries, with the exception of head-on collisions. We present the first numerical relativity investigation of mergers of eccentric equal-mass neutron-star binaries that probes the regime between head-on and circular. In addition to gravitational waves generated by the orbital motion, we find that the signal also contains a strong component due to stellar oscillations (f-modes) induced by tidal forces, extending a classical result for Newtonian binaries. The merger can lead to rather massive disks on the order of 10% of the total initial mass.

#### Sep 30, 2011

1109.5128 (/preprints)
2011-09-30, 11:55 

## [1109.4978] Detecting Double Degenerate Progenitors of SNe Ia with LISA

Authors: Alexander Stroeer, Matthew Benacquista, Frank Ceballos

Date: 23 Sep 2011

Abstract: The Galactic population of close white dwarf binaries is expected to provide the largest number of gravitational wave sources for low frequency detectors such as the Laser Interferometer Space Antenna (LISA). Current data analysis techniques have demonstrated the capability of resolving on the order of $10ˆ4$ white dwarf binaries from a 2 year observation. Resolved binaries are either at high frequencies or large amplitudes. Such systems are more likely to be high-mass binaries, a subset of which will be progenitors of SNe Ia in the double degenerate scenario. We report on results of a study of the properties of resolved binaries using a population synthesis model of the Galactic white dwarf binaries and a LISA data analysis algorithm using Mock LISA Data Challenge tools.

#### Sep 30, 2011

1109.4978 (/preprints)
2011-09-30, 11:52 

## [1109.5685] Price for Environmental Neutrino-Superluminality

Authors: Gia Dvali, Alexander Vikman

Date: 26 Sep 2011

Abstract: We ask whether the resent OPERA results on neutrino superluminality could be an environmental effect characteristic of the local neighborhood of our planet, without the need of violation of the Poincaré-invariance at a fundamental level. This explanation requires the existence of a new spin-2 field of a planetary Compton wave-length that is coupled to neutrinos and the rest of the matter asymmetrically, both in the magnitude and in the sign. Sourced by the earth this field creates an effective metric on which neutrinos propagate superluminally, whereas other species are much less sensitive to the background. Such a setup, at an effective field theory level, passes all immediate phenomenological tests and its natural prediction is an inevitable appearance of a testable long-range gravity-type fifth force. We then prove that under the assumption of the weakly-coupled Poincaré-invariant physics, the asymmetrically-coupled second massive graviton is the only possible environmental explanation. Despite phonemonolgical viability, the sign assymetry of the coupling we identify as the main potential obstacle for a consistent UV-completion. We also discuss the possible identification of this field with a Kaluza-Klein state of an extra dimension in which neutrino can propagate.

#### Sep 30, 2011

1109.5685 (/preprints)
2011-09-30, 11:52 

## [1109.5384] The torquing of circumnuclear accretion disks by stars and the evolution of massive black holes

Authors: Michal Bregman, Tal Alexander (Weizmann Institute of Science)

Date: 25 Sep 2011

Abstract: An accreting massive black hole (MBH) in a galactic nucleus is surrounded by a dense stellar cluster. We analyze and simulate numerically the evolution of a thin accretion disk due to its internal viscous torques, due to the frame-dragging torques of a spinning MBH (the Bardeen-Petterson effect) and due to the orbit-averaged gravitational torques by the stars (Resonant Relaxation). We show that the evolution of the MBH mass accretion rate, the MBH spin growth rate, and the covering fraction of the disk relative to the central ionizing continuum source, are all strongly coupled to the stochastic fluctuations of the stellar potential via the warps that the stellar torques excite in the disk. These lead to fluctuations by factors of up to a few in these quantities on a typical timescale of ~(M_bh/M_d)P(R_d), where M_bh and M_d are the masses of the MBH and disk, and P is the orbital period at the disk's mass-weighted mean radius R_d. The response of the disk is stronger the lighter it is and the more centrally concentrated the stellar cusp. As proof of concept, we simulate the evolution of the low-mass maser disk in NGC 4258, and show that its observed O(10 deg) warp can be driven by the stellar torques. We also show that the frame-dragging of a massive AGN disk couples the stochastic stellar torques to the MBH spin and can excite a jitter of a few degrees in its direction relative to that of the disk's outer regions.

#### Sep 30, 2011

1109.5384 (/preprints)
2011-09-30, 11:52 

## [1109.5642] Reduced Basis representations of multi-mode black hole ringdown gravitational waves

Authors: Sarah Caudill, Scott E. Field, Chad R. Galley, Frank Herrmann, Manuel Tiglio

Date: 26 Sep 2011

Abstract: We construct compact and high accuracy Reduced Basis (RB) representations of single and multiple quasinormal modes (QNMs). The RB method determines a hierarchical and relatively small set of the most relevant waveforms. We find that the exponential convergence of the method allows for a dramatic compression of template banks used for ringdown searches. Compressing a catalog with a minimal match $\MMm=0.99$, we find that the selected RB waveforms are able to represent {\em any} QNM, including those not in the original bank, with extremely high accuracy, typically less than $10ˆ{-13}$. We then extend our studies to two-mode QNMs. Inclusion of a second mode is expected to help with detection, and might make it possible to infer details of the progenitor of the final black hole. We find that the number of RB waveforms needed to represent any two-mode ringdown waveform with the above high accuracy is {\em smaller} than the number of metric-based, one-mode templates with $\MMm=0.99$. For unconstrained two-modes, which would allow for consistency tests of General Relativity, our high accuracy RB has around $10ˆ4$ {\em fewer} waveforms than the number of metric-based templates for $\MMm=0.99$. The number of RB elements grows only linearly with the number of multipole modes versus exponentially with the standard approach, resulting in very compact representations even for multiple modes. The results of this paper open the possibility of searches of multi-mode ringdown gravitational waves.

#### Sep 30, 2011

1109.5642 (/preprints)
2011-09-30, 11:51 

## [1109.5177] The merger of binary white dwarf--neutron stars: Simulations in full general relativity

Authors: Vasileios Paschalidis, Yuk Tung Liu, Zachariah Etienne, Stuart L. Shapiro

Date: 23 Sep 2011

Abstract: We present fully general relativistic (GR) simulations of binary white dwarf-neutron star (WDNS) inspiral and merger. The initial binary is in a circular orbit at the Roche critical separation. The goal is to determine the ultimate fate of such systems. We focus on binaries whose total mass exceeds the maximum mass (Mmax) a cold, degenerate EOS can support against gravitational collapse. The time and length scales span many orders of magnitude, making fully general relativistic hydrodynamic (GRHD) simulations computationally prohibitive. For this reason, we model the WD as a "pseudo-white dwarf" (pWD) as in our binary WDNS head-on collisions study [PRD83:064002,2011]. Our GRHD simulations of a pWDNS system with a 0.98-solar-mass WD and a 1.4-solar-mass NS show that the merger remnant is a spinning Thorne-Zytkow-like Object (TZlO) surrounded by a massive disk. The final total rest mass exceeds Mmax, but the remnant does not collapse promptly. To assess whether the object will ultimately collapse after cooling, we introduce radiative thermal cooling. We first apply our cooling algorithm to TZlOs formed in WDNS head-on collisions, and show that these objects collapse and form black holes on the cooling time scale, as expected. However, when we cool the spinning TZlO formed in the merger of a circular-orbit WDNS binary, the remnant does not collapse, demonstrating that differential rotational support is sufficient to prevent collapse. Given that the final total mass exceeds Mmax, magnetic fields and/or viscosity may redistribute angular momentum and ultimately lead to delayed collapse to a BH. We infer that the merger of realistic massive WDNS binaries likely will lead to the formation of spinning TZlOs that undergo delayed collapse.

#### Sep 30, 2011

1109.5177 (/preprints)
2011-09-30, 11:50 

## [1109.5224] Efficient asymptotic frame selection for binary black hole spacetimes using asymptotic radiation

Authors: R. O&#x27;Shaughnessy (1), B. Vaishnav (2), J. Healy (3), Z. Meeks (3), D. Shoemaker (3) ((1) University of Wisconsin, Milwaukee, (2) Georgia Southern University, (3) Center for Relativistic Astrophysics, Georgia Tech)

Date: 24 Sep 2011

Abstract: Previous studies have demonstrated that gravitational radiation reliably encodes information about the natural emission direction of the source (e.g., the orbital plane). In this paper, we demonstrate that these orientations can be efficiently estimated by the principal axes of <L_a L_b>, an average of the action of rotation group generators on the Weyl tensor at asymptotic infinity. Evaluating this average at each time provides the instantaneous emission direction. Further averaging across the entire signal yields an average orientation, closely connected to the angular components of the Fisher matrix. The latter direction is well-suited to data analysis and parameter estimation when the instantaneous emission direction evolves significantly. Finally, in the time domain, the average <L_a L_b> provides fast, invariant diagnostics of waveform quality.

#### Sep 30, 2011

1109.5224 (/preprints)
2011-09-30, 11:50 

## [1109.6312] Relativistic Superluminal Neutrinos

Authors: Alex Kehagias

Date: 28 Sep 2011

Abstract: We present a possible solution to the OPERA collaboration anomaly for the speed of neutrinos, based on the idea that it is a local effect caused by a scalar field sourced by the earth. The coupling of the scalar to neutrinos effectively changes the background metric where they propagate, leading to superluminality. The strength of the coupling is set by a new mass scale, which is at $1\, {\rm TeV}$ to account for the OPERA anomaly. Moreover, if this scenario is valid, the neutrino velocity depends on the baseline distance between the emission and detection points in such a way that superluminal signals are turn to subluminal for baseline distances roughly larger than the earth radius.

#### Sep 30, 2011

1109.6312 (/preprints)
2011-09-30, 11:49 

## [1109.6201] A relativistic navigation system for space

Authors: Angelo Tartaglia, Matteo Luca Ruggiero, Emiliano Capolongo

Date: 28 Sep 2011

Abstract: We present here a method for the relativistic positioning in spacetime based on the reception of pulses from sources of electromagnetic signals whose worldline is known. The method is based on the use of a four-dimensional grid covering the whole spacetime and made of the null hypersurfaces representing the propagating pulses. In our first approach to the problem of positioning we consider radio-pulsars at infinity as primary sources of the required signals. The reason is that, besides being very good clocks, pulsars can be considered as being fixed stars for reasonably long times. The positioning is obtained linearizing the worldline of the observer for times of the order of a few periods of the signals. We present an exercise where the use of our method applied to the signals from four real pulsars permits the reconstruction of the motion of the Earth with respect to the fixed stars during three days. The uncertainties and the constraints of the method are discussed and the possibilities of using moving artificial sources carried around by celestial bodies or spacecrafts in the Solar System is also discussed.

#### Sep 30, 2011

1109.6201 (/preprints)
2011-09-30, 11:48 

## [1109.6339] SDSS J163030.58 423305.8: A 40 minute Orbital Period Detached White Dwarf Binary

Authors: Mukremin Kilic, Warren R. Brown, J. J. Hermes, Carlos Allende Prieto, S. J. Kenyon, D. E. Winget, K. I. Winget

Date: 28 Sep 2011

Abstract: We report the discovery of a new detached, double white dwarf system with an orbital period of 39.8 min. We targeted SDSS J163030.58+423305.8 (hereafter J1630) as part of our radial velocity program to search for companions around low-mass white dwarfs using the 6.5m MMT. We detect peak-to-peak radial velocity variations of 576 km/s. The mass function and optical photometry rule out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companion is most likely another white dwarf. Unlike the other 39 min binary SDSS J010657.39-100003.3, follow-up high speed photometric observations of J1630 obtained at the McDonald 2.1m telescope do not show significant ellipsoidal variations, indicating a higher primary mass and smaller radius. The absence of eclipses constrain the inclination angle to <82deg. J1630 contains a pair of white dwarfs, 0.3 Msun primary + >0.3 Msun invisible secondary, at a separation of >0.32 Rsun. The two white dwarfs will merge in less than 31 Myr. Depending on the core composition of the companion, the merger will form either a single core-He burning subdwarf star or a rapidly rotating massive white dwarf. The gravitational wave strain from J1630 is detectable by instruments like the Laser Interferometer Space Antenna (LISA) within the first year of operation.

#### Sep 30, 2011

1109.6339 (/preprints)
2011-09-30, 11:47 

## [1109.6324] Black holes in scalar-tensor gravity

Authors: Thomas P. Sotiriou, Valerio Faraoni

Date: 28 Sep 2011

Abstract: Hawking has proven that black holes which are stationary as the endpoint of gravitational collapse in Brans--Dicke theory (without a potential) are no different than in general relativity. We extend this proof to the much more general class of scalar-tensor and f(R) gravity theories, without assuming any additional symmetries.

#### Sep 30, 2011

1109.6324 (/preprints)
2011-09-30, 11:45 

## [1109.6329] Tidal disruption rate of stars by spinning supermassive black holes

Authors: Michael Kesden

Date: 28 Sep 2011

Abstract: A supermassive black hole can disrupt a star when its tidal field exceeds the star's self-gravity, and can directly capture stars that cross its event horizon. For black holes with mass M > 10ˆ7 solar masses, tidal disruption of main-sequence stars occurs close enough to the event horizon that a Newtonian treatment of the tidal field is no longer valid. The fraction of stars that are directly captured is also no longer negligible. We calculate generically oriented stellar orbits in the Kerr metric, and evaluate the relativistic tidal tensor at pericenter for those stars not directly captured by the black hole. We combine this relativistic analysis with previous calculations of how these orbits are populated to determine tidal-disruption rates for spinning black holes. We find, consistent with previous results, that black-hole spin increases the upper limit on the mass of a black hole capable of tidally disrupting solar-like stars to ~7 x 10ˆ8 solar masses. More quantitatively, we find that direct stellar capture reduces tidal-disruption rates by a factor 2/3 (1/10) at M = 10ˆ7 (10ˆ8) solar masses. The strong dependence of tidal-disruption rates on black-hole spin for M > 10ˆ8 solar masses implies that future surveys like LSST that discover thousands of tidal disruption events can constrain supermassive black-hole spin demographics.

#### Sep 30, 2011

1109.6329 (/preprints)
2011-09-30, 11:45 

## [1109.6021] Floating and sinking: the imprint of massive scalars around rotating black holes

Authors: Vitor Cardoso, Sayan Chakrabarti, Paolo Pani, Emanuele Berti, Leonardo Gualtieri

Date: 27 Sep 2011

Abstract: We study the coupling of massive scalar fields to matter in orbit around rotating black holes. It is generally expected that orbiting bodies will lose energy in gravitational waves, slowly inspiralling into the black hole. Instead, we show that the coupling of the field to matter leads to a surprising effect: because of superradiance, matter can hover into "floating orbits" for which the net gravitational energy loss at infinity is entirely provided by the black hole's rotational energy. Orbiting bodies remain floating until they extract sufficient angular momentum from the black hole, or until perturbations or nonlinear effects disrupt the orbit. For slowly rotating and nonrotating black holes floating orbits are unlikely to exist, but resonances at orbital frequencies corresponding to quasibound states of the scalar field can speed up the inspiral, so that the orbiting body "sinks". These effects could be a smoking gun of deviations from general relativity.

#### Sep 29, 2011

1109.6021 (/preprints)
2011-09-29, 15:57 

## [1109.4585] The post-Newtonian limit in C-theories of gravitation

Authors: Tomi S. Koivisto

Date: 21 Sep 2011

Abstract: C-theory provides a unified framework to study metric, metric-affine and more general theories of gravity. In the vacuum weak-field limit of these theories, the parameterized post-Newtonian (PPN) parameters $\beta$ and $\gamma$ can differ from their general relativistic values. However, there are several classes of models featuring long-distance modifications of gravity but nevertheless passing the Solar system tests. Here it is shown how to compute the PPN parameters in C-theories and also in nonminimally coupled curvature theories, correcting previous results in the literature for the latter.

#### Sep 22, 2011

1109.4585 (/preprints)
2011-09-22, 17:13 

## [1109.4256] Testing the Kerr black hole hypothesis

Authors: Cosimo Bambi

Date: 20 Sep 2011

Abstract: It is thought that the final product of the gravitational collapse is a Kerr black hole and astronomers have discovered several good astrophysical candidates. While there are some indirect evidences suggesting that the latter have an event horizon, and therefore that they are black holes, a proof that the space-time around these objects is described by the Kerr geometry is still lacking. Recently, there has been an increasing interest in the possibility of testing the Kerr black hole hypothesis with present and future experiments. In this paper, I briefly review the state of art of the field, focussing on some recent results and works in progress.

#### Sep 22, 2011

1109.4256 (/preprints)
2011-09-22, 16:03 

## [1109.3707] Massive black hole binary plane reorientation in rotating stellar systems

Authors: Alessia Gualandris, Massimo Dotti, Alberto Sesana

Date: 16 Sep 2011

Abstract: We study the evolution of the orientation of the orbital plane of massive black hole binaries (BHBs) in rotating stellar systems in which the total angular momentum of the stellar cusp is misaligned with respect to that of the binary. We compare results from direct summation N-body simulations with predictions from a simple theoretical model. We find that the same encounters between cusp stars and the BHB that are responsible for the hardening and eccentricity evolution of the binary, lead to a reorientation of the binary orbital plane. In particular, binaries whose angular momentum is initially misaligned with respect to that of the stellar cusp tend to realign their orbital planes with the angular momentum of the cusp on a timescale of a few hardening times. This is due to angular momentum exchange between stars and the BHB during close encounters, and may have important implications for the relative orientation of host galaxies and radio jets.

#### Sep 22, 2011

1109.3707 (/preprints)
2011-09-22, 16:02 

## [1109.4170] Repeated Bursts from Relativistic Scattering of Compact Objects in Galactic Nuclei

Authors: Bence Kocsis, Janna Levin

Date: 19 Sep 2011

Abstract: Galactic nuclei are densely populated by stellar mass compact objects such as black holes and neutron stars. Bound, highly eccentric binaries form as a result of gravitational wave (GW) losses during close flybys between these objects. We study the evolution of these systems using 2.5 and 3.5 order post-Newtonian equations of motion. The GW signal consists of many thousand repeated bursts (RB) for minutes to days (depending on the impact parameter and masses), followed by a powerful GW chirp and an eccentric merger. We show that a significant signal to noise ratio (SNR) accumulates already in the RB phase, corresponding to a detection limit around 200--300 Mpc and 300--600 Mpc for Advanced LIGO for an average orientation BH/NS or BH/BH binary, respectively. The theoretical errors introduced by the inaccuracy of the PN templates are typically much less severe for the RB phase than in the following eccentric merger. The GW signal in the RB phase is broadband; we show that encounters involving intermediate mass black holes are detectable in multiple frequency bands coincidentally using LIGO and LISA.

#### Sep 22, 2011

1109.4170 (/preprints)
2011-09-22, 16:02 

## [1109.3611] Accuracy of numerical relativity waveforms from binary neutron star mergers and their comparison with post-Newtonian waveforms

Authors: Sebastiano Bernuzzi, Marcus Thierfelder, Bernd Bruegmann

Date: 16 Sep 2011

Abstract: We present numerical relativity simulations of nine-orbit equal-mass binary neutron star covering the quasi-circular late inspiral and merger. The extracted gravitational waveforms are analyzed for convergence and accuracy. Second order convergence is observed up to contact, i.e. about 3-4 cycles to merger, thus error estimates can be made up to this point. The uncertainties on the phase and the amplitude are dominated by truncation errors and can be minimized to 0.13 rad and less then 1 %, respectively, by using several simulations and extrapolating in resolution. In the latter case finite radius extraction uncertainties become a source of error of the same order and have to be taken into account. The waveforms are tested against accuracy standards for data analysis. The uncertainties on the waveforms are such that accuracy standards are generically not met for signal-to-noise ratios relevant for detection, except for some best cases and optimistic (but rigorous) choice of error bars. A detailed analysis of the errors is thus imperative for the use of numerical relativity waveforms from binary neutron stars in quantitative studies. The waveforms are compared with the post-Newtonian Taylor T4 approximants both for point-particle and including the analytically known tidal corrections. After alignment, the T4 approximants maintain the phasing for three to four cycles, but later they rapidly accumulate about 2.5 rad at contact and about 6 rad at merger.

#### Sep 19, 2011

1109.3611 (/preprints)
2011-09-19, 10:31 

## [1109.3506] Reply to the "Comment on: Detecting Vanishing Dimensions Via Primordial Gravitational Wave Astronomy"

Authors: Jonas Mureika, Dejan Stojkovic

Date: 15 Sep 2011

Abstract: The "Comment on: Detecting Vanishing Dimensions Via Primordial Gravitational Wave Astronomy" [arXiv:1104.1223] is misleading and premised on a misinterpretation of the main content of Phys. Rev. Lett. 106, 101101 (2011) [arXiv:1102.3434]. The main claim in the comment - that in some exotic theories different from general relativity (GR) there might be local degrees of freedom even in lower dimensional spaces - is trivial. Nevertheless, the authors of the Comment fail to come-up with a single self-consistent example. This claim, however, has no implications for our paper, in which we make it clear we are working within the framework of "vanishing" or "evolving" dimensions as defined in arXiv:1003.5914.

#### Sep 19, 2011

1109.3506 (/preprints)
2011-09-19, 10:31 

## [1109.3498] Implementation and testing of the first prompt search for electromagnetic counterparts to gravitational wave transients

Authors: The LIGO Scientific Collaboration, Virgo Collaboration, M. Boer, R. Fender, N. Gehrels, A. Klotz, E. O. Ofek, M. Smith, M. Sokolowski, B. W. Stappers, I. Steele, J. Swinbank, R. A. M. J. Wijers

Date: 15 Sep 2011

Abstract: Aims. A transient astrophysical event observed in both gravitational wave (GW) and electromagnetic (EM) channels would yield rich scientific rewards. A first program initiating EM follow-ups to possible transient GW events has been developed and exercised by the LIGO and Virgo community in association with several partners. In this paper, we describe and evaluate the methods used to promptly identify and localize GW event candidates and to request images of targeted sky locations.
Methods. During two observing periods (Dec 17 2009 to Jan 8 2010 and Sep 2 to Oct 20 2010), a low-latency analysis pipeline was used to identify GW event candidates and to reconstruct maps of possible sky locations. A catalog of nearby galaxies and Milky Way globular clusters was used to select the most promising sky positions to be imaged, and this directional information was delivered to EM observatories with time lags of about thirty minutes. A Monte Carlo simulation has been used to evaluate the low-latency GW pipeline's ability to reconstruct source positions correctly.
Results. For signals near the detection threshold, our low-latency algorithms often localized simulated GW burst signals to tens of square degrees, while neutron star/neutron star inspirals and neutron star/black hole inspirals were localized to a few hundred square degrees. Localization precision improves for moderately stronger signals. The correct sky location of signals well above threshold and originating from nearby galaxies may be observed with ~50% or better probability with a few pointings of wide-field telescopes.

#### Sep 19, 2011

1109.3498 (/preprints)
2011-09-19, 10:30 

## [1109.3492] Gravitational Wave Astrophysics: Opening the New Frontier

Authors: Joan Centrella

Date: 15 Sep 2011

Abstract: The gravitational wave window onto the universe is expected to open in ~ 5 years, when ground-based detectors make the first detections in the high-frequency regime. Gravitational waves are ripples in spacetime produced by the motions of massive objects such as black holes and neutron stars. Since the universe is nearly transparent to gravitational waves, these signals carry direct information about their sources - such as masses, spins, luminosity distances, and orbital parameters - through dense, obscured regions across cosmic time. This article explores gravitational waves as cosmic messengers, highlighting key sources, detection methods, and the astrophysical payoffs across the gravitational wave spectrum.

#### Sep 19, 2011

1109.3492 (/preprints)
2011-09-19, 10:29 

## [1109.3402] Extracting equation of state parameters from black hole-neutron star mergers. I. Nonspinning black holes

Authors: Benjamin D. Lackey, Koutarou Kyutoku, Masaru Shibata, Patrick R. Brady, John L. Friedman

Date: 15 Sep 2011

Abstract: The late inspiral, merger, and ringdown of a black hole-neutron star (BHNS) system can provide information about the neutron-star equation of state (EOS). Candidate EOSs can be approximated by a parametrized piecewise-polytropic EOS above nuclear density, matched to a fixed low-density EOS; and we report results from a large set of BHNS inspiral simulations that systematically vary two parameters. To within the accuracy of the simulations, we find that, apart from the neutron-star mass, a single physical parameter Lambda, describing its deformability, can be extracted from the late inspiral, merger, and ringdown waveform. This parameter is related to the radius, mass, and l=2 Love number, k_2, of the neutron star by Lambda = 2k_2 Rˆ5/3M_{NS}ˆ5, and it is the same parameter that determines the departure from point-particle dynamics during the early inspiral. Observations of gravitational waves from BHNS inspiral thus restrict the EOS to a surface of constant Lambda in the parameter space, thickened by the measurement error. Using various configurations of a single Advanced LIGO detector, we find that neutron stars are distinguishable from black holes of the same mass and that Lambdaˆ{1/5} or equivalently R can be extracted to 10-40% accuracy from single events for mass ratios of Q=2 and 3 at a distance of 100 Mpc, while with the proposed Einstein Telescope, EOS parameters can be extracted to accuracy an order of magnitude better.

#### Sep 16, 2011

1109.3402 (/preprints)
2011-09-16, 10:23 

## [1109.2284] Triple Disruptions in The Galactic Centre: Captured and Ejected Binaries, Rejuvenated Stars, and Correlated Orbits

Authors: Idan Ginsburg, Hagai B. Perets

Date: 11 Sep 2011

Abstract: The disruption of a binary star by a massive black hole (MBH) typically leads to the capture of one component around the MBH and the ejection of its companion at a high velocity, possibly producing a hypervelocity star. The high fraction of observed triples ($\sim10$% for F/G/K stars and $\sim50$% for OB stars) give rise to the possibility of the disruption of triples by a MBH. Here we study this scenario, and use direct $N$-body integrations to follow the orbits of thousands of triples, during and following their disruption by a MBH (of $4\times10ˆ6$ M$_\odot$, similar to the MBH existing in the Galactic Centre; SgrA$ˆ*$). We find that triple disruption can lead to several outcomes and we discuss their relative frequency. Beside the ejection/capture of single stars, similar to the binary disruption case, the outcomes of triple disruption include the ejection of hypervelocity binaries; capture of binaries around the MBH; collisions between two or all of the triple components (with low enough velocities that could lead to their merger); and the capture of two or even three stars at close orbits around the MBH. The orbits of single stars captured in a single disruption event are found to be correlated. The eccentricity of the mutual orbits of captured/ejected binaries is typically excited to higher values. Stellar evolution of captured/ejected binaries may later result in their coalescence/strong interaction and the formation of hypervelocity blue stragglers or merger remnants in orbits around SgrA*. Finally, the capture of binaries close to the MBH can replenish and increase the binary frequency near the MBH, which is otherwise very low.

#### Sep 14, 2011

1109.2284 (/preprints)
2011-09-14, 13:46 

## [1109.2744] Tidal Perturbations to the Gravitational Inspiral of J0651 2844

Authors: Matthew Benacquista

Date: 13 Sep 2011

Abstract: The recently discovered J0651+2844 is a detached, eclipsing white dwarf binary with an orbital period of 765 s. We investigate the prospects for the detection of gravitational radiation from this system and estimate the effect of the tidal deformation of the low-mass component on the period evolution of the system. Because of the high inclination of the system, the amplitude of the gravitational waves at Earth will be as much as a factor of two lower than that from an optimally oriented system. The dominant contribution of tidal corrections to the period evolution comes from the increase in rotational energy of the components as they spin up to remain tied to the orbital period. This contribution results in an advance of the timing of the eclipses by an additional 0.3 s after one year.

#### Sep 14, 2011

1109.2744 (/preprints)
2011-09-14, 13:45 

## [1108.6056] What is the Most Promising Electromagnetic Counterpart of a Neutron Star Binary Merger?

Authors: Brian D. Metzger, Edo Berger

Date: 30 Aug 2011

#### Sep 09, 2011

1108.6056 (/preprints)
2011-09-09, 07:00 

## [1108.5161] Hubble without the Hubble: cosmology using advanced gravitational-wave detectors alone

Authors: Stephen R. Taylor, Jonathan R. Gair, Ilya Mandel

Date: 25 Aug 2011

Abstract: We propose a novel approach to measuring the Hubble constant using gravitational-wave signals from compact binaries by exploiting the narrowness of the distribution of masses of the underlying neutron-star population. The Advanced LIGO gravitational wave detector is due to come online in 2015 with a factor of ~10 sensitivity increase over its predecessor. The volume-averaged range at which an inspiraling double-neutron-star binary can be detected should increase from ~15 Mpc to ~200 Mpc, providing a thousand-fold gain in the volume sensitivity of the detector. Incorporating AdLIGO into a global network (through AdVirgo or LIGO-Australia) will boost the directional sensitivity and permit source distance determination. In this paper, we explore what we can learn about the background cosmology and the mass distribution of neutron stars from the set of neutron star (NS) mergers detected by such a network. We use a Bayesian formalism to analyse catalogues of NS-NS inspiral detections. We find that it is possible to constrain the Hubble constant, H_0, and the parameters of the NS mass function using gravitational-wave data alone, without relying on electromagnetic counterparts. Under reasonable assumptions, we will be able to determine H_0 to +/- 10% using ~100 observations, provided the Gaussian half-width of the underlying double NS mass distribution is less than 0.04 M_{\odot}. The expected precision depends linearly on the intrinsic width of the NS mass function, but has only a weak dependence on H_0 near the default parameter values. Finally, we consider what happens if, for some fraction of our data catalogue, we have an electromagnetically measured redshift. The detection, and cataloguing, of these compact object mergers will allow precision astronomy, and provide a determination of H_0 which is independent of the local distance scale.

#### Sep 09, 2011

1108.5161 (/preprints)
2011-09-09, 06:59 

## [1108.5618] Interpolating compact binary waveforms using the singular value decomposition

Authors: Kipp Cannon, Chad Hanna, Drew Keppel

Date: 29 Aug 2011

Abstract: Compact binary systems with total masses between tens and hundreds of solar masses will produce gravitational waves during their merger phase that are detectable by second-generation ground-based gravitational-wave detectors. In order to model the gravitational waveform of the merger epoch of compact binary coalescence, the full Einstein equations must be solved numerically for the entire mass and spin parameter space. However, this is computationally expensive. Several models have been proposed to interpolate the results of numerical relativity simulations. In this paper we propose a numerical interpolation scheme that stems from the singular value decomposition. This algorithm shows promise in allowing one to construct arbitrary waveforms within a certain parameter space given a sufficient density of numerical simulations covering the same parameter space. We also investigate how similar approaches could be used to interpolate waveforms in the context of parameter estimation.

#### Sep 09, 2011

1108.5618 (/preprints)
2011-09-09, 06:58 

## [1108.5486] Visualizing Spacetime Curvature via Frame-Drag Vortexes and Tidal Tendexes I. General Theory and Weak-Gravity Applications

Authors: David A. Nichols, Robert Owen, Fan Zhang, Aaron Zimmerman, Jeandrew Brink, Yanbei Chen, Jeffrey D. Kaplan, Geoffrey Lovelace, Keith D. Matthews, Mark A. Scheel, Kip S. Thorne

Date: 28 Aug 2011

Abstract: When one splits spacetime into space plus time, the Weyl curvature tensor (vacuum Riemann tensor) gets split into two spatial, symmetric, and trace-free (STF) tensors: (i) the Weyl tensor's so-called "electric" part or tidal field, and (ii) the Weyl tensor's so-called "magnetic" part or frame-drag field. Being STF, the tidal field and frame-drag field each have three orthogonal eigenvector fields which can be depicted by their integral curves. We call the integral curves of the tidal field's eigenvectors tendex lines, we call each tendex line's eigenvalue its tendicity, and we give the name tendex to a collection of tendex lines with large tendicity. The analogous quantities for the frame-drag field are vortex lines, their vorticities, and vortexes. We build up physical intuition into these concepts by applying them to a variety of weak-gravity phenomena: a spinning, gravitating point particle, two such particles side by side, a plane gravitational wave, a point particle with a dynamical current-quadrupole moment or dynamical mass-quadrupole moment, and a slow-motion binary system made of nonspinning point particles. [Abstract is abbreviated; full abstract also mentions additional results.]

#### Sep 09, 2011

1108.5486 (/preprints)
2011-09-09, 06:57 

## [1109.0081] Hybrid method for understanding black-hole mergers: Inspiralling case

Authors: David A. Nichols, Yanbei Chen

Date: 1 Sep 2011

Abstract: We adapt a method of matching post-Newtonian and black-hole-perturbation theories on a timelike surface (which proved useful for understanding head-on black-hole-binary collisions) to treat equal-mass, inspiralling black-hole binaries. We first introduce a radiation-reaction potential into this method, and we show that it leads to a self-consistent set of equations that describe the simultaneous evolution of the waveform and of the timelike matching surface. This allows us to produce a full inspiral-merger-ringdown waveform of the l=2, m=2,-2 modes of the gravitational waveform of an equal-mass black-hole-binary inspiral. These modes match those of numerical-relativity simulations well in phase, though less well in amplitude for the inspiral. As a second application of this method, we study a merger of black holes with spins antialigned in the orbital plane (the "superkick" configuration). During the ringdown of the superkick, the phases of the mass- and current-quadrupole radiation become locked together, because they evolve at the same quasinormal mode frequencies. We argue that this locking begins during merger, and we show that if the spins of the black holes evolve via geodetic precession in the perturbed black-hole spacetime of our model, then the spins precess at the orbital frequency during merger. In turn, this gives rise to the correct behavior of the radiation, and produces a kick similar to that observed in numerical simulations.

#### Sep 09, 2011

1109.0081 (/preprints)
2011-09-09, 06:55 

## [1109.0068] Constraints on a scalar-tensor theory with an intermediate-range force by binary pulsars

Authors: Xue-Mei Deng

Date: 1 Sep 2011

Abstract: Searching for an intermediate-range force has been considerable interests in gravity experiments. In this paper, aiming at a scalar-tensor theory with an intermediate-range force, we have derived the metric and equations of motion (EOMs) in the first post-Newtonian (1PN) approximation for general matter without specific equation of state and $N$ point masses firstly. Subsequently, the secular periastron precession $\dot{\omega}$ of binary pulsars in harmonic coordinates is given. After that, $\dot{\omega}$ of four binary pulsars data (PSR B1913+16, PSR B1534+12, PSR J0737-3039 and PSR B2127+11C) have been used to constrain the intermediate-range force, namely, the parameters $\alpha$ and $\lambda$. $\alpha$ and $\lambda$ respectively represent the strength of the intermediate-range force coupling and its length scale. The limits from four binary pulsars data are respectively $\lambda=(4.95\pm0.02)\times10ˆ{8}$m and $\alpha=(2.30\pm0.01)\times10ˆ{-8}$ if $\beta=1$ where $\beta$ is a parameter like standard parametrized post-Newtonian parameter $\beta_{PPN}$. When three degrees of freedom ($\alpha$, $\lambda$ and $\bar{\beta}\equiv\beta-1$) in 1$\sigma$ confidence level are considered, it yields $\alpha=(4.21\pm0.01)\times10ˆ{-4}$, $\lambda=(4.51\pm0.01)\times10ˆ{7}$m and $\bar{\beta}=(-3.30\pm0.01)\times10ˆ{-3}$. Through our research on the scalar-tensor theory with the intermediate-range force, it shows that the parameter $\alpha$ is directly related to the parameter $\gamma$ ($\alpha=(1-\gamma)/(1+\gamma)$). Thus, this presents the constraints on $1-\gamma$ by binary pulsars which is about $10ˆ{-4}$ for three degrees of freedom.

#### Sep 09, 2011

1109.0068 (/preprints)
2011-09-09, 06:55 

## [1109.0501] A semi-coherent search strategy for known continuous wave sources in binary systems

Authors: Chris Messenger

Date: 2 Sep 2011

Abstract: We present a method for detection of weak continuous signals from sources in binary systems via the incoherent combination of many "short" coherently-analyzed segments. The main focus of the work is on the construction of a metric on the parameter space for such signals for use in matched-filter based searches. The metric is defined using a maximum likelihood detection statistic applied to a binary orbit phase model including eccentricity. We find that this metric can be accurately approximated by its diagonal form in the regime where the segment length is << the orbital period. Hence correlations between parameters are effectively removed by the combination of many independent observation. We find that the ability to distinguish signal parameters is independent of the total semi-coherent observation span (for the semi-coherent span >> the segment length) for all but the orbital angular frequency. Increased template density for this parameter scales linearly with the observation span. We also present two example search schemes. The first uses a re parameterized phase model upon which we compute the metric on individual short coherently analyzed segments. The second assumes long >> the orbital period segment lengths from which we again compute the coherent metric and find it to be approximately diagonal. In this latter case we also show that the semi-coherent metric is equal to the coherent metric.

#### Sep 09, 2011

1109.0501 (/preprints)
2011-09-09, 06:54 

## [1109.0442] A Student-t based filter for robust signal detection

Authors: Christian R&#xf6;ver

Date: 2 Sep 2011

Abstract: The search for gravitational-wave signals in detector data is often hampered by the fact that many data analysis methods are based on the theory of stationary Gaussian noise, while actual measurement data frequently exhibit clear departures from these assumptions. Deriving methods from models more closely reflecting the data's properties promises to yield more sensitive procedures. The commonly used matched filter is such a detection method that may be derived via a Gaussian model. In this paper we propose a generalized matched filtering technique based on a Student-t distribution that is able to account for heavier-tailed noise and is robust against outliers in the data. On the technical side, it generalizes the matched-filter's least-squares method to an iterative, or adaptive, variation. In a simplified Monte Carlo study we show that when applied to simulated signals buried in actual interferometer noise it leads to a higher detection rate than the usual (Gaussian) matched filter.

#### Sep 09, 2011

1109.0442 (/preprints)
2011-09-09, 06:53 

## [1109.0928] Compact stars in alternative theories of gravity. Einstein-Dilaton-Gauss-Bonnet gravity

Authors: Paolo Pani, Emanuele Berti, Vitor Cardoso, Jocelyn Read

Date: 5 Sep 2011

Abstract: We develop a theoretical framework to study slowly rotating compact stars in a rather general class of alternative theories of gravity, with the ultimate goal of investigating constraints on alternative theories from electromagnetic and gravitational-wave observations of compact stars. Our Lagrangian includes as special cases scalar-tensor theories (and indirectly f(R) theories) as well as models with a scalar field coupled to quadratic curvature invariants. As a first application of the formalism, we discuss (for the first time in the literature) compact stars in Einstein-Dilaton-Gauss-Bonnet gravity. We show that compact objects with central densities typical of neutron stars cannot exist for certain values of the coupling constants of the theory. In fact, the existence and stability of compact stars sets more stringent constraints on the theory than the existence of black hole solutions. This work is a first step in a program to systematically rule out (possibly using Bayesian model selection) theories that are incompatible with astrophysical observations of compact stars.

#### Sep 09, 2011

1109.0928 (/preprints)
2011-09-09, 06:53 

## [1109.0572] The Chimera Scheme: Approximate Waveforms for Extreme-Mass-Ratio Inspirals

Authors: Carlos F. Sopuerta, Nicol&#xe1;s Yunes

Date: 2 Sep 2011

Abstract: We introduce the Chimera scheme, a new framework to model the dynamics of generic extreme mass-ratio inspirals (stellar compact objects spiraling into a spinning super-massive black hole) and to produce the gravitational waveforms that describe the gravitational wave emission of these systems. The Chimera scheme combines techniques from black hole perturbation theory and post-Minkowskian theory. The orbital evolution is approximated as a sequence of osculating geodesics that shrink due to the stellar compact object's self-acceleration. Lacking a general prescription for this self-force, we here approximate it locally in time via a post-Minkowskian expansion. The orbital evolution is thus equivalent to evolving the geodesic equations with time-dependent orbital elements, as dictated by this post-Minkowskian radiation-reaction prescription. Gravitational radiation is modeled via a multipolar expansion in post-Minkowskian theory, here taken up to mass hexadecapole and current octopole order. To complete the scheme, both the orbital evolution and wave generation require to map the Boyer-Lindquist coordinates of the orbits to the harmonic coordinates in which the different post-Minkowskian quantities have been derived, a mapping that we provide explicitly in this paper. The Chimera scheme is thus a combination of approximations that can be used to model generic inspirals of systems with extreme mass ratios to systems with more moderate mass ratios, and hence can provide valuable information for future space-based gravitational-wave observatories like the Laser Interferometer Space Antenna and even for advanced ground detectors. Finally, due to the local character in time of our post-Minkowskian self-force, the Chimera scheme can be used to perform studies of the possible appearance of transient resonances in generic inspirals.

#### Sep 09, 2011

1109.0572 (/preprints)
2011-09-09, 06:51 

## [1109.0542] New Sources of Gravitational Waves during Inflation

Authors: Leonardo Senatore, Eva Silverstein, Matias Zaldarriaga

Date: 2 Sep 2011

Abstract: We point out that detectable inflationary tensor modes can be generated by particle or string sources produced during inflation, consistently with the requirements for inflation and constraints from scalar fluctuations. We show via examples that this effect can dominate over the contribution from quantum fluctuations of the metric, occurring even when the inflationary potential energy is too low to produce a comparable signal. Thus a detection of tensor modes from inflation does not automatically constitute a determination of the inflationary Hubble scale.

#### Sep 09, 2011

1109.0542 (/preprints)
2011-09-09, 06:51 

## [1109.1050] Blindly detecting orbital modulations of jets from merging supermassive black holes

Authors: R. O&#x27;Shaughnessy (1), D.L. Kaplan (1), A. Sesana (2), A. Kamble (1) ((1) University of Wisconsin-Milwaukee, (2) Albert Einstein Institute, Golm)

Date: 6 Sep 2011

Abstract: In the last few years before merger, supermassive black hole binaries will rapidly inspiral and precess in a magnetic field imposed by a surrounding circumbinary disk. Multiple simulations suggest this relative motion will convert some of the local energy to a Poynting-dominated outflow, with a luminosity 10ˆ{43} erg/s * (B/10ˆ4 G)ˆ2(M/10ˆ8 Msun)ˆ2 (v/0.4 c)ˆ2, some of which may emerge as synchrotron emission at frequencies near 1 GHz where current and planned wide-field radio surveys will operate. On top of a secular increase in power on the gravitational wave inspiral timescale, orbital motion will produce significant, detectable modulations, both on orbital periods and (if black hole spins are not aligned with the binary's total angular momenta) spin-orbit precession timescales. Because the gravitational wave merger time increases rapidly with separation, we find vast numbers of these transients are ubiquitously predicted, unless explicitly ruled out (by low efficiency $\epsilon$) or obscured (by accretion geometry f_{geo}). If the fraction of Poynting flux converted to radio emission times the fraction of lines of sight accessible $f_{geo}$ is sufficiently large (f_{geo} \epsilon > 2\times 10ˆ{-4} for a 1 year orbital period), at least one event is accessible to future blind surveys at a nominal 10ˆ4 {deg}ˆ2 with 0.5 mJy sensitivity. Our procedure generalizes to other flux-limited surveys designed to investigate EM signatures associated with many modulations produced by merging SMBH binaries.

#### Sep 09, 2011

1109.1050 (/preprints)
2011-09-09, 06:50 

Authors: Han Wang, Jan Steinhoff, Jing Zeng, Gerhard Sch&#xe4;fer

Date: 6 Sep 2011

Abstract: In the present paper the leading-order post-Newtonian spin-orbit and spin(1)-spin(2) radiation-reaction Hamiltonians are calculated. We utilize the canonical formalism of Arnowitt, Deser, and Misner (ADM), which has shown to be valuable for this kind of calculations. The results are valid for arbitrary many objects. The energy loss is then computed and compared to well known results for the energy flux as a check.

#### Sep 09, 2011

1109.1182 (/preprints)
2011-09-09, 06:50 

## [1108.5175] Kicking massive black holes off clusters: Intermediate-mass ratio inspirals

Authors: Symeon Konstantinidis, Pau Amaro-Seoane, Kostas D. Kokkotas

Date: 25 Aug 2011

Abstract: Contrary to supermassive and stellar-mass black holes (SBHs), the existence of intermediate-mass black holes (IMBHs) with masses ranging between 100 and 10,000 Msun has not yet been confirmed. The main problem in the detection is that the innermost stellar kinematics of globular clusters (GCs), the natural loci to IMBHs, are very difficult to resolve. However, if IMBHs reside in the center of GCs, a possibility is that they interact dynamically with their enviroment. A binary formed with the IMBH and a compact object of the GC would naturally lead to a prominent source of gravitational radiation, detectable with future observatories. We run for the first time direct-summation integrations of GCs with an IMBH including the dynamical evolution of the IMBH with the stellar system and relativistic effects, such as energy loss in gravitational waves (GWs) and periapsis shift, and gravitational recoil. We find in one of our models an intermediate-mass ratio inspiral (IMRI), which leads to a merger with a recoiling velocity higher than the escape velocity of the GC. The GWs emitted fall in the range of frequencies that a LISA-like observatory could detect, like the European eLISA or in mission options considered in the recent preliminary mission study conducted in China. The merger has an impact on the global dynamics of the cluster, as an important heating source is removed when the merged system leaves the GC. The detection of one IMRI would constitute a test of GR, as well as an irrefutable proof of the existence of IMBHs.

#### Sep 09, 2011

1108.5175 (/preprints)
2011-09-09, 06:48 

## [1108.5588] Recovering MOND from extended metric theories of gravity

Authors: T. Bernal, S. Capozziello, J.C. Hidalgo, S. Mendoza

Date: 29 Aug 2011

Abstract: We show that the Modified Newtonian Dynamics (MOND) regime can be fully recovered as the weak-field limit of a particular theory of gravity formulated in the metric approach. This is possible when Milgrom's acceleration constant is taken as a fundamental quantity which couples to the theory in a very consistent manner. As a consequence, the scale invariance of the gravitational interaction is naturally broken. In this sense, Newtonian gravity is the weak-field limit of general relativity and MOND is the weak-field limit of that particular extended theory of gravity. We also prove that a Noether's symmetry approach to the problem yields a conserved quantity coherent with this relativistic MONDian extension.

#### Sep 09, 2011

1108.5588 (/preprints)
2011-09-09, 06:48 

## [1108.5174] The butterfly effect in the extreme-mass ratio inspiral problem

Authors: Pau Amaro-Seoane, Patrick Brem, Jorge Cuadra, Philip J. Armitage

Date: 25 Aug 2011

Abstract: Measurements of gravitational waves from the inspiral of a stellar-mass compact object into a massive black hole (MBH) are unique probes to test General Relativity (GR) and MBH properties, as well as the stellar distribution about these holes in galactic nuclei. Current data analysis techniques can provide us with parameter estimation with very narrow errors. However, an EMRI is not a two-body problem, since other stellar bodies orbiting nearby will influence the capture orbit. Any deviation from the isolated inspiral of the binary will induce a small, though observable deviation from the idealised waveform which could be misinterpreted as a failure of GR. Based on conservative analysis of mass segregation in a Milky Way like nucleus, we estimate that the possibility that a star has a semi-major axis comparable to that of the EMRI is non-negligible. This star introduces an observable perturbation in the orbit in the case in which we consider only loss of energy via gravitational radiation at periapsis. When considering the two first-order non-dissipative post-Newtonian contributions (the periapsis shift of the orbit) the evolution of the orbital elements of the EMRI turns out to be chaotic in nature. The implications of this study are twofold. From the one side, the application to testing GR and measuring MBHs parameters with the detection of EMRIs in galactic nuclei with a millihertz mission will be even more challenging than believed. From the other side, this behaviour could in principle be used as a signature of mass segregation in galactic nuclei.

#### Sep 09, 2011

1108.5174 (/preprints)
2011-09-09, 06:48 

## [1108.4421] Intermediate-mass-ratio black hole binaries II: Modeling Trajectories and Gravitational Waveforms

Authors: Hiroyuki Nakano, Yosef Zlochower, Carlos O. Lousto, Manuela Campanelli

Date: 22 Aug 2011

#### May 17, 2011

1105.2821 (/preprints)
2011-05-17, 14:07 

## [1105.2825] Radiative efficiency and thermal spectrum of accretion onto Schwarzschild black holes

Authors: Scott C. Noble (Rochester Institute of Technology), Julian H. Krolik (Johns Hopkins University), Jeremy D. Schnittman (NASA/Goddard Space Flight Center), John F. Hawley (University of Virginia)

Date: 13 May 2011

Abstract: Recent general relativistic magneto-hydrodynamic (MHD) simulations of accretion onto black holes have shown that, contrary to the basic assumptions of the Novikov-Thorne model, there can be substantial magnetic stress throughout the plunging region. Additional dissipation and radiation can therefore be expected. We use data from a particularly well-resolved simulation of accretion onto a non-spinning black hole to compute both the radiative efficiency of such a flow and its spectrum if all emitted light is radiated with a thermal spectrum whose temperature matches the local effective temperature. This disk is geometrically thin enough (H/r ~= 0.06) that little heat is retained in the flow. In terms of light reaching infinity (i.e., after allowance for all relativistic effects and for photon capture by the black hole), we find that the radiative efficiency is at least ~=6-10% greater than predicted by the Novikov-Thorne model (complete radiation of all heat might yield another ~6%). We also find that the spectrum more closely resembles the Novikov-Thorne prediction for a/M ~= 0.2--0.3 than for the correct value, a/M=0. As a result, if the spin of a non-spinning black hole is inferred by model-fitting to a Novikov-Thorne model with known black hole mass, distance, and inclination, the inferred a/M is too large by ~= 0.2--0.3.

#### May 17, 2011

1105.2825 (/preprints)
2011-05-17, 14:07 

## [1105.0009] Revisiting the double-binary-pulsar probe of non-dynamical Chern-Simons gravity

Authors: Yacine Ali-Ha&#xef;moud

Date: 29 Apr 2011

Abstract: One of the popular modifications to the theory of general relativity is non-dynamical Chern-Simons (CS) gravity, in which the metric is coupled to an externally prescribed scalar field. Setting accurate constraints to the parameters of the theory is important owing to their implications for the scalar field and/or the underlying fundamental theory. The current best constraints rely on measurements of the periastron precession rate in the double-binary-pulsar system and place a very tight bound on the characteristic CS lengthscale k_csˆ{-1} <~ 3*10ˆ{-9} km. This paper considers several effects that were not accounted for when deriving this bound and lead to a substantial suppression of the predicted rate of periastron precession. It is shown, in particular, that the point mass approximation for extended test bodies does not apply in this case. The constraint to the characteristic CS lengthscale is revised to k_csˆ{-1} <~ 0.4 km, eight orders of magnitude weaker than what was previously found.

#### May 17, 2011

1105.0009 (/preprints)
2011-05-17, 09:06 

## [1105.1992] An upper limit to the secular variation of the gravitational constant from white dwarf stars

Authors: Enrique Garc&#xed;a-Berro, Pablo Lor&#xe9;n-Aguilar, Santiago Torres, Leandro G. Althaus, Jordi Isern

Date: 10 May 2011

Abstract: A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10ˆ{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.

#### May 17, 2011

1105.1992 (/preprints)
2011-05-17, 09:06 

## [1105.0368] Gravitational waves and nonaxisymmetric oscillation modes in mergers of compact object binaries

Authors: Nikolaos Stergioulas, Andreas Bauswein, Kimon Zagkouris, Hans-Thomas Janka

Date: 2 May 2011

Abstract: We study the excitation of nonaxisymmetric modes in the post-merger phase of binary compact object mergers and the associated gravitational wave emission. Our analysis is based on general-relativistic simulations, in the spatial conformal flatness approximation, using smoothed-particle-hydrodynamics for the evolution of matter, and we use a set of equal and unequal mass models, described by two nonzero-temperature hadronic equations of state and by one strange star equation of state. Through Fourier transforms of the evolution of matter variables, we can identify a number of oscillation modes, as well as several nonlinear components (combination frequencies). We focus on the dominant m=2 mode, which forms a triplet with two nonlinear components that are the result of coupling to the quasiradial mode. A corresponding triplet of frequencies is identified in the gravitational wave spectrum, when the individual masses of the compact objects are in the most likely range of 1.2 to 1.35 $M_\odot$. We can thus associate, through direct analysis of the dynamics of the fluid, a specific frequency peak in the gravitational wave spectrum with the nonlinear component resulting from the difference between the m=2 mode and the quasiradial mode. Once such observations become available, both the m=2 and quasiradial mode frequencies could be extracted, allowing for the application of gravitational-wave asteroseismology to the post-merger remnant and leading to tight constraints on the equation of state of high-density matter.

#### May 06, 2011

1105.0368 (/preprints)
2011-05-06, 22:21 

## [1105.0109] Studying null and time-like geodesics in the classroom

Authors: Thomas M&#xfc;ller, J&#xf6;rg Frauendiener

Date: 30 Apr 2011

Abstract: In a first course of general relativity it is usually quite difficult for students to grasp the concept of a geodesic. It is supposed to be straight (auto-parallel) and yet it 'looks' curved. In these situations it is very useful to have some explicit examples available which show the different behaviour of geodesics. In this paper we present the GeodesicViewer, an interactive tool for studying the behaviour of geodesics in many different space-times. The geodesics can be represented in several ways, depending on the space-time in question. The use of a local reference frame and 'Cartesian-like' coordinates helps the students to develop some intuition in various situations. We present the various features of the GeodesicViewer in the form of readily formulated exercises for the students.

#### May 06, 2011

1105.0109 (/preprints)
2011-05-06, 22:19 

## [1105.0670] Massive black hole binary eccentricity in rotating stellar systems

Authors: Alberto Sesana, Alessia Gualandris, Massimo Dotti

Date: 3 May 2011

Abstract: In this letter we study the eccentricity evolution of a massive black hole (MBH) binary (MBHB) embedded in a rotating stellar cusp. Following the observation that stars on counter-rotating (with respect to the MBHB) orbits extract angular momentum from the binary more efficiently then their co-rotating counterparts, the eccentricity evolution of the MBHB must depend on the degree of co-rotation (counter-rotation) of the surrounding stellar distribution. Using an hybrid scheme that couples numerical three-body scatterings to an analytical formalism for the cusp-binary interaction, we verify this hypothesis by evolving the MBHB in spherically symmetric cusps with different fractions F of co-rotating stars. Consistently with previous works, binaries in isotropic cusps (F=0.5) tend to increase their eccentricity, and when F approaches zero (counter-rotating cusps) the eccentricity rapidly increases to almost unity. Conversely, binaries in cusps with a significant degree of co-rotation (F>0.7) tend to become less and less eccentric, circularising quite quickly for F approaching unity. Direct N-body integrations performed to test the theory, corroborate the results of the hybrid scheme, at least at a qualitative level. We discuss quantitative differences, ascribing their origin to the oversimplified nature of the hybrid approach.

#### May 06, 2011

1105.0670 (/preprints)
2011-05-06, 22:18 

## [1105.0918] Galactic-center S-Stars as a prospective test of the Einstein Equivalence Principle

Authors: R. Angelil, P. Saha

Date: 4 May 2011

Abstract: The S-Stars in the Galactic-center region are found to be on near-perfect Keplerian orbits around presumably a supermassive black hole, with periods of 15-50 yr. Since these stars reach a few percent of light speed at pericenter, various relativistic effects are expected, and have been discussed in the literature. We argue that an elegant test of the Einstein equivalence principle should be possible with existing instruments, through spectroscopic monitoring of an S-star concentrated during the months around pericenter, supplemented with an already-adequate astrometric determination of the inclination. In essence, the spectrum of an S-star can be considered a heterogeneous ensemble of clocks in a freely-falling frame, which near pericenter is moving at relativistic speeds.

#### May 06, 2011

1105.0918 (/preprints)
2011-05-06, 22:17 

## [1104.5552] Toward an optimal search strategy of optical and gravitational wave emissions from binary neutron star coalescence

Authors: D.M. Coward, B. Gendre, P.J. Sutton, E.J. Howell, T. Regimbau, M. Laas-Bourez, A. Klotz, M. Boer, M. Branchesi

Date: 29 Apr 2011

Abstract: Observations of an optical source coincident with gravitational wave emission detected from a binary neutron star coalescence will improve the confidence of detection, provide host galaxy localisation, and test models for the progenitors of short gamma ray bursts. We employ optical observations of three short gamma ray bursts, 050724, 050709, 051221, to estimate the detection rate of a coordinated optical and gravitational wave search of neutron star mergers. Model R-band optical afterglow light curves of these bursts that include a jet-break are extrapolated for these sources at the sensitivity horizon of an Advanced LIGO/Virgo network. Using optical sensitivity limits of three telescopes, namely TAROT (m=18), Zadko (m=21) and an (8-10) meter class telescope (m=26), we approximate detection rates and cadence times for imaging. We find a median coincident detection rate of 4 yrˆ{-1} for the three bursts. GRB 050724 like bursts, with wide opening jet angles, offer the most optimistic rate of 13 coincident detections yrˆ{-1}, and would be detectable by Zadko up to five days after the trigger. Late time imaging to m=26 could detect off-axis afterglows for GRB 051221 like bursts several months after the trigger. For a broad distribution of beaming angles, the optimal strategy for identifying the optical emissions triggered by gravitational wave detectors is rapid response searches with robotic telescopes followed by deeper imaging at later times if an afterglow is not detected within several days of the trigger.

#### May 02, 2011

1104.5552 (/preprints)
2011-05-02, 13:57 

## [1104.5650] Sky localization of complete inspiral-merger-ringdown signals for nonspinning massive black hole binaries

Authors: Sean T. McWilliams, Ryan N. Lang, John G. Baker, James Ira Thorpe

Date: 29 Apr 2011

Abstract: We investigate the capability of LISA to measure the sky position of equal-mass, nonspinning black hole binaries, combining for the first time the entire inspiral-merger-ringdown signal, the effect of the LISA orbits, and the complete three-channel LISA response. We consider an ensemble of systems near the peak of LISA's sensitivity band, with total rest mass of 2\times10ˆ6 M\odot, a redshift of z = 1, and randomly chosen orientations and sky positions. We find median sky localization errors of approximately \sim3 arcminutes. This is comparable to the field of view of powerful electromagnetic telescopes, such as the James Webb Space Telescope, that could be used to search for electromagnetic signals associated with merging massive black holes. We investigate the way in which parameter errors decrease with measurement time, focusing specifically on the additional information provided during the merger-ringdown segment of the signal. We find that this information improves all parameter estimates directly, rather than through diminishing correlations with any subset of well- determined parameters. Although we have employed the baseline LISA design for this study, many of our conclusions regarding the information provided by mergers will be applicable to alternative mission designs as well.

#### May 02, 2011

1104.5650 (/preprints)
2011-05-02, 13:56 

## [1104.5615] Gravitational radiation for extreme mass ratio inspirals to the 14th post-Newtonian order

Authors: Ryuichi Fujita

Date: 29 Apr 2011

Abstract: We derive gravitational waveforms needed to compute the 14th post-Newtonian (14PN) order energy flux, i.e. $vˆ{28}$ beyond Newtonian approximation where $v$ is the orbital velocity of a test particle, in a circular orbit around a Schwarzschild black hole. We exhibit clearly the convergence of the energy flux in the PN expansion and suggest the fitting formula which can be used for more general case. The phase difference between the 14PN waveforms and numerical waveforms after two years inspiral becomes about $10ˆ{-7}$ for $\mu/M=10ˆ{-4}$ and $10ˆ{-3}$ for $\mu/M=10ˆ{-5}$ where $\mu$ and $M$ are the masses of a compact object and a supermassive black hole at the centers of galaxies respectively. The 14PN expressions will lead to the parameter estimation comparable to numerical waveforms for extreme mass ratio inspirals, which are one of the main targets of Laser Interferometer Space Antenna.

#### May 02, 2011

1104.5615 (/preprints)
2011-05-02, 13:56 

## [1104.5659] Tail-induced spin-orbit effect in the gravitational radiation of compact binaries

Authors: Luc Blanchet (IAP), Alessandra Buonanno, Guillaume Faye (IAP)

Date: 29 Apr 2011

Abstract: Gravitational waves contain tail effects which are due to the back-scattering of linear waves in the curved space-time geometry around the source. In this paper we improve the knowledge and accuracy of the two-body inspiraling post-Newtonian (PN) dynamics and gravitational-wave signal by computing the spin-orbit terms induced by tail effects. Notably, we derive those terms at 3PN order in the gravitational-wave energy flux, and 2.5PN and 3PN orders in the wave polarizations. This is then used to derive the spin-orbit tail effects in the phasing through 3PN order. Our results can be employed to carry out more accurate comparisons with numerical-relativity simulations and to improve the accuracy of analytical templates aimed at describing the all process of inspiral, merger and ringdown.

#### May 02, 2011

1104.5659 (/preprints)
2011-05-02, 13:55 

## [1104.4751] Numerical relativity simulations of binary neutron stars

Authors: Marcus Thierfelder, Sebastiano Bernuzzi, Bernd Bruegmann

Date: 25 Apr 2011

Abstract: We present a new numerical relativity code designed for simulations of compact binaries involving matter. The code is an upgrade of the BAM code to include general relativistic hydrodynamics and implements state-of-the-art high-resolution-shock-capturing schemes on a hierarchy of mesh refined Cartesian grids with moving boxes. We test and validate the code in a series of standard experiments involving single neutron star spacetimes. We present test evolutions of quasi-equilibrium equal-mass irrotational binary neutron star configurations in quasi-circular orbits which describe the late inspiral to merger phases. Neutron star matter is modeled as a zero-temperature fluid; thermal effects can be included by means of a simple ideal-gas prescription. We analyze the impact that the use of different values of damping parameter in the Gamma-driver shift condition has on the dynamics of the system. The use of different reconstruction schemes and their impact in the post-merger dynamics is investigated. We compute and characterize the gravitational radiation emitted by the system. Self-convergence of the waves is tested, and we consistently estimate error-bars on the numerically generated waveforms in the inspiral phase.

#### Apr 26, 2011

1104.4751 (/preprints)
2011-04-26, 16:42 

## [1104.4608] Nordstrom's scalar theory of gravity and the equivalence principle

Authors: Nathalie Deruelle

Date: 24 Apr 2011

Abstract: Nordstrom's theory of gravity, which describes gravity by a scalar field in flat spacetime, is observationally ruled out. It is however the only theory of gravity with General Relativity to obey the strong equivalence principle. I show in this paper that this remarkable property is true beyond post-newtonian level and can be related to the existence of a 'Nordstrom-Katz' superpotential.

#### Apr 26, 2011

1104.4608 (/preprints)
2011-04-26, 16:42 

## [1104.3565] Stochastic Gravitational Wave Background from Coalescing Binary Black Holes

Authors: Xing-Jiang Zhu, Eric Howell, Tania Regimbau, David Blair, Zong-Hong Zhu

Date: 18 Apr 2011

Abstract: We estimate the stochastic gravitational wave (GW) background signal from the field population of coalescing binary stellar mass black holes (BHs) throughout the Universe. This study is motivated by recent observations of BH-Wolf-Rayet star systems and by new estimates in the metallicity abundances of star forming galaxies that imply BH-BH systems are more common than previously assumed. Using recent analytical results of the inspiral-merger-ringdown waveforms for coalescing binary BH systems, we estimate the resulting stochastic GW background signal. Assuming average quantities for the single source energy emissions, we explore the parameter space of chirp mass and local rate density required for detection by advanced and third generation interferometric GW detectors. For an average chirp mass of 8.7$M_{\odot}$, we find that detection through 3 years of cross-correlation by two advanced detectors will require a rate density, $r_0 \geq 0.5 \rm{Mpc}ˆ{-3} \rm{Myr}ˆ{-1}$. Combining data from multiple pairs of detectors can reduce this limit by up to 40%. Investigating the full parameter space we find that detection could be achieved at rates $r_0 \sim 0.1 \rm{Mpc}ˆ{-3} \rm{Myr}ˆ{-1}$ for populations of coalescing binary BH systems with average chirp masses of $\sim 15M_{\odot}$ which are predicted by recent studies of BH-Wolf-Rayet star systems \citep{Bulik08}. While this scenario is at the high end of theoretical estimates, cross-correlation of data by two Einstein Telescopes could detect this signal under the condition $r_0 \geq 10ˆ{-3} \rm{Mpc}ˆ{-3} \rm{Myr}ˆ{-1}$. Such a signal could potentially mask a primordial GW background signal of dimensionless energy density, $\Omega_{\rm{GW}}\sim 10ˆ{-10}$, around the (1--500) Hz frequency range.

#### Apr 22, 2011

1104.3565 (/preprints)
2011-04-22, 15:08 

## [1104.3303] Random Walks in Curved Spacetimes

Authors: Matteo Smerlak

Date: 17 Apr 2011

Abstract: We introduce the master equation describing random walks in curved spacetimes, and derive the corresponding Fokker-Planck equation. By a combination of redshift and spatial curvature effects, the latter generates subleading corrections to Einstein's square-root law for the RMS displacement. We compute the first correction explicitely, and evaluate it for the cases of the Schwarzschild constant-density star, the Kerr black hole and the Friedmann-Robertson-Walker universe: in the first two cases, gravity turns out to enhance diffusion at small times, while in the third case the sign of the correction depends on the curvature of space, and diverges at the Big Bang - unless space is flat.

#### Apr 22, 2011

1104.3303 (/preprints)
2011-04-22, 15:07 

## [1104.3741] Key problems in black hole physics today

Authors: Pankaj S. Joshi

Date: 19 Apr 2011

Abstract: We review here some of the major open issues and challenges in black hole physics today, and the current progress on the same. It is pointed out that to secure a concrete foundation for the basic theory as well as astrophysical applications for black hole physics, it is essential to gain a suitable insight into these questions. In particular, we discuss the recent results investigating the final fate of a massive star within the framework of the Einstein gravity, and the stability and genericity aspects of the gravitational collapse outcomes in terms of black holes and naked singularities. Recent developments such as spinning up a black hole by throwing matter into it, and physical effects near naked singularities are considered. It is pointed out that some of the new results obtained in recent years in the theory of gravitational collapse imply interesting possibilities and understanding for the theoretical advances in gravity as well as towards new astrophysical applications.

#### Apr 22, 2011

1104.3741 (/preprints)
2011-04-22, 15:06 

## [1104.3079] Next-to-next-to-leading order post-Newtonian spin-orbit Hamiltonian for self-gravitating binaries

Authors: Johannes Hartung, Jan Steinhoff

Date: 15 Apr 2011

Abstract: We present the next-to-next-to-leading order post-Newtonian (PN) spin-orbit Hamiltonian for two self-gravitating spinning compact objects. If at least one of the objects is rapidly rotating, then the corresponding interaction is comparable in strength to a 3.5PN effect. The result in the present paper in fact completes the knowledge of the post-Newtonian Hamiltonian for binary spinning black holes up to and including 3.5PN. The Hamiltonian is checked via known results for the test-spin case and via the global Poincaré algebra with the center-of-mass vector uniquely determined by an ansatz.

#### Apr 18, 2011

1104.3079 (/preprints)
2011-04-18, 15:29 

## [1104.2968] Gravitational wave astronomy - astronomy of the 21st century

Authors: S. V. Dhurandhar

Date: 15 Apr 2011

Abstract: An enigmatic prediction of Einstein's general theory of relativity is gravitational waves. With the observed decay in the orbit of the Hulse-Taylor binary pulsar agreeing within a fraction of a percent with the theoretically computed decay from Einstein's theory, the existence of gravitational waves was firmly established. Currently there is a worldwide effort to detect gravitational waves with interferometric gravitational wave observatories or detectors and several such detectors have been built or being built. The initial detectors have reached their design sensitivities and now the effort is on to construct advanced detectors which are expected to detect gravitational waves from astrophysical sources. The era of gravitational wave astronomy has arrived. This article describes the worldwide effort which includes the effort on the Indian front - the IndIGO project -, the principle underlying interferometric detectors both on ground and in space, the principal noise sources that plague such detectors, the astrophysical sources of gravitational waves that one expects to detect by these detectors and some glimpse of the data analysis methods involved in extracting the very weak gravitational wave signals from detector noise.

#### Apr 18, 2011

1104.2968 (/preprints)
2011-04-18, 15:22 

## [1104.2938] An improved, "phase-relaxed" F-statistic for gravitational-wave data analysis

Authors: Curt Cutler

Date: 14 Apr 2011

Abstract: Rapidly rotating, slightly non-axisymmetric neutron stars emit nearly periodic gravitational waves (GWs), quite possibly at levels detectable by ground-based GW interferometers. We refer to these sources as "GW pulsars". For any given sky position and frequency evolution, the F-statistic is the optimal (frequentist) statistic for the detection of GW pulsars. However, in "all-sky" searches for previously unknown GW pulsars, it would be computationally intractable to calculate the (fully coherent) F-statistic at every point of a (suitably fine) grid covering the parameter space: the number of gridpoints is many orders of magnitude too large for that. Here we introduce a "phase-relaxed" F-statistic, which we denote F_pr, for incoherently combining the results of fully coherent searches over short time intervals. We estimate (very roughly) that for realistic searches, our F_pr is ~10-15% more sensitive than the "semi-coherent" F-statistic that is currently used. Moreover, as a byproduct of computing F_pr, one obtains a rough determination of the time-evolving phase offset between one's template and the true signal imbedded in the detector noise. Almost all the ingredients that go into calculating F_pr are already implemented in LAL, so we expect that relatively little additional effort would be required to develop a search code that uses F_pr.

#### Apr 18, 2011

1104.2938 (/preprints)
2011-04-18, 14:10 

## [1104.2700] N-body Simulation for Astronomical Collisional Systems with a New SIMD Instruction Set Extension to the x86 Architecture, Advanced Vector Extensions

Authors: Ataru Tanikawa, Kohji Yoshikawa, Takashi Okamoto, Keigo Nitadori

Date: 14 Apr 2011

Abstract: We present a high-performance N-body code for astronomical collisional systems accelerated with the aid of a new SIMD instruction set extension of the x86 architecture: Advanced Vector eXtensions (AVX), an enhanced version of the Streaming SIMD Extensions (SSE). With one processor core of Intel Core i7-2600 processor (8MB cache and 3.40 GHz) based on Sandy Bridge micro-architecture, we achieved the performance of ~ 20 giga floating point number operations per second (GFlops) for double-precision accuracy, which is two times and five times higher than that of the previously developed code implemented with the SSE instructions (Nitadori et al., 2006b), and that of a code implemented without any explicit use of SIMD instructions with the same processor core. We have parallelized the collisional N-body code by using so-called NINJA scheme (Nitadori et al., 2006a), and achieved ~ 90 GFlops for a system containing more than N = 8192 particles with 8 MPI processes on four cores. We can expect to achieve about 10 tera Flops (TFlops) for an astronomical collisional system with N ~ 10ˆ5 on massively parallel systems with at most 800 cores with Sandy Bridge micro-architecture. This performance will be comparable to that of Graphic Processing Unit (GPU) cluster systems. This paper offers an alternative to collisional N-body simulations with GRAPEs and GPUs.

#### Apr 15, 2011

1104.2700 (/preprints)
2011-04-15, 13:52 

## [1104.2712] Beating the spin-down limit on gravitational wave emission from the Vela pulsar

Date: 14 Apr 2011

Abstract: We present direct upper limits on continuous gravitational wave emission from the Vela pulsar using data from the Virgo detector's second science run. These upper limits have been obtained using three independent methods that assume the gravitational wave emission follows the radio timing. Two of the methods produce frequentist upper limits for an assumed known orientation of the star's spin axis and value of the wave polarization angle of, respectively, $1.9\ee{-24}$ and $2.2\ee{-24}$, with 95% confidence. The third method, under the same hypothesis, produces a Bayesian upper limit of $2.1\ee{-24}$, with 95% degree of belief. These limits are below the indirect {\it spin-down limit} of $3.3\ee{-24}$ for the Vela pulsar, defined by the energy loss rate inferred from observed decrease in Vela's spin frequency, and correspond to a limit on the star ellipticity of $\sim 10ˆ{-3}$. Slightly less stringent results, but still well below the spin-down limit, are obtained assuming the star's spin axis inclination and the wave polarization angles are unknown.

#### Apr 15, 2011

1104.2712 (/preprints)
2011-04-15, 13:51 

## [1104.2650] A blind hierarchical coherent search for gravitational-wave signals from coalescing compact binaries in a network of interferometric detectors

Authors: Sukanta Bose, Thilina Dayanga, Shaon Ghosh, Dipongkar Talukder

Date: 14 Apr 2011

Abstract: We describe a hierarchical data analysis pipeline for coherently searching for gravitational wave (GW) signals from non-spinning compact binary coalescences (CBCs) in the data of multiple earth-based detectors. It assumes no prior information on the sky position of the source or the time of occurrence of its transient signals and, hence, is termed "blind". The pipeline computes the coherent network search statistic that is optimal in stationary, Gaussian noise, and allows for the computation of a suite of alternative statistics and signal-based discriminators that can improve its performance in real data. Unlike the coincident multi-detector search statistics employed so far, the coherent statistics are different in the sense that they check for the consistency of the signal amplitudes and phases in the different detectors with their different orientations and with the signal arrival times in them. The first stage of the hierarchical pipeline constructs coincidences of triggers from the multiple interferometers, by requiring their proximity in time and component masses. The second stage follows up on these coincident triggers by computing the coherent statistics. The performance of the hierarchical coherent pipeline on Gaussian data is shown to be better than the pipeline with just the first (coincidence) stage.

#### Apr 15, 2011

1104.2650 (/preprints)
2011-04-15, 13:51 

## [1104.2044] Gravitational wave background from coalescence of black hole binaries population

Authors: Stefania Marassi, Raffaella Schneider, Giovanni Corvino, Valeria Ferrari, Simon Portergies Zwart

Date: 11 Apr 2011

Abstract: We compute the stochastic gravitational wave background(GWB) generated by a cosmological population of (BH-BH) binaries. Using an updated version of the SeBa population synthesis code, we simulate a large sample of binary systems. Adopting a set of "standard" conservative assumptions calibrated to reproduce the observed properties of single Wolf-Rayet stars and double pulsars, we extract fundamental statistical information on (BH-BH) physical parameters (primary and secondary BH masses, orbital separations and eccentricities, formation and merger timescales). We then derive the binary birth rate from the cosmic star formation history obtained from a numerical study which reproduces the available observations at redshifts $z < 8$. Making a significant step forward to previous calculations, where only the inspiral signal was considered, we include the contribution to the GWB coming from the merging of the two BHs and from the ring-down of the final BH. We find that the GWB from the inspiral phase is characterized by a maximum amplitude in the range $\Omega_{\rm GW} \sim [0.88-1.7]\times 10ˆ{-7}$ at frequencies $[80 - 100]$ Hz; this signal could be detected with a (S/N)$> 100$ by a second generation interferometer, such as Advanced LIGO/VIRGO, with 1-3 years of integration. Third generation detectors, such as the Einstein Telescope, could easily detect the GWB generated by the emission during all the three phases of the evolution. The frequency dependence and amplitude of the GWB generated during the merger and ring-down is very sensitive to the adopted core mass threshold for BH formation. This opens up the possibility to better understand the final stages of the evolution of massive stellar binaries using observational constraints on the associated gravitational wave emission.

#### Apr 14, 2011

1104.2044 (/preprints)
2011-04-14, 21:35 

## [1104.2322] Observable Signatures of EMRI Black Hole Binaries Embedded in Thin Accretion Disks

Authors: Bence Kocsis, Nicolas Yunes, Abraham Loeb

Date: 12 Apr 2011

Abstract: We examine the electromagnetic (EM) and gravitational wave (GW) signatures of stellar-mass compact objects (COs) spiraling into a supermassive black hole (extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure dominated, accretion disk. At large separations, the tidal effect of the secondary CO clears a gap. We show that the gap refills during the late GW-driven phase of the inspiral, leading to a sudden EM brightening of the source. The accretion disk leaves an imprint on the GW through its angular momentum exchange with the binary, the mass increase of the binary members due to accretion, and its gravity. We compute the disk-modified GWs both in an analytical Newtonian approximation and in a numerical effective-one-body approach. We find that disk-induced migration provides the dominant perturbation to the inspiral, with weaker effects from the mass accretion onto the CO and hydrodynamic drag. Depending on whether a gap is present, the perturbation of the GW phase is between 10 and 1000 radians per year, detectable with the future Laser Interferometer Space Antenna (LISA) at high significance. The Fourier transform of the disk-modified GW in the stationary phase approximation is sensitive to disk parameters with a frequency trend different from post-Newtonian vacuum corrections. Our results suggest that observations of EMRIs may place new sensitive constraints on the physics of accretion disks.

#### Apr 14, 2011

1104.2322 (/preprints)
2011-04-14, 21:35 

## [1012.4869] Frame-Dragging Vortexes and Tidal Tendexes Attached to Colliding Black Holes: Visualizing the Curvature of Spacetime

Authors: Robert Owen, Jeandrew Brink, Yanbei Chen, Jeffrey D. Kaplan, Geoffrey Lovelace, Keith D. Matthews, David A. Nichols, Mark A. Scheel, Fan Zhang, Aaron Zimmerman, Kip S. Thorne

Date: 22 Dec 2010

Abstract: When one splits spacetime into space plus time, the spacetime curvature (Weyl tensor) gets split into an "electric" part E_{jk} that describes tidal gravity and a "magnetic" part B_{jk} that describes differential dragging of inertial frames. We introduce tools for visualizing B_{jk} (frame-drag vortex lines, their vorticity, and vortexes) and E_{jk} (tidal tendex lines, their tendicity, and tendexes), and also visualizations of a black-hole horizon's (scalar) vorticity and tendicity. We use these tools to elucidate the nonlinear dynamics of curved spacetime in merging black-hole binaries.

#### Apr 14, 2011

1012.4869 (/preprints)
2011-04-14, 21:35 

## [1104.1183] Gravitational waves from extreme mass-ratio inspirals in Dynamical Chern-Simons gravity

Authors: Paolo Pani, Vitor Cardoso, Leonardo Gualtieri

Date: 6 Apr 2011

Abstract: Dynamical Chern-Simons gravity is an interesting extension of General Relativity, which finds its way in many different contexts, including string theory, cosmological settings and loop quantum gravity. In this theory, the gravitational field is coupled to a scalar field by a parity-violating term, which gives rise to characteristic signatures. Here we investigate how Chern-Simons gravity would affect the quasi-circular inspiralling of a small, stellar-mass object into a large non-rotating supermassive black hole, and the accompanying emission of gravitational and scalar waves. We find the relevant equations describing the perturbation induced by the small object, and we solve them through the use of Green's function techniques. Our results show that for a wide range of coupling parameters, the Chern-Simons coupling gives rise to an increase in total energy flux, which translates into a fewer number of gravitational-wave cycles over a certain bandwidth. For space-based gravitational-wave detectors such as LISA, this effect can be used to constrain the coupling parameter effectively.

#### Apr 12, 2011

1104.1183 (/preprints)
2011-04-12, 14:30 

## [1104.1977] Cosmography with strong lensing of LISA gravitational wave sources

Authors: M. Sereno (POLITO), Ph. Jetzer, A. Sesana, M. Volonteri

Date: 11 Apr 2011

Abstract: LISA might detect gravitational waves from mergers of massive black hole binaries strongly lensed by intervening galaxies (Sereno et al. 2010). The detection of multiple gravitational lensing events would provide a new tool for cosmography. Constraints on cosmological parameters could be placed by exploiting either lensing statistics of strongly lensed sources or time delay measurements of lensed gravitational wave signals. These lensing methods do not need the measurement of the redshifts of the sources and the identification of their electromagnetic counterparts. They would extend cosmological probes to redshift z <= 10 and are then complementary to other lower or higher redshift tests, such as type Ia supernovae or cosmic microwave background. The accuracy of lensing tests strongly depends on the formation history of the merging binaries, and the related number of total detectable multiple images. Lensing amplification might also help to find the host galaxies. Any measurement of the source redshifts would allow to exploit the distance-redshift test in combination with lensing methods. Time-delay analyses might measure the Hubble parameter H_0 with accuracy of >= 10 km sˆ{-1}Mpcˆ{-1}. With prior knowledge of H_0, lensing statistics and time delays might constrain the dark matter density (delta Omega_M >= 0.08, due to parameter degeneracy). Inclusion of our methods with other available orthogonal techniques might significantly reduce the uncertainty contours for Omega_M and the dark energy equation of state.

#### Apr 12, 2011

1104.1977 (/preprints)
2011-04-12, 14:30 

## [1104.1704] Search method for long-duration gravitational-wave transients from neutron stars

Authors: Reinhard Prix, Stefanos Giampanis, Chris Messenger

Date: 9 Apr 2011

Abstract: We introduce a search method for a new class of gravitational-wave signals, namely long-duration O(hours - weeks) transients from spinning neutron stars. We discuss the astrophysical motivation from glitch relaxation models and we derive a rough estimate for the maximal expected signal strength based on the superfluid excess rotational energy. The transient signal model considered here extends the traditional class of infinite-duration continuous-wave signals by a finite start-time and duration. We derive a multi-detector Bayes factor for these signals in Gaussian noise using $\F$-statistic amplitude priors, which simplifies the detection statistic and allows for an efficient implementation. We consider both a fully coherent statistic, which is computationally limited to directed searches for known pulsars, and a cheaper semi-coherent variant, suitable for wide parameter-space searches for transients from unknown neutron stars. We have tested our method by Monte-Carlo simulation, and we find that it outperforms orthodox maximum-likelihood approaches both in sensitivity and in parameter-estimation quality.

#### Apr 12, 2011

1104.1704 (/preprints)
2011-04-12, 14:29 

## [1104.1122] Effective field theory calculation of conservative binary dynamics at third post-Newtonian order

Authors: S. Foffa, R. Sturani

Date: 6 Apr 2011

Abstract: We reproduce the two-body gravitational conservative dynamics at third post-Newtonian order for spin-less sources by using the effective field theory methods for the gravitationally bound two-body system, proposed by Goldberger and Rothstein. This result has been obtained by automatizing the computation of Feynman amplitudes within a Mathematica algorithm, paving the way for higher-order computations not yet performed by traditional methods.

#### Apr 07, 2011

1104.1122 (/preprints)
2011-04-07, 13:25 

## [1104.0387] Systematics of black hole binary inspiral kicks and the slowness approximation

Authors: Richard H. Price, Gaurav Khanna, Scott A. Hughes

Date: 3 Apr 2011

Abstract: During the inspiral and merger of black holes, the interaction of gravitational wave multipoles carries linear momentum away, thereby providing an astrophysically important recoil, or "kick" to the system and to the final black hole remnant. It has been found that linear momentum during the last stage (quasinormal ringing) of the collapse tends to provide an "antikick" that in some cases cancels almost all the kick from the earlier (quasicircular inspiral) emission. We show here that this cancellation is not due to peculiarities of gravitational waves, black holes, or interacting multipoles, but simply to the fact that the rotating flux of momentum changes its intensity slowly. We show furthermore that an understanding of the systematics of the emission allows good estimates of the net kick for numerical simulations started at fairly late times, and is useful for understanding qualitatively what kinds of systems provide large and small net kicks.

#### Apr 06, 2011

1104.0387 (/preprints)
2011-04-06, 12:17 

## [1104.0819] Linearized f(R) Gravity: Gravitational Radiation & Solar System Tests

Authors: Christopher P. L. Berry, Jonathan R. Gair

Date: 5 Apr 2011

Abstract: We investigate the linearized form of metric f(R)-gravity, assuming that f(R) is analytic about R = 0 so it may be expanded as f(R) = R + a_2 Rˆ2/2 + … Gravitational radiation is modified, admitting an extra mode of oscillation, that of the Ricci scalar. We derive an effective energy-momentum tensor for the radiation. We also present weak-field metrics for simple sources. These demonstrate that Kerr (or Schwarzschild) black holes do not exist in f(R)-gravity. We apply the metrics to tests that could constrain f(R). We show that light deflection experiments cannot distinguish f(R)-gravity from general relativity as both have an effective post-Newtonian parameter gamma = 1. We find that planetary precession rates are enhanced relative to general relativity; from the orbit of Mercury we derive the bound |a_2| < 1.2 \times 10ˆ{18} mˆ2. Gravitational wave astronomy may be more useful: considering the phase of a gravitational waveform we estimate deviations from general relativity could be measurable for an extreme-mass-ratio inspiral about a 10ˆ6 M_sol black hole if |a_2| > 10ˆ{17} mˆ2. However Eot-Wash experiments provide the strictest bound |a_2| < 2 \times 10ˆ{-9} mˆ2. Although the astronomical bounds are weaker, they are still of interest in the case that the effective form of f(R) is modified in different regions, perhaps through the chameleon mechanism. Assuming the laboratory bound is universal, we conclude that the propagating Ricci scalar mode cannot be excited by astrophysical sources.

#### Apr 06, 2011

1104.0819 (/preprints)
2011-04-06, 12:16 

## [1104.0385] Formation scenarios and mass-radius relation for neutron stars

Authors: J.L. Zdunik, P. Haensel

Date: 3 Apr 2011

Abstract: Neutron star crust, formed via accretion of matter from a companion in a low-mass X-ray binary (LMXB), has an equation of state (EOS) stiffer than that of catalyzed matter. At a given neutron star mass, M, the radius of a star with an accreted crust is therefore larger, by DR(M), than for usually considered star built of catalyzed matter. Using a compressible liquid drop model of nuclei, we calculate, within the one-component plasma approximation, the EOSs corresponding to different nuclear compositions of ashes of X-ray bursts in LMXB. These EOSs are then applied for studying the effect of different formation scenarios on the neutron-star mass-radius relation. Assuming the SLy EOS for neutron star's liquid core, derived by Douchin & Haensel (2001), we find that at M=1.4 M_sun the star with accreted crust has a radius more than 100 m larger that for the crust of catalyzed matter. Using smallness of the crust mass compared to M, we derive a formula that relates DR(M) to the difference in the crust EOS. This very precise formula gives also analytic dependence of DR on M and R of the reference star built of catalyzed matter. The formula is valid for any EOS of the liquid core. Rotation of neutron star makes DR(M) larger. We derive an approximate but very precise formula that gives difference in equatorial radii, DR_eq(M), as a function of stellar rotation frequency.

#### Apr 06, 2011

1104.0385 (/preprints)
2011-04-06, 12:16 

## [1104.0060] Astrodynamical Space Test of Relativity using Optical Devices I (ASTROD I) - A class-M fundamental physics mission proposal for Cosmic Vision 2015-2025: 2010 Update

Authors: Claus Braxmaier, Hansj&#xf6;rg Dittus, Bernard Foulon, Ertan G&#xf6;kl&#xfc;, Catia Grimani, Jian Guo, Sven Herrmann, Claus L&#xe4;mmerzahl, Wei-Tou Ni, Achim Peters, Benny Rievers, &#xc9;tienne Samain, Hanns Selig, Diana Shaul, Drazen Svehla, Pierre Touboul, Gang Wang, An-Ming Wu, Alexander F. Zakharov

Date: 1 Apr 2011

Abstract: This paper on ASTROD I is based on our 2010 proposal submitted for the ESA call for class-M mission proposals, and is a sequel and an update to our previous paper [Experimental Astronomy 23 (2009) 491-527; designated as Paper I] which was based on our last proposal submitted for the 2007 ESA call. In this paper, we present our orbit selection with one Venus swing-by together with orbit simulation. In Paper I, our orbit choice is with two Venus swing-bys. The present choice takes shorter time (about 250 days) to reach the opposite side of the Sun. We also present a preliminary design of the optical bench, and elaborate on the solar physics goals with the radiation monitor payload. We discuss telescope size, trade-offs of drag-free sensitivities, thermal issues and present an outlook. ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test General Relativity with an improvement in sensitivity of over 3 orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system; and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals.

#### Apr 04, 2011

1104.0060 (/preprints)
2011-04-04, 10:08 

## [1103.5867] Prospects of observing continuous gravitational waves from known pulsars

Authors: Matthew Pitkin

Date: 30 Mar 2011

Abstract: Several searches for gravitational waves from a selection of known pulsars have been performed with data from the science runs of the LIGO gravitational wave detectors. So far these have lead to no detection, but upper limits on the gravitational wave amplitudes have been set. Here we study our intrinsic ability to detect, and estimate the gravitational wave amplitude for non-accreting pulsars. Using spin-down limits on emission as a guide we examine amplitudes that would be required to observe known pulsars with future detectors (Advanced LIGO, Advanced Virgo and the Einstein Telescope), assuming that they are triaxial stars emitting at precisely twice the known rotation frequency. Maximum allowed amplitudes depend on the stars' equation of state (e.g. a normal neutron star, a quark star, a hybrid star) and the theoretical mass quadrupoles that they can sustain. We study what range of quadrupoles, and therefore equations of state, would be consistent with being able to detect these sources. For globular cluster pulsars, with spin-downs masked by accelerations within the cluster, we examine what spin-down values gravitational wave observations would be able to set. For all pulsars we also alternatively examine what internal magnetic fields they would need to sustain observable ellipticities.

#### Apr 02, 2011

1103.5867 (/preprints)
2011-04-02, 22:33 

## [1103.6041] Accuracy of the post-Newtonian approximation. II. Optimal asymptotic expansion of the energy flux for quasicircular, extreme mass-ratio inspirals into a Kerr black hole

Authors: Zhongyang Zhang, Nicolas Yunes, Emanuele Berti

Date: 30 Mar 2011

Abstract: We study the effect of black hole spin on the accuracy of the post-Newtonian approximation. We focus on the gravitational energy flux for the quasicircular, equatorial, extreme mass-ratio inspiral of a compact object into a Kerr black hole of mass M and spin J. For a given dimensionless spin a=J/Mˆ2 (in geometrical units), the energy flux depends only on the orbital velocity v or (equivalently) on the Boyer-Lindquist orbital radius r. We investigate the formal region of validity of the Taylor post-Newtonian expansion of the energy flux (which is known up to order vˆ8 beyond the quadrupole formula), generalizing previous work by two of us. The "error function" used to determine the region of validity of the post-Newtonian expansion can have two qualitatively different kinds of behavior, and we deal with these two cases separately. We find that, at any fixed post-Newtonian order, the edge of the region of validity (as measured by v/v_{ISCO}, where v_{ISCO} is the orbital velocity at the innermost stable circular orbit) is only weakly dependent on a. Unlike in the nonspinning case, the lack of sufficiently high order terms does not allow us to determine if there is a convergent to divergent transition at order vˆ6. Independently of a, the inclusion of angular multipoles up to and including l=5 in the numerical flux is necessary to achieve the level of accuracy of the best-known (N=8) PN expansion of the energy flux.

#### Apr 02, 2011

1103.6041 (/preprints)
2011-04-02, 22:26 

## [1103.5587] Status of Horava gravity: A personal perspective

Authors: Matt Visser (Victoria University of Wellington)

Date: 29 Mar 2011

Abstract: Horava gravity is a relatively recent (Jan 2009) idea in theoretical physics for trying to develop a quantum field theory of gravity. It is not a string theory, nor loop quantum gravity, but is instead a traditional quantum field theory that breaks Lorentz invariance at ultra-high (presumably trans-Planckian) energies, while retaining approximate Lorentz invariance at low and medium (sub-Planckian) energies. The challenge is to keep the Lorentz symmetry breaking controlled and small - small enough to be compatible with experiment. I will give a very general overview of what is going on in this field, paying particular attention to the disturbing role of the scalar graviton.

#### Mar 30, 2011

1103.5587 (/preprints)
2011-03-30, 08:04 

## [1103.5446] Stellar Dynamics and Black Holes

Authors: David Merritt

Date: 28 Mar 2011

Abstract: Chandrasekhar's most important contribution to stellar dynamics was the concept of dynamical friction. I briefly review that work, then discuss some implications of Chandrasekhar's theory of gravitational encounters for motion in galactic nuclei.

#### Mar 28, 2011

1103.5446 (/preprints)
2011-03-28, 22:47 

## [1103.5135] Evolution of the spin parameter of accreting compact objects with non-Kerr quadrupole moment

Authors: Cosimo Bambi

Date: 26 Mar 2011

Abstract: There is robust observational evidence supporting the existence of $5 - 20$ $M_\odot$ compact bodies in X-ray binary systems and of $10ˆ5 - 10ˆ9$ $M_\odot$ bodies at the center of many galaxies. All these objects are commonly interpreted as black holes, even is there is no direct evidence that they have an event horizon. A fundamental limit for a black hole in 4-dimensional general relativity is the Kerr bound $|a_*| \le 1$, where $a_*$ is the spin parameter. This is just the condition for the existence of the event horizon. The accretion process can spin a black hole up to $a_* \approx 0.998$ and some super-massive objects in galactic nuclei could be rapidly rotating black holes with spin parameter close to this limit. However, if these super-massive objects are not black holes, the Kerr bound does not hold and the accretion process can spin them up to $a_* > 1$. In this paper, I consider compact bodies with non-Kerr quadrupole moment. I study the evolution of the spin parameter due to accretion and I find its equilibrium value. Future experiments like the gravitational wave detector LISA will be able to test if the super-massive objects at the center of galaxies are the black holes predicted by general relativity. If they are not black holes, some of them may be super-spinning objects with $a_* > 1$.

#### Mar 28, 2011

1103.5135 (/preprints)
2011-03-28, 22:47 

## [1103.5088] The uncertainty in hybrid gravitational waveforms: Optimizing initial orbital frequencies for binary black-hole simulations

Authors: Michael Boyle

Date: 25 Mar 2011

Abstract: A general method is presented for estimating the uncertainty in hybrid models of gravitational waveforms from binary black-hole systems with arbitrary physical parameters, and thence the highest allowable initial orbital frequency for a numerical-relativity simulation such that the combined analytical and numerical waveform meets some minimum desired accuracy. The key strength of this estimate is that no prior numerical simulation in the relevant region of parameter space is needed. The method is demonstrated for a selection of extreme physical parameters. It is shown that optimal initial orbital frequencies depend roughly linearly on the mass of the binary, and therefore useful accuracy criteria must depend on the mass. The results indicate that accurate estimation of the parameters of stellar-mass black-hole binaries in Advanced LIGO data or calibration of waveforms for detection will require much longer numerical simulations than are currently available or more accurate post-Newtonian approximations -- or both -- especially for comparable-mass systems with high spin.

#### Mar 28, 2011

1103.5088 (/preprints)
2011-03-28, 22:46 

## [1103.4609] Imprint of Accretion Disk-Induced Migration on Gravitational Waves from Extreme Mass Ratio Inspirals

Authors: Nicol&#xe1;s Yunes, Bence Kocsis, Abraham Loeb, Zolt&#xe1;n Haiman

Date: 23 Mar 2011

Abstract: We study the effects of a thin gaseous accretion disk on the inspiral of a stellar--mass black hole into a supermassive black hole. We construct a phenomenological angular momentum transport equation that reproduces known disk effects. Disk torques modify the gravitational wave phase evolution to detectable levels with LISA for reasonable disk parameters. The Fourier transform of disk-modified waveforms acquires a correction with a different frequency trend than post-Newtonian vacuum terms. Such inspirals could be used to detect accretion disks with LISA and to probe their physical parameters.

#### Mar 24, 2011

1103.4609 (/preprints)
2011-03-24, 22:06 

## [1103.3526] Will black hole-neutron star binary inspirals tell us about the neutron star equation of state?

Authors: Francesco Pannarale, Luciano Rezzolla, Frank Ohme, Jocelyn S. Read

Date: 17 Mar 2011

Abstract: The strong tidal forces that arise during the last stages of the life of a black hole-neutron star binary may severely distort, and possibly disrupt, the star. Both phenomena will imprint signatures about the stellar structure in the emitted gravitational radiation. The information from the disruption, however, is confined to very high frequencies, where detectors are not very sensitive. We thus assess whether the lack of tidal distortion corrections in data-analysis pipelines will affect the detection of the inspiral part of the signal and whether these may yield information on the equation of state of matter at nuclear densities. Using recent post-Newtonian expressions and realistic equations of state to model these scenarios, we find that point-particle templates are sufficient for the detection of black hole-neutron star inspiralling binaries, with a loss of signals below 1% for both second and third-generation detectors. Such detections may be able to constrain particularly stiff equations of state, but will be unable to reveal the presence of a neutron star with a soft equation of state.

#### Mar 24, 2011

1103.3526 (/preprints)
2011-03-24, 16:33 

## [1103.4215] Testing feasibility of scalar-tensor gravity by scale dependent mass and coupling to matter

Authors: D. F. Mota, V. Salzano, S. Capozziello

Date: 22 Mar 2011

Abstract: We investigate whether there are any cosmological evidences for a scalar field with a mass and cou- pling to matter which change accordingly to the properties of the astrophysical system it "lives in", without directly focusing on the underlying mechanism that drives the scalar field scale-dependent- properties. We assume a Yukawa type of coupling between the field and matter and also that the scalar field mass grows with density, in order to overcome all gravity constraints within the solar system. We analyse three different gravitational systems assumed as "cosmological indicators": su- pernovae type Ia, low surface brightness spiral galaxies and clusters of galaxies. Results show that: a. a quite good fit to the rotation curves of low surface brightness galaxies only using visible stellar and gas mass components is obtained; b. a scalar field can fairly well reproduce the matter profile in clusters of galaxies, estimated by X-ray observations and without the need of any additional dark matter; c. there is an intrinsic difficulty in extracting information about the possibility of a scale-dependent massive scalar field (or more generally about a varying gravitational constant) from supernovae type Ia.

#### Mar 24, 2011

1103.4215 (/preprints)
2011-03-24, 16:33 

## [1103.3874] Accurate numerical simulations of inspiralling binary neutron stars and their comparison with effective-one-body analytical models

Authors: Luca Baiotti, Thibault Damour, Bruno Giacomazzo, Alessandro Nagar, Luciano Rezzolla

Date: 20 Mar 2011

Abstract: Binary neutron-star systems represent one of the most promising sources of gravitational waves. In order to be able to extract important information, notably about the equation of state of matter at nuclear density, it is necessary to have in hands an accurate analytical model of the expected waveforms. Following our recent work, we here analyze more in detail two general-relativistic simulations spanning about 20 gravitational-wave cycles of the inspiral of equal-mass binary neutron stars with different compactnesses, and compare them with a tidal extension of the effective-one-body (EOB) analytical model. The latter tidally extended EOB model is analytically complete up to the 1.5 post-Newtonian level, and contains an analytically undetermined parameter representing a higher-order amplification of tidal effects. We find that, by calibrating this single parameter, the EOB model can reproduce, within the numerical error, the two numerical waveforms essentially up to the merger. By contrast, analytical models (either EOB, or Taylor-T4) that do not incorporate such a higher-order amplification of tidal effects, build a dephasing with respect to the numerical waveforms of several radians.

#### Mar 22, 2011

1103.3874 (/preprints)
2011-03-22, 14:06 

## [1103.2987] Bayesian versus frequentist upper limits

Authors: Christian R&#xf6;ver, Chris Messenger, Reinhard Prix

Date: 15 Mar 2011

Abstract: While gravitational waves have not yet been measured directly, data analysis from detection experiments commonly includes an upper limit statement. Such upper limits may be derived via a frequentist or Bayesian approach; the theoretical implications are very different, and on the technical side, one notable difference is that one case requires maximization of the likelihood function over parameter space, while the other requires integration. Using a simple example (detection of a sinusoidal signal in white Gaussian noise), we investigate the differences in performance and interpretation, and the effect of the "trials factor", or "look-elsewhere effect".

#### Mar 17, 2011

1103.2987 (/preprints)
2011-03-17, 21:20 

## [1103.2808] Constraining the coalescence rate of supermassive black-hole binaries using pulsar timing

Authors: Z. L. Wen, F. A. Jenet, D. Yardley, G. B. Hobbs, R. N. Manchester

Date: 14 Mar 2011

Abstract: Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift. In contrast to the indirect constraints obtained from other techniques, pulsar timing observations provide a direct constraint on the black-hole merger rate. This is possible since pulsar timing is sensitive to the gravitational waves (GWs) emitted by these sources in the final stages of their evolution. We find that upper bounds calculated from the recently published Parkes Pulsar Timing Array data are just above theoretical predictions for redshifts below 10. In the future, with improved timing precision and longer data spans, we show that a non-detection of GWs will rule out some of the available parameter space in a particular class of SMBH binary merger models. We also show that if we can time a set of pulsars to 10ns timing accuracy, for example, using the proposed Square Kilometre Array, it should be possible to detect one or more individual SMBH binary systems.

#### Mar 16, 2011

1103.2808 (/preprints)
2011-03-16, 09:49 

## [1103.2728] The Advanced LIGO Gravitational Wave Detector

Authors: S. J. Waldman (the LIGO Scientific Collaboration)

Date: 14 Mar 2011

Abstract: The Advanced LIGO gravitational wave detectors will be installed starting in 2011, with completion scheduled for 2015. The new detectors will improve the strain sensitivity of current instruments by a factor of ten, with a thousandfold increase in the observable volume of space. Here we describe the design and limiting noise sources of these second generation, long-baseline, laser interferometers.

#### Mar 16, 2011

1103.2728 (/preprints)
2011-03-16, 09:47 

## [0911.1777] Computing the Bayesian Factor from a Markov chain Monte Carlo Simulation of the Posterior Distribution

Authors: Martin D. Weinberg

Date: 9 Nov 2009

Abstract: Computation of the marginal likelihood from a simulated posterior distribution is central to Bayesian model selection but is computationally difficult. I argue that the marginal likelihood can be reliably computed from a posterior sample by careful attention to the numerics of the probability integral. Posing the expression for the marginal likelihood as a Lebesgue integral, we may convert the harmonic mean approximation from a sample statistic to a quadrature rule. As a quadrature, the harmonic mean approximation suffers from enormous truncation error as consequence . In addition, I demonstrate that the integral expression for the harmonic-mean approximation converges slowly at best for high-dimensional problems with uninformative prior distributions. These observations lead to two computationally-modest families of quadrature algorithms that use the full generality sample posterior but without the instability. The first algorithm automatically eliminates the part of the sample that contributes large truncation error. The second algorithm uses the posterior sample to assign probability to a partition of the sample space and performs the marginal likelihood integral directly. This eliminates convergence issues. The first algorithm is analogous to standard quadrature but can only be applied for convergent problems. The second is a hybrid of cubature: it uses the posterior to discover and tessellate the subset of that sample space was explored and uses quantiles to compute a representive field value. Neither algorithm makes strong assumptions about the shape of the posterior distribution and neither is sensitive outliers. [abridged]

#### Mar 14, 2011

0911.1777 (/preprints)
2011-03-14, 15:04 

## [1103.2149] Time-domain modelling of Extreme-Mass-Ratio Inspirals for the Laser Interferometer Space Antenna

Authors: Priscilla Canizares, Carlos F. Sopuerta

Date: 10 Mar 2011

Abstract: When a stellar-mass compact object is captured by a supermassive black hole located in a galactic centre, the system losses energy and angular momentum by the emission of gravitational waves. Subsequently, the stellar compact object evolves inspiraling until plunging onto the massive black hole. These EMRI systems are expected to be one of the main sources of gravitational waves for the future space-based Laser Interferometer Space Antenna (LISA). However, the detection of EMRI signals will require of very accurate theoretical templates taking into account the gravitational self-force, which is the responsible of the stellar-compact object inspiral. Due to its potential applicability on EMRIs, the obtention of an efficient method to compute the scalar self-force acting on a point-like particle orbiting around a massive black hole is being object of increasing interest. We present here a review of our time-domain numerical technique to compute the self-force acting on a point-like particle and we show its suitability to deal with both circular and eccentric orbits.

#### Mar 13, 2011

1103.2149 (/preprints)
2011-03-13, 18:23 

## [1103.2271] Introduction to Early Universe Cosmology

Authors: Robert H. Brandenberger (McGill University)

Date: 11 Mar 2011

Abstract: Observational cosmology is in its "golden age" with a vast amount of recent data on the distribution of matter and light in the universe. This data can be used to probe theories of the very early universe. It is small amplitude cosmological fluctuations which encode the information about the very early universe and relate it to current data. Hence, a central topic in these lectures is the "theory of cosmological perturbations", the theory which describes the generation of inhomogeneities in the very early universe and their evolution until the current time. I will apply this theory to three classes of models of the very early universe. The first is "Inflationary Cosmology", the current paradigm for understanding the early evolution of the universe. I will review the successes of inflationary cosmology, but will also focus on some conceptual challenges which inflationary cosmology is facing, challenges which motivate the search for possible alternatives. I will introduce two alternative scenarios, the "Matter Bounce" model and "String Gas Cosmology", and I will discuss how cosmological fluctuations which can explain the current data are generated in those models.

#### Mar 13, 2011

1103.2271 (/preprints)
2011-03-13, 18:22 

## [1103.0576] Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data

Authors: R. van Haasteren, Y. Levin, G.H. Janssen, K. Lazaridis, M. Kramer B.W. Stappers, G. Desvignes, M.B. Purver, A.G. Lyne, R.D. Ferdman, A. Jessner, I. Cognard, G. Theureau, N. D&#x27;Amico, A. Possenti, M. Burgay, A. Corongiu, J.W.T. Hessels, R. Smits, J.P.W. Verbiest

Date: 2 Mar 2011

Abstract: Direct detection of low-frequency gravitational waves ($10ˆ{-9} - 10ˆ{-8}$ Hz) is the main goal of pulsar timing array (PTA) projects. One of the main targets for the PTAs is to measure the stochastic background of gravitational waves (GWB) whose characteristic strain is expected to approximately follow a power-law of the form $h_c(f)=A (f/\hbox{yr}ˆ{-1})ˆ{\alpha}$, where $f$ is the gravitational-wave frequency. In this paper we use the current data from the European PTA to determine an upper limit on the GWB amplitude $A$ as a function of the unknown spectral slope $\alpha$ with a Bayesian algorithm, by modelling the GWB as a random Gaussian process. For the case $\alpha=-2/3$, which is expected if the GWB is produced by supermassive black-hole binaries, we obtain a 95% confidence upper limit on $A$ of $6\times 10ˆ{-15}$, which is 1.8 times lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our approach to the data analysis incorporates the multi-telescope nature of the European PTA and thus can serve as a useful template for future intercontinental PTA collaborations.

#### Mar 09, 2011

1103.0576 (/preprints)
2011-03-09, 23:08 

## [1103.1608] New Symbolic Tools for Differential Geometry, Gravitation, and Field Theory

Authors: I. M. Anderson, C. G. Torre

Date: 8 Mar 2011

Abstract: DifferentialGeometry is a Maple software package which symbolically performs fundamental operations of calculus on manifolds, differential geometry, tensor calculus, Lie algebras, Lie groups, transformation groups, jet spaces, and the variational calculus. These capabilities, combined with dramatic recent improvements in symbolic approaches to solving algebraic and differential equations, have allowed for development of powerful new tools for solving research problems in gravitation and field theory. The purpose of this paper is to describe some of these new tools and present some advanced applications involving: Killing vector fields and isometry groups, Killing tensors and other tensorial invariants, algebraic classification of curvature, and symmetry reduction of field equations.

#### Mar 09, 2011

1103.1608 (/preprints)
2011-03-09, 23:08 

## [1103.1301] An all-sky search algorithm for continuous gravitational waves from spinning neutron stars in binary systems

Authors: E. Goetz, K. Riles

Date: 7 Mar 2011

Abstract: Rapidly spinning neutron stars with non-axisymmetric mass distributions are expected to generate quasi-monochromatic continuous gravitational waves. While search methods for unknown isolated spinning neutron stars are approaching maturity, there have been no previous searches for unknown sources in binary systems. Since current search methods for unknown, isolated neutron stars are already computationally limited, expanding the parameter space searched to include binary systems is a formidable challenge. We present a new hierarchical binary search method called TwoSpect, which exploits the periodic orbital modulations of the continuous waves by searching for patterns in doubly Fourier-transformed data. We will describe the TwoSpect search pipeline, including its mitigation of detector noise variations and corrections for Doppler frequency modulation caused by changing detector velocity. Tests on Gaussian noise and on a set of simulated signals will be presented.

#### Mar 08, 2011

1103.1301 (/preprints)
2011-03-08, 09:42 

## [1103.0983] Effective Field Theory Methods in Gravitational Physics and Tests of Gravity

Authors: Umberto Cannella

Date: 5 Mar 2011

Abstract: In this PhD thesis I make use of the effective field theory approach to General Relativity to investigate theories of gravity and to take a different point of view on the physical information that can be extracted from experiments. In the first work I present, I study a scalar-tensor theory of gravity and I address the renormalization of the energy-momentum tensor for point-like and string-like sources. The second and third study I report are set in the context of testing gravity. So far experiments have tested dynamical regimes only up to order (v/c)ˆ5 in the post-Newtonian expansion, i.e. the very first term of the radiative sector in General Relativity. In contrast, by means of gravitational-wave astronomy, one aims at testing General Relativity up to (v/c)ˆ(12)! It is then relevant to envisage testing frameworks which are appropriate to this strong-field/radiative regime. In the last two chapters of this thesis a new such framework is presented. Using the effective field theory approach, General Relativity non-linearities are described by Feynman diagrams in which classical gravitons interact with matter sources and among themselves. Tagging the self-interaction vertices of gravitons with parameters it is possible, for example, to translate the measure of the period decay of Hulse-Taylor pulsar in a constraint on the three-graviton vertex at the 0.1% level; for comparison, LEP constraints on the triple-gauge-boson couplings of weak interactions are accurate at 3%. With future observations of gravitational waves, higher order graviton vertices can in principle be constrained through a Fisher matrix analysis.

#### Mar 08, 2011

1103.0983 (/preprints)
2011-03-08, 09:42 

## [1103.0768] Gravitational power from cosmic string loops with many kinks

Authors: Alejandro Bohe

Date: 3 Mar 2011

Abstract: We investigate the effect of a large number of kinks on the gravitational power radiated by cosmic string loops. We show that the total power radiated by a loop with N left-moving and right-moving kinks is proportional to N and increases with the typical kink angle. We then apply these results to loops containing junctions which give rise to a proliferation of the number of sharp kinks. We show that the time of gravitational decay of these loops is smaller than previously assumed. In light of this we revisit the gravitational wave burst predictions from a network containing such loops. We find there is no parameter regime in which the rate of individual kink bursts is enhanced with respect to standard networks. By contrast, there remains a region of parameter space for which the kink-kink bursts dominate the stochastic background. Finally, we discuss the order of magnitude of the typical number of sharp kinks resulting from kink proliferation on loops with junctions.

#### Mar 03, 2011

1103.0768 (/preprints)
2011-03-03, 20:45 

## [1103.0287] Self force on a scalar charge in Kerr spacetime: eccentric equatorial orbits

Authors: Niels Warburton, Leor Barack

Date: 1 Mar 2011

Abstract: We present a numerical code for calculating the self force on a scalar charge moving in a bound (eccentric) geodesic in the equatorial plane of a Kerr black hole. We work in the frequency domain and make use of the method of extended homogeneous solutions [Phys.\ Rev.\ D {\bf 78}, 084021 (2008)], in conjunction with mode-sum regularization. Our work is part of a program to develop a computational architecture for fast and efficient self-force calculations, alternative to time-domain methods. We find that our frequency-domain method outperforms existing time-domain schemes for small eccentricities, and, remarkably, remains competitive up to eccentricities as high as $\sim 0.7$. As an application of our code we (i) compute the conservative scalar-field self-force correction to the innermost stable circular equatorial orbit, as a function of the Kerr spin parameter; and (ii) calculate the variation in the rest mass of the scalar particle along the orbit, caused by the component of the self force tangent to the four-velocity.

#### Mar 02, 2011

1103.0287 (/preprints)
2011-03-02, 23:59 

## [1103.0282] Model Independent Tests of Cosmic Gravity

Authors: Eric V. Linder

Date: 1 Mar 2011

Abstract: Gravitation governs the expansion and fate of the universe, and the growth of large scale structure within it, but has not been tested in detail on these cosmic scales. The observed acceleration of the expansion may provide signs of gravitational laws beyond general relativity. Since the form of any such extension is not clear, from either theory or data, we adopt a model independent approach to parametrising deviations to the Einstein framework. We explore the phase space dynamics of two key post-GR functions and derive a classification scheme and an absolute criterion on accuracy necessary for distinguishing classes of gravity models. Future surveys will be able to constrain the post-GR functions' amplitudes and forms to the required precision, and hence reveal new aspects of gravitation.

#### Mar 02, 2011

1103.0282 (/preprints)
2011-03-02, 23:59 

## [1103.0518] A Bayesian parameter estimation approach to pulsar time-of-arrival analysis

Authors: C. Messenger, A. Lommen, P. Demorest, S. Ransom

Date: 2 Mar 2011

Abstract: The increasing sensitivities of pulsar timing arrays to ultra-low frequency (nHz) gravitational waves promises to achieve direct gravitational wave detection within the next 5-10 years. While there are many parallel efforts being made in the improvement of telescope sensitivity, the detection of stable millisecond pulsars and the improvement of the timing software, there are reasons to believe that the methods used to accurately determine the time-of-arrival (TOA) of pulses from radio pulsars can be improved upon. More specifically, the determination of the uncertainties on these TOAs, which strongly affect the ability to detect GWs through pulsar timing, may be unreliable. We propose two Bayesian methods for the generation of pulsar TOAs starting from pulsar "search-mode" data and pre-folded data. These methods are applied to simulated toy-model examples and in this initial work we focus on the issue of uncertainties in the folding period. The final results of our analysis are expressed in the form of posterior probability distributions on the signal parameters (including the TOA) from a single observation.

#### Mar 02, 2011

1103.0518 (/preprints)
2011-03-02, 23:58 

## [1103.0115] Gravitational wave astronomy of single sources with a pulsar timing array

Authors: K. J. Lee, N. Wex, M. Kramer, B. W. Stappers, C. G. Bassa, G. H. Janssen, R.Karuppusamy, R. Smits

Date: 1 Mar 2011

Abstract: Abbreviated:
We investigate the potential of detecting the gravitational wave from individual binary black hole systems using pulsar timing arrays (PTAs) and calculate the accuracy for determining the GW properties. This is done in a consistent analysis, which at the same time accounts for the measurement of the pulsar distances via the timing parallax.
We find that, at low redshift, a PTA is able to detect the nano-Hertz GW from super massive black hole binary systems with masses of $\sim10ˆ8 - 10ˆ{10}\,M_{\sun}$ less than $\sim10ˆ5$\,years before the final merger, and those with less than $\sim10ˆ3 - 10ˆ4$ years before merger may allow us to detect the evolution of binaries.
We derive an analytical expression to describe the accuracy of a pulsar distance measurement via timing parallax. We consider five years of bi-weekly observations at a precision of 15\,ns for close-by ($\sim 0.5 - 1$\,kpc) pulsars. Timing twenty pulsars would allow us to detect a GW source with an amplitude larger than $5\times 10ˆ{-17}$. We calculate the corresponding GW and binary orbital parameters and their measurement precision. The accuracy of measuring the binary orbital inclination angle, the sky position, and the GW frequency are calculated as functions of the GW amplitude. We note that the "pulsar term", which is commonly regarded as noise, is essential for obtaining an accurate measurement for the GW source location.
We also show that utilizing the information encoded in the GW signal passing the Earth also increases the accuracy of pulsar distance measurements. If the gravitational wave is strong enough, one can achieve sub-parsec distance measurements for nearby pulsars with distance less than $\sim 0.5 - 1$\,kpc.

#### Mar 01, 2011

1103.0115 (/preprints)
2011-03-01, 23:29 

## [1102.4830] Magnetar Asteroseismology with Long-Term Gravitational Waves

Authors: Kazumi Kashiyama, Kunihito Ioka

Date: 23 Feb 2011

Abstract: Magnetic flares and induced oscillations of magnetars (super-magnetized neutron stars) are promising sources of gravitational waves (GWs). We suggest that the GW emission, if any, would last longer than the observed X-ray quasi-periodic oscillations (X-QPOs), calling for the longer-term GW analyses from a day to months than the current searches. Like the pulsar timing, the oscillation frequency would also evolve with time because of the decay or reconfiguration of magnetic field, which is crucial for the GW detection. With the observed GW frequency and its time-derivatives, we can probe the interior magnetic field strength and its evolution to open a new GW asteroseismology with the next generation interferometers like advanced LIGO, advanced VIRGO, LCGT and ET.

#### Feb 28, 2011

1102.4830 (/preprints)
2011-02-28, 23:28 

## [1102.4855] Fast coalescence of massive black hole binaries from mergers of galactic nuclei: implications for low-frequency gravitational-wave astrophysics

Authors: Miguel Preto, Ingo Berentzen, Peter Berczik, Rainer Spurzem

Date: 23 Feb 2011

Abstract: We investigate a purely stellar dynamical solution to the Final Parsec Problem. Galactic nuclei resulting from major mergers are not spherical, but show some degree of triaxiality. With $N$-body simulations, we show that massive black hole binaries (MBHB) hosted by them will continuously interact with stars on centrophilic orbits and will thus inspiral — in much less than a Hubble time — down to separations at which gravitational wave (GW) emission is strong enough to drive them to coalescence. Such coalescences will be important sources of GWs for future space-borne detectors such as the {\it Laser Interferometer Space Antenna} (LISA). Based on our results, we expect that LISA will see between $\sim 10$ to $\sim {\rm few} \times 10ˆ2$ such events every year, depending on the particular MBH seed model as obtained in recent studies of merger trees of galaxy and MBH co-evolution. Orbital eccentricities in the LISA band will be clearly distinguishable from zero with $e \gtrsim 0.001-0.01$.

#### Feb 28, 2011

1102.4855 (/preprints)
2011-02-28, 23:28 

## [1102.4965] Numerical simulation of time delay interferometry for LISA with one arm dysfunctional

Authors: S. V. Dhurandhar, W.-T. Ni, G. Wang

Date: 24 Feb 2011

Abstract: In order to attain the requisite sensitivity for LISA, laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. In a previous paper (Dhurandhar et al., Class. Quantum Grav., 27, 135013, 2010), we have found a large family of second generation analytic solutions of time delay interferometry with one arm dysfunctional and also estimated the laser noise due to residual time-delay semi-analytically from orbit perturbations due to Earth. Since other planets and solar-system bodies also perturb the orbits of LISA spacecraft and affect the time delay interferometry (TDI), we simulate the time delay numerically in this paper for all solutions with n \leq 3. To conform to the actual LISA planning, we have worked out a set of 3-year optimized mission orbits of LISA spacecraft starting at June 21, 2021 using CGC2.7 ephemeris framework. We then use this numerical solution to calculate the residual optical path differences in the second generation solutions of our previous paper, and compare with the semi-analytic error estimate. The accuracy of this calculation is better than 1 cm (or 30 ps). The maximum path length difference, for all configuration calculated, is below 1 m (3 ns). This is well below the limit under which the laser frequency noise is required to be suppressed.

#### Feb 28, 2011

1102.4965 (/preprints)
2011-02-28, 23:28 

## [1102.5646] Evidence for gravitational quadrupole moment variations in the companion of PSR J2051-0827

Authors: K. Lazaridis, J.P.W. Verbiest, T.M. Tauris, B.W. Stappers, M. Kramer, N. Wex, A. Jessner, I. Cognard, G. Desvignes, G.H. Janssen, M.B. Purver, G. Theureau, C.G. Bassa, R. Smits

Date: 28 Feb 2011

Abstract: We have conducted radio timing observations of the eclipsing millisecond binary pulsar J2051-0827 with the European Pulsar Timing Array network of telescopes and the Parkes radio telescope, spanning over 13 years. The increased data span allows significant measurements of the orbital eccentricity, e = (6.2 {\pm} 1.3) {\times} 10ˆ{-5} and composite proper motion, {\mu}_t = 7.3 {\pm} 0.4 mas/yr. Our timing observations have revealed secular variations of the projected semi-major axis of the pulsar orbit which are much more extreme than those previously published; and of the orbital period of the system. Investigation of the physical mechanisms producing such variations confirm that the variations of the semi-major axis are most probably caused by classical spin-orbit coupling in the binary system, while the variations in orbital period are most likely caused by tidal dissipation leading to changes in the gravitational quadrupole moment of the companion.

#### Feb 28, 2011

1102.5646 (/preprints)
2011-02-28, 23:27 

## [1102.5470] Observability of pulsar beam bending by the Sgr~A* black hole

Authors: Kevin Stovall, Teviet Creighton, Richard H. Price, Fredrick A. Jenet

Date: 27 Feb 2011

Abstract: According to some models, there may be a significant population of radio pulsars in the Galactic center. In principle, a beam from one of these pulsars could pass close to the supermassive black hole (SMBH) at the center, be deflected, and be detected by Earth telescopes. Such a configuration would be an unprecedented probe of the properties of spacetime in the moderate- to strong-field regime of the SMBH. We present here background on the problem, and approximations for the probability of detection of such beams. We conclude that detection is marginally probable with current telescopes, but that telescopes that will be operating in the near future, with an appropriate multiyear observational program, will have a good chance of detecting a beam deflected by the SMBH.

#### Feb 28, 2011

1102.5470 (/preprints)
2011-02-28, 23:27 

## [1102.5340] Arecibo PALFA Survey and Einstein@Home: Binary Pulsar Discovery by Volunteer Computing

Authors: B. Knispel, P. Lazarus, B. Allen, D. Anderson, C. Aulbert, N. D. R. Bhat, O. Bock, S. Bogdanov, A. Brazier, F. Camilo, S. Chatterjee, J. M. Cordes, F. Crawford, J. S. Deneva, G. Desvignes, H. Fehrmann, P. C. C. Freire, D. Hammer, J. W. T. Hessels, F. A. Jenet, V. M. Kaspi, M. Kramer, J. van Leeuwen, D. R. Lorimer, A. G. Lyne, B. Machenschalk, M. A. McLaughlin, C. Messenger, D. J. Nice, M. A. Papa, H. J. Pletsch, R. Prix, S. M. Ransom, X. Siemens, I. H. Stairs, B. W. Stappers, K. Stovall, A. Venkataraman

Date: 25 Feb 2011

Abstract: We report the discovery of the 20.7-ms binary pulsar J1952+2630, made using the distributed computing project Einstein@Home in Pulsar ALFA survey observations with the Arecibo telescope. Follow-up observations with the Arecibo telescope confirm the binary nature of the system. We obtain a circular orbital solution with an orbital period of 9.4 hr, a projected orbital radius of 2.8 lt-s, and a mass function of f = 0.15 solar masses by analysis of spin period measurements. No evidence of orbital eccentricity is apparent; we set a 2-sigma upper limit e < 1.7e-3. The orbital parameters suggest a massive white dwarf companion with a minimum mass of 0.95 solar masses, assuming a pulsar mass of 1.4 solar masses. Most likely, this pulsar belongs to the rare class of intermediate mass binary pulsars. Future timing observations will aim to determine the parameters of this system further, measure relativistic effects, and elucidate the nature of the companion star.

#### Feb 28, 2011

1102.5340 (/preprints)
2011-02-28, 23:27 

## [1102.5192] On the unreasonable effectiveness of the post-Newtonian approximation in gravitational physics

Authors: Clifford M. Will

Date: 25 Feb 2011

Abstract: The post-Newtonian approximation is a method for solving Einstein's field equations for physical systems in which motions are slow compared to the speed of light and where gravitational fields are weak. Yet it has proven to be remarkably effective in describing certain strong-field, fast-motion systems, including binary pulsars containing dense neutron stars and binary black hole systems inspiraling toward a final merger. The reasons for this effectiveness are largely unknown. When carried to high orders in the post-Newtonian sequence, predictions for the gravitational-wave signal from inspiraling compact binaries will play a key role in gravitational-wave detection by laser-interferometric observatories.

#### Feb 28, 2011

1102.5192 (/preprints)
2011-02-28, 23:12 

## [1102.5332] Binary Inspirals in Nordstr\"om's Second Theory

Authors: Travis Garrett

Date: 25 Feb 2011

Abstract: We investigate Nordstr\"om's second theory of gravitation, with a focus on utilizing it as a testbed for developing techniques in numerical relativity. Numerical simulations of inspiraling compact star binaries are performed for this theory, and compared to the predictions of semi-analytic calculations (which are similar to Peters and Mathews' results for GR). The simulations are based on a co-rotating spherical coordinate system, where both finite difference and pseudo-spectral methods are used. We also adopt the "Hydro without Hydro" approximation, and the Weak Radiation Reaction approximation when the orbital motion is quasi-circular. We evolve a binary with quasi-circular initial data for hundreds of orbits and find that the resulting inspiral closely matches the ¼ power law profile given by the semi-analytical calculations. We additionally find that an eccentric binary circularizes and precesses at the expected rates. The methods investigated thus provide a promising line of attack for the numerical modeling of long binary inspirals in general relativity.

#### Feb 28, 2011

1102.5332 (/preprints)
2011-02-28, 23:12 

## [1102.5094] Reassessing The Fundamentals: New Constraints on the Evolution, Ages and Masses of Neutron Stars

Authors: Bulent Kiziltan

Date: 24 Feb 2011

Abstract: The ages and masses of neutron stars (NSs) are two fundamental threads that make pulsars accessible to other sub-disciplines of astronomy and physics. A realistic and accurate determination of these two derived parameters play an important role in understanding of advanced stages of stellar evolution and the physics that govern relevant processes. Here I summarize new constraints on the ages and masses of NSs with an evolutionary perspective. I show that the observed P-Pdot demographics is more diverse than what is theoretically predicted for the standard evolutionary channel. In particular, standard recycling followed by dipole spin-down fails to reproduce the population of millisecond pulsars with higher magnetic fields (B > 4 x 10ˆ{8} G) at rates deduced from observations. A proper inclusion of constraints arising from binary evolution and mass accretion offers a more realistic insight into the age distribution. By analytically implementing these constraints, I propose a "modified" spin-down age for millisecond pulsars that gives estimates closer to the true age. Finally, I independently analyze the peak, skewness and cutoff values of the underlying mass distribution from a comprehensive list of radio pulsars for which secure mass measurements are available. The inferred mass distribution shows clear peaks at 1.35 Msun and 1.50 Msun for NSs in double neutron star (DNS) and neutron star-white dwarf (NS-WD) systems respectively. I find a mass cutoff at 2 Msun for NSs with WD companions, which establishes a firm lower bound for the maximum mass of NSs.

#### Feb 28, 2011

1102.5094 (/preprints)
2011-02-28, 23:11 

## [1102.4654] Designing a cross-correlation search for continuous-wave gravitational radiation from a neutron star in the supernova remnant SNR 1987A

Authors: Christine Chung, Andrew Melatos, Badri Krishnan, John T. Whelan

Date: 23 Feb 2011

Abstract: A strategy is devised for a semi-coherent cross-correlation search for a young neutron star in the supernova remnant SNR 1987A, using science data from the Initial LIGO and/or Virgo detectors. An astrophysical model for the gravitational wave phase is introduced which describes the star's spin down in terms of its magnetic field strength $B$ and ellipticity $\epsilon$, instead of its frequency derivatives. The model accurately tracks the gravitational wave phase from a rapidly decelerating neutron star under the restrictive but computationally unavoidable assumption of constant braking index, an issue which has hindered previous searches for such young objects. The theoretical sensitivity is calculated and compared to the indirect, age-based wave strain upper limit. The age-based limit lies above the detection threshold in the frequency band 75\,Hz $\lesssim \nu \lesssim 450$\,Hz. The semi-coherent phase metric is also calculated and used to estimate the optimal search template spacing for the search. The range of search parameters that can be covered given our computational resources ($\sim 10ˆ9$ templates) is also estimated. For Initial LIGO sensitivity, in the frequency band between 50\,Hz and 500\,Hz, in the absence of a detected signal, we should be able to set limits of $B \gtrsim 10ˆ{11}$\,G and $\epsilon \lesssim 10ˆ{-4}$.

#### Feb 24, 2011

1102.4654 (/preprints)
2011-02-24, 15:38 

## [1102.4614] Stellar mass black holes in star clusters: gravitational wave emission and detection rates

Authors: Sambaran Banerjee (AIfA, Bonn)

Date: 22 Feb 2011

Abstract: We investigate the dynamics of stellar-mass black holes (BH) in star clusters focusing on the dynamical formation of BH-BH binaries, which are very important sources of gravitational waves (GW). We examine the properties of these BH-BH binaries through direct N-body computations of Plummer clusters, having initially N(0) <= 10ˆ5 low mass stars and a population of stellar mass BHs, using the state-of-the-art N-body integrator "NBODY6". We find that the stellar mass BHs segregate rapidly into the cluster core to form a central dense sub-cluster of BHs in which BH-BH binaries form via 3-body encounters. While most of the BH binaries finally escape from the cluster by recoils due to super-elastic encounters with the single BHs, we find that for clusters with N(0) >= 5 X 10ˆ4, typically a few of them dynamically harden to the extent that they can merge via GW emission within the cluster. Also, for each of such clusters, there are a few escaped BH binaries that merge within a Hubble time, most of the mergers happening within a few Gyr of cluster evolution. These results imply that the intermediate-aged massive clusters constitute the most important class of star clusters that can produce dynamical BH-BH mergers at the present epoch. The BH-BH merger rates obtained from our computations imply a significant detection rate (~ 30/yr) for the "Advanced LIGO" GW detector that will become operative in the near future. Finally, we briefly discuss our ongoing development on this work incorporating the formation of BHs in star clusters from stellar evolution. In particular, we highlight the effect of stellar metallicity on the BH sub-cluster driven expansion of a star cluster's core.

#### Feb 24, 2011

1102.4614 (/preprints)
2011-02-24, 15:38 

## [1102.4824] Pulsar Timing Sensitivities to Gravitational Waves from Relativistic Metric Theories of Gravity

Authors: Marcio Eduardo da Silva Alves, Massimo Tinto

Date: 23 Feb 2011

Abstract: Pulsar timing experiments aimed at the detection of gravitational radiation have been performed for decades now. With the forthcoming construction of large arrays capable of tracking multiple millisecond pulsars, it is very likely we will be able to make the first detection of gravitational radiation in the nano-Hertz band, and test Einstein's theory of relativity by measuring the polarization components of the detected signals. Since a gravitational wave predicted by the most general relativistic metric theory of gravity accounts for {\it six} polarization modes (the usual two Einstein's tensor polarizations as well as two vector and two scalar wave components), we have estimated the single-antenna sensitivities to these six polarizations. We find pulsar timing experiments to be significantly more sensitive, over their entire observational frequency band ($\approx 10ˆ{-9} - 10ˆ{-6}$ Hz), to scalar-longitudinal and vector waves than to scalar-transverse and tensor waves. At $10ˆ{-7}$ Hz and with pulsars at a distance of $1$ kpc, for instance, we estimate an average sensitivity to scalar-longitudinal waves that is more than two orders of magnitude better than the sensitivity to tensor waves. Our results imply that a direct detection of gravitational radiation by pulsar timing will result into a test of the theory of general relativity that is more stringent than that based on monitoring the decay of the orbital period of a binary system.

#### Feb 24, 2011

1102.4824 (/preprints)
2011-02-24, 15:38 

## [1102.4362] Equivalence Principle and Gravitational Redshift

Authors: Michael A. Hohensee, Steven Chu, Achim Peters, Holger Mueller

Date: 21 Feb 2011

Abstract: We investigate leading order deviations from general relativity that violate the Einstein equivalence principle (EEP) in the gravitational standard model extension (SME). We show that redshift experiments based on matter waves and clock comparisons are equivalent to one another. Consideration of torsion balance tests, along with matter wave, microwave, optical, and M\"ossbauer clock tests yields comprehensive limits on spin-independent EEP-violating SME terms at the $10ˆ{-6}$ level.

#### Feb 23, 2011

1102.4362 (/preprints)
2011-02-23, 13:48 

## [1102.4348] Cosmic recycling of millisecond pulsars

Authors: Wynn C. G. Ho, Thomas J. Maccarone, Nils Andersson (University of Southampton)

Date: 21 Feb 2011

Abstract: We compare the rotation rate of neutron stars in low-mass X-ray binaries (LMXBs) with the orbital period of the binaries. We find that, while short orbital period LMXBs span a range of neutron star rotation rates, all the long period LMXBs have fast rotators. We also find that the rotation rates are highest for the systems with the highest mean mass accretion rates, as can be expected if the accretion rate correlates with the orbital period. We show that these properties can be understood by a balance between spin-up due to accretion and spin-down due to gravitational radiation. Our scenario indicates that the gravitational radiation emitted by these systems may be detectable by future ground-based gravitational wave detectors.

#### Feb 23, 2011

1102.4348 (/preprints)
2011-02-23, 13:48 

## [1102.3706] Bumpy Black Holes in Alternate Theories of Gravity

Authors: Sarah Vigeland, Nicol&#xe1;s Yunes, Leo Stein

Date: 17 Feb 2011

Abstract: We generalize the bumpy black hole framework to allow for alternative theory deformations. We construct two model-independent parametric deviations from the Kerr metric: one built from a generalization of the quasi-Kerr and bumpy metrics and one built directly from perturbations of the Kerr spacetime in Lewis-Papapetrou form. We find the conditions that these "bumps" must satisfy for there to exist an approximate second-order Killing tensor so that the perturbed spacetime still possesses three constants of the motion (a deformed energy, angular momentum and Carter constant) and the geodesic equations can be written in first-order form. We map these parameterized metrics to each other via a diffeomorphism and to known analytical black hole solutions in alternative theories of gravity. The parameterized metrics presented here serve as frameworks for the systematic calculation of extreme-mass ratio inspiral waveforms in parameterized non-GR theories and the investigation of the accuracy to which space-borne gravitational wave detectors can constrain such deviations.

#### Feb 20, 2011

1102.3706 (/preprints)
2011-02-20, 18:36 

## [1102.3781] Search for gravitational waves from binary black hole inspiral, merger and ringdown

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

Date: 18 Feb 2011

Abstract: We present the first modeled search for gravitational waves using the complete binary black hole gravitational waveform from inspiral through the merger and ringdown for binaries with negligible component spin. We searched approximately 2 years of LIGO data taken between November 2005 and September 2007 for systems with component masses of 1-99 solar masses and total masses of 25-100 solar masses. We did not detect any plausible gravitational-wave signals but we do place upper limits on the merger rate of binary black holes as a function of the component masses in this range. We constrain the rate of mergers for binary black hole systems with component masses between 19 and 28 solar masses and negligible spin to be no more than 2.0 per Mpcˆ3 per Myr at 90% confidence.

#### Feb 20, 2011

1102.3781 (/preprints)
2011-02-20, 18:36 

## [1102.3647] Gravitational Waves Notes, Issue #5 : "The Capra research programme for capture of small compact objects by massive black holes"

Authors: Pau Amaro-Seoane, Bernard Schutz, Jonathan Thornburg

Date: 17 Feb 2011

Abstract: GW Notes was born from the need for a journal where the distinct communities involved in gravitation wave research might gather. While these three communities - astrophysics, general relativity and data analysis - have made significant collaborative progress over recent years, we believe that it is indispensable to future advancement that they draw closer, and that they speak a common idiom. In this 6th GW Note (since we started numbering with #0), we present the work of Jonathan Thornburg, who has been fully-refereed, on the Capra research programme for capture of small compact objects by massive black holes.

#### Feb 20, 2011

1102.3647 (/preprints)
2011-02-20, 10:29 

## [1102.3612] Violations of Einstein's Relativity: Motivations, Theory, and Phenomenology

Authors: Ralf Lehnert

Date: 17 Feb 2011

Abstract: One of the most difficult questions in present-day physics concerns a fundamental theory of space, time, and matter that incorporates a consistent quantum description of gravity. There are various theoretical approaches to such a quantum-gravity theory. Nevertheless, experimental progress is hampered in this research field because many models predict deviations from established physics that are suppressed by some power of the Planck scale, which currently appears to be immeasurably small. However, tests of relativity theory provide one promising avenue to overcome this phenomenological obstacle: many models for underlying physics can accommodate a small breakdown of Lorentz symmetry, and numerous feasible Lorentz-symmetry tests have Planck reach. Such mild violations of Einstein's relativity have therefore become a focus of recent research efforts. This presentation provides a brief survey of the key ideas in this research field and is geared at both experimentalists and theorists. In particular, several theoretical mechanisms leading to deviations from relativity theory are presented; the standard theoretical framework for relativity violations at currently accessible energy scales (i.e., the SME) is reviewed, and various present and near-future experimental efforts within this field are discussed.

#### Feb 20, 2011

1102.3612 (/preprints)
2011-02-20, 10:28 

## [1102.3434] Detecting Vanishing Dimensions Via Primordial Gravitational Wave Astronomy

Authors: Jonas R. Mureika, Dejan Stojkovic

Date: 16 Feb 2011

Abstract: Lower-dimensionality at higher energies has manifold theoretical advantages as recently pointed out. Moreover, it appears that experimental evidence may already exists for it - a statistically significant planar alignment of events with energies higher than TeV has been observed in some earlier cosmic ray experiments. We propose a robust and independent test for this new paradigm. Since (2+1)-dimensional spacetimes have no gravitational degrees of freedom, gravity waves cannot be produced in that epoch. This places a universal maximum frequency at which primordial waves can propagate, marked by the transition between dimensions. We show that this cut-off frequency may be accessible to future gravitational wave detectors such as LISA.

#### Feb 20, 2011

1102.3434 (/preprints)
2011-02-20, 10:28 

## [1102.3355] Gravitational Wave Detection by Interferometry (Ground and Space)

Authors: Matthew Pitkin, Stuart Reid, Sheila Rowan, Jim Hough

Date: 16 Feb 2011

Abstract: Significant progress has been made in recent years on the development of gravitational wave detectors. Sources such as coalescing compact binary systems, neutron stars in low-mass X-ray binaries, stellar collapses and pulsars are all possible candidates for detection. The most promising design of gravitational wave detector uses test masses a long distance apart and freely suspended as pendulums on Earth or in drag-free craft in space. The main theme of this review is a discussion of the mechanical and optical principles used in the various long baseline systems in operation around the world - LIGO (USA), Virgo (Italy/France), TAMA300 and LCGT (Japan), and GEO600 (Germany/U.K.) - and in LISA, a proposed space-borne interferometer. A review of recent science runs from the current generation of ground-based detectors will be discussed, in addition to highlighting the astrophysical results gained thus far. Looking to the future, the major upgrades to LIGO (Advanced LIGO), Virgo (Advanced Virgo), LCGT and GEO600 (GEO-HF) will be completed over the coming years, which will create a network of detectors with significantly improved sensitivity required to detect gravitational waves. Beyond this, the concept and design of possible future "third generation" gravitational wave detectors, such as the Einstein Telescope (ET), will be discussed.

#### Feb 16, 2011

1102.3355 (/preprints)
2011-02-16, 21:37 

## [1102.3180] Stellar Dynamics of Extreme-Mass-Ratio Inspirals

Authors: David Merritt, Tal Alexander, Seppo Mikkola, Clifford Will

Date: 15 Feb 2011

Abstract: Inspiral of compact stellar remnants into massive black holes (MBHs) is accompanied by the emission of gravitational waves at frequencies that are potentially detectable by the proposed laser interferometer space antenna. Event rates computed from statistical (Fokker-Planck, Monte-Carlo) approaches span a wide range due to uncertaintities about the rate coefficients. Here we present results from direct integration of the post-Newtonian N-body equations of motion descrbing dense clusters of compact stars around Schwarzschild and Kerr MBHs. These simulations embody an essentially exact (at the post-Newtonian level) treatment of the interplay between stellar dynamical relaxation, relativistic precession, and gravitational-wave energy loss. The rate of capture of stars by the MBH is found to be greatly reduced by relativistic precession, which limits the ability of torques from the stellar potential to change orbital angular momenta. Penetration of this "Schwarzschild barrier" does occasionally occur, resulting in capture of stars onto orbits that gradually inspiral due to gravitational wave emission; we discuss two mechanisms for barrier penetration and find evidence for both in the simulations. We derive an approximate formula for the capture rate, which predicts that captures would be strongly disfavored from orbits with semi-major axes below a certain value; this prediction, as well as the predicted rate, are verified in the N-body integrations. Adding spin to the MBH does not substantially change the capture rate; the back-reaction of the stellar torques on the spin of the MBH is evaluated and shown to be potentially observable. We discuss the implications of our results for the detection of extreme-mass-ratio inspirals from galactic nuclei with a range of physical properties.

#### Feb 16, 2011

1102.3180 (/preprints)
2011-02-16, 14:54 

## [1102.2857] The Capra Research Program for Modelling Extreme Mass Ratio Inspirals

Authors: Jonathan Thornburg

Date: 14 Feb 2011

Abstract: Suppose a small compact object (black hole or neutron star) of mass $m$ orbits a large black hole of mass $M \gg m$. This system emits gravitational waves (GWs) that have a radiation-reaction effect on the particle's motion. EMRIs (extreme--mass-ratio inspirals) of this type will be important GW sources for LISA; LISA's data analysis will require highly accurate EMRI GW templates. In this article I outline the "Capra" research program to try to model EMRIs and calculate their GWs \textit{ab initio}, assuming only that $m \ll M$ and that the Einstein equations hold. Here we treat the EMRI spacetime as a perturbation of the large black hole's "background" (Schwarzschild or Kerr) spacetime and use the methods of black-hole perturbation theory, expanding in the small parameter $m/M$. The small body's motion can be described either as the result of a radiation-reaction "self-force" acting in the background spacetime or as geodesic motion in a perturbed spacetime. Several different lines of reasoning lead to the (same) basic $\O(m/M)$ "MiSaTaQuWa" equations of motion for the particle. Surprisingly, for a nonlinear field theory such as general relativity, modelling the small body as a point particle works well. The particle's own field is singular along the particle worldline so it's difficult to formulate a meaningful "perturbation" theory or equations of motion there. I discuss "mode-sum" and "puncture-function" regularization schemes that resolve this difficulty and allow practical self-force calculations, and I outline an important recent calculation of this type.
Most Capra research to date has used 1st order perturbation theory. To obtain the very high accuracies needed to fully exploit LISA's observations of the strongest EMRIs, 2nd order perturbation theory will probably be needed.

#### Feb 15, 2011

1102.2857 (/preprints)
2011-02-15, 17:28 

## [1102.2230] On detection of the stochastic gravitational-wave background using the Parkes pulsar timing array

Authors: D. R. B. Yardley, W. A. Coles, G. B. Hobbs, J. P. W. Verbiest, R. N. Manchester, W. van Straten, F. A. Jenet, M. Bailes, N. D. R. Bhat, S. Burke-Spolaor, D. J. Champion, A. W. Hotan, S. Oslowski, J. E. Reynolds, J. M. Sarkissian

Date: 10 Feb 2011

Abstract: We search for the signature of an isotropic stochastic gravitational-wave background in pulsar timing observations using a frequency-domain correlation technique. These observations, which span roughly 12 yr, were obtained with the 64-m Parkes radio telescope augmented by public domain observations from the Arecibo Observatory. A wide range of signal processing issues unique to pulsar timing and not previously presented in the literature are discussed. These include the effects of quadratic removal, irregular sampling, and variable errors which exacerbate the spectral leakage inherent in estimating the steep red spectrum of the gravitational-wave background. These observations are found to be consistent with the null hypothesis, that no gravitational-wave background is present, with 76 percent confidence. We show that the detection statistic is dominated by the contributions of only a few pulsars because of the inhomogeneity of this data set. The issues of detecting the signature of a gravitational-wave background with future observations are discussed.

#### Feb 13, 2011

1102.2230 (/preprints)
2011-02-13, 23:10 

## [1102.2001] Elementary analysis of the special relativistic combination of velocities, Wigner rotation, and Thomas precession

Authors: Kane O&#x27;Donnell (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)

Date: 9 Feb 2011

Abstract: The purpose of this paper is to provide an elementary introduction to the qualitative and quantitative results of velocity combination in special relativity, including the Wigner rotation and Thomas precession. We utilize only the most familiar tools of special relativity, in arguments presented at three differing levels: (1) utterly elementary, which will suit a first course in relativity; (2) intermediate, to suit a second course; and (3) advanced, to suit higher level students. We then give a summary of useful results, and suggest further reading in this often obscure field.

#### Feb 10, 2011

1102.2001 (/preprints)
2011-02-10, 22:19 

## [1102.1749] On the use of higher order waveforms in the search for gravitational waves emitted by compact binary coalescences

Authors: D. J. A. McKechan

Date: 8 Feb 2011

Abstract: This thesis concerns the use, in gravitational wave data analysis, of higher order waveform models of the gravitational radiation emitted by compact binary coalescences. We begin with an introductory chapter that includes an overview of the theory of general relativity, gravitational radiation and ground-based interferometric gravitational wave detectors. We then discuss, in Chapter 2, the gravitational waves emitted by compact binary coalescences, with an explanation of higher order waveforms and how they differ from leading order waveforms; we also introduce the post-Newtonian formalism. In Chapter 3 the method and results of a gravitational wave search for low mass compact binary coalescences using a subset of LIGO's 5th science run data are presented and in the subsequent chapter we examine how one could use higher order waveforms in such analyses. We follow the development of a new search algorithm that incorporates higher order waveforms with promising results for detection efficiency and parameter estimation. In Chapter 5, a new method of windowing time-domain waveforms that offers benefit to gravitational wave searches is presented. The final chapter covers the development of a game designed as an outreach project to raise public awareness and understanding of the search for gravitational waves.

#### Feb 10, 2011

1102.1749 (/preprints)
2011-02-10, 09:28 

## [1102.0769] Constraining the dark energy equation of state using LISA observations of spinning Massive Black Hole binaries

Authors: Antoine Petiteau, Stanislav Babak, Alberto Sesana

Date: 3 Feb 2011

Abstract: Gravitational wave signals from coalescing Massive Black Hole (MBH) binaries could be used as standard sirens to measure cosmological parameters. The future space based gravitational wave observatory Laser Interferometer Space Antenna (LISA) will detect up to a hundred of those events, providing very accurate measurements of their luminosity distances. To constrain the cosmological parameters we also need to measure the redshift of the galaxy (or cluster of galaxies) hosting the merger. This requires the identification of a distinctive electromagnetic event associated to the binary coalescence. However, putative electromagnetic signatures may be too weak to be observed. Instead, we study here the possibility of constraining the cosmological parameters by enforcing statistical consistency between all the possible hosts detected within the measurement error box of a few dozen of low redshift (z<3) events. We construct MBH populations using merger tree realizations of the dark matter hierarchy in a LambdaCDM Universe, and we use data from the Millennium simulation to model the galaxy distribution in the LISA error box. We show that, assuming that all the other cosmological parameters are known, the parameter w describing the dark energy equation of state can be constrained to a 4-8% level (2sigma error), competitive with current uncertainties obtained by type Ia supernovae measurements, providing an independent test of our cosmological model.

#### Feb 09, 2011

1102.0769 (/preprints)
2011-02-09, 12:00 

## [1102.1020] Radio Remnants of Compact Binary Mergers - the Electromagnetic Signal that will follow the Gravitational Waves

Authors: Ehud Nakar, Tsvi Piran

Date: 4 Feb 2011

Abstract: The question "what is the observable electromagnetic (EM) signature of a compact binary merger?" is an intriguing one with crucial consequences to the quest for gravitational waves (GW). Compact binary mergers are prime sources of GW, targeted by current and next generation detectors. Numerical simulations have demonstrated that these mergers eject energetic sub-relativistic (or even relativistic) outflows. This is certainly the case if the mergers produce short GRBs, but even if not, significant outflows are expected. The interaction of such outflows with the surround matter inevitably leads to a long lasting radio signal. We calculate the expected signal from these outflows (our calculations are also applicable to short GRB orphan afterglows) and we discuss their detectability. We show that the optimal search for such signal should, conveniently, take place around 1.4 GHz. Realistic estimates of the outflow parameters yield signals of a few hundred $\mu$Jy, lasting a few weeks, from sources at the detection horizon of advanced GW detectors. Followup radio observations, triggered by GW detection, could reveal the radio remnant even under unfavorable conditions. Upcoming all sky surveys can detect a few dozen, and possibly even thousands, merger remnants at any give time, thereby providing robust merger rate estimates even before the advanced GW detectors become operational. In fact, the radio transient RT 19870422 fits well the overall properties predicted by our model and we suggest that its most probable origin is a compact binary merger radio remnant.

#### Feb 09, 2011

1102.1020 (/preprints)
2011-02-09, 11:59 

## [1102.0327] Growing Massive Black Holes in a Local Group Environment: the Central Supermassive, Slowly Sinking, and Ejected Populations

Authors: Miroslav Micic, Kelly Holley-Bockelmann, Steinn Sigurdsson

Date: 2 Feb 2011

Abstract: We explore the growth of < 10ˆ7 Msun black holes that reside at the centers of spiral and field dwarf galaxies in a Local Group type of environment. We use merger trees from a cosmological N-body simulation known as Via Lactea II (VL-2) as a framework to test two merger-driven semi-analytic recipes for black hole growth that include dynamical friction, tidal stripping, and gravitational wave recoil in over 20,000 merger tree realizations. First, we apply a Fundamental Plane limited (FPL) model to the growth of Sgr A*, which drives the central black hole to a maximum mass limited by the Black Hole Fundamental Plane after every merger. Next, we present a new model that allows for low-level Prolonged Gas Accretion (PGA) during the merger. We find that both models can generate a Sgr A* mass black hole. We predict a population of massive black holes in local field dwarf galaxies - if the VL-2 simulation is representative of the growth of the Local Group, we predict up to 35 massive black holes (< 10ˆ6 Msun) in Local Group field dwarfs. We also predict that hundreds of < 10ˆ5 Msun black holes fail to merge, and instead populate the Milky Way halo, with the most massive of them at roughly the virial radius. In addition, we find that there may be hundreds of massive black holes ejected from their hosts into the nearby intergalactic medium due to gravitational wave recoil. We discuss how the black hole population in the Local Group field dwarfs may help to constrain the growth mechanism for Sgr A*.

#### Feb 03, 2011

1102.0327 (/preprints)
2011-02-03, 12:34 

## [1102.0262] Physics of Coevolution of Galaxies and Supermassive Black Holes

Authors: Renyue Cen (Princeton University Observatory)

Date: 1 Feb 2011

Abstract: A model for coevolution of galaxies and supermassive black holes (SMBH) is presented that is physically based. The starting point is a gas-rich major merger that triggers a starburst and the endpoint is a quiescent elliptical galaxy many gigayears later. There is an approximate coevolution between starburst galaxies and elliptical galaxies, although it is not exact in several important ways. Starburst precedes the onset of main SMBH growth with a gap of time equal to ~100Myr and is responsible for shutting down its own activities; AGN has little to do with it. While starburst occurs earlier and lasts for only about 100Myrs, the AGN accretion occurs later and lasts for ~1 Gyr or longer with a diminishing Eddington ratio. The main AGN growth in post-starburst phase is fueled by recycled gas from inner bulge stars and self-regulated. The predicted relation between SMBH mass and bulge mass/velocity dispersion is consistent with observations. A suite of testable and falsifiable predictions and implications with respect to relationships between various types of galaxies and AGN are made. Where comparisons to extant observations are possible, the model is in agreement with them.

#### Feb 03, 2011

1102.0262 (/preprints)
2011-02-03, 12:33 

## Preprints and articles of interest

→ Previously: To Check

#### Feb 02, 2011

home (/preprints)
2011-02-02, 18:37 

## [1101.6072] Likelihood Methods for the Detection and Characterization of Gamma-ray Pulsars with the Fermi Large Area Telescope

Authors: Matthew Kerr

Date: 31 Jan 2011

Abstract: The sensitivity of the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope allows detection of thousands of new gamma-ray sources and detailed characterization of the spectra and variability of bright sources. Unsurprisingly, this increased capability leads to increased complexity in data analysis. Likelihood methods are ideal for connecting models with data, but the computational cost of folding the model input through the multi-scale instrument response function is appreciable. Both interactive analysis and large projects — such as analysis of the full gamma-ray sky — can be prohibitive or impossible, reducing the scope of the science possible with the LAT. To improve on this situation, we have developed pointlike, a software package for fast maximum likelihood analysis of LAT data. It is interactive by design and its rapid evaluation of the likelihood facilitates exploratory and large-scale, all-sky analysis. We detail its implementation and validate its performance on simulated data. We demonstrate its capability for interactive analysis and present several all-sky analyses. These include a search for new gamma-ray sources and the selection of LAT sources with pulsar-like characteristics for targeted radio pulsation searches. We conclude by developing sensitive periodicity tests incorporating spectral information obtained from pointlike.

#### Jan 31, 2011

1101.6072 (/preprints)
2011-01-31, 22:25 

## [1101.6019] The Geometry Of Modified Newtonian Dynamics

Authors: C. Skordis, T.G. Zlosnik

Date: 31 Jan 2011

Abstract: Modified Newtonian Dynamics is an empirical modification to Poisson's equation which has had success in accounting for the ‘gravitational field’ $\Phi$ in a variety of astrophysical systems. The field $\Phi$ may be interpreted in terms of the weak field limit of a variety of spacetime geometries. Here we consider three of these geometries in a more comprehensive manner and look at the effect on timelike and null geodesics. In particular we consider the Aquadratic Lagrangian (AQUAL) theory, Tensor-Vector-Scalar (TeVeS) theory and Generalized Einstein-{\AE}ther (GEA) theory. We uncover a number of novel features, some of which are specific to the theory considered while others are generic. In the case of AQUAL and TeVeS theories, the spacetime exhibits an excess (AQUAL) or deficit (TeVeS) solid angle akin to the case of a Barriola-Vilenkin global monopole. In the case of GEA, a disformal symmetry of the action emerges in the limit of $\grad\Phi\rightarrow 0$. Finally, in all theories studied, massive particles can never reach spatial infinity while photons can do so only after experiencing infinite redshift.

#### Jan 31, 2011

1101.6019 (/preprints)
2011-01-31, 22:25 

## [1101.5803] Gravitational Lens Modeling with Genetic Algorithms and Particle Swarm Optimizers

Authors: Adam Rogers, Jason D. Fiege

Date: 30 Jan 2011

Abstract: Strong gravitational lensing of an extended object is described by a mapping from source to image coordinates that is nonlinear and cannot generally be inverted analytically. Determining the structure of the source intensity distribution also requires a description of the blurring effect due to a point spread function. This initial study uses an iterative gravitational lens modeling scheme based on the semilinear method to determine the linear parameters (source intensity profile) of a strongly lensed system. Our 'matrix-free' approach avoids construction of the lens and blurring operators while retaining the least squares formulation of the problem. The parameters of an analytical lens model are found through nonlinear optimization by an advanced genetic algorithm (GA) and particle swarm optimizer (PSO). These global optimization routines are designed to explore the parameter space thoroughly, mapping model degeneracies in detail. We develop a novel method that determines the L-curve for each solution automatically, which represents the trade-off between the image chi-square and regularization effects, and allows an estimate of the optimally regularized solution for each lens parameter set. In the final step of the optimization procedure, the lens model with the lowest chi-square is used while the global optimizer solves for the source intensity distribution directly. This allows us to accurately determine the number of degrees of freedom in the problem to facilitate comparison between lens models and enforce positivity on the source profile. In practice we find that the GA conducts a more thorough search of the parameter space than the PSO.

#### Jan 31, 2011

1101.5803 (/preprints)
2011-01-31, 22:25 

## [1101.4298] The missing link: Merging neutron stars naturally produce jet-like structures and can power short Gamma-Ray Bursts

Authors: Luciano Rezzolla, Bruno Giacomazzo, Luca Baiotti, Jonathan Granot, Chryssa Kouveliotou, Miguel A. Aloy

Date: 22 Jan 2011

Abstract: Short Gamma-Ray Bursts (SGRBs) are among the most luminous explosions in the universe, releasing in less than one second the energy emitted by our Galaxy over one year. Despite decades of observations, the nature of their "central-engine" remains unknown. Considering a generic binary of magnetized neutron stars and solving Einstein equations, we show that their merger results in a rapidly spinning black hole surrounded by a hot and highly magnetized torus. Lasting over 35 ms and much longer than previous simulations, our study reveals that magnetohydrodynamical instabilities amplify an initially turbulent magnetic field of ~ 10ˆ{12} G to produce an ordered poloidal field of ~ 10ˆ{15} G along the black-hole spin-axis, within a half-opening angle of ~ 30 deg, which may naturally launch a relativistic jet. The broad consistency of our ab-initio calculations with SGRB observations shows that the merger of magnetized neutron stars can provide the basic physical conditions for the central-engine of SGRBs.

#### Jan 31, 2011

1101.4298 (/preprints)
2011-01-31, 22:25 

## [1101.5971] Prospects for accurate distance measurements of pulsars with the SKA: enabling fundamental physics

Authors: R. Smits, S.J. Tingay, N. Wex, M. Kramer, B. Stappers

Date: 31 Jan 2011

Abstract: Parallax measurements of pulsars allow for accurate measurements of the interstellar electron density and contribute to accurate tests of general relativity using binary systems. The Square Kilometre Array (SKA) will be an ideal instrument for measuring the parallax of pulsars, having a very high sensitivity as well as baselines extending up to several thousands of kilometres. We have performed simulations to estimate the number of pulsars for which the parallax can be measured with the SKA and to what distance a parallax can be measured. We compare two different methods. The first method measures the parallax directly by utilising the long baselines of the SKA to form high angular resolution images. The second method uses the arrival times of the radio signals of pulsars to fit a transformation between time coordinates in the terrestrial frame and the comoving pulsar frame which directly yields the parallax. We find that with the first method a parallax with an accuracy of 20% or less can be measured up to a maximum distance of 13 kpc, which would include 9,000 pulsars. By timing pulsars with the most stable arrival times for the radio emission, parallaxes can be measured for about 3,600 millisecond pulsars up to a distance of 9 kpc with an accuracy of 20%.

#### Jan 31, 2011

1101.5971 (/preprints)
2011-01-31, 22:25 

## [1101.5408] Localization of gravitational wave sources with networks of advanced detectors

Authors: S. Klimenko, G. Vedovato, M. Drago, G. Mazzolo, G. Mitselmakher, C. Pankow, G. Prodi, V. Re, F. Salemi, I. Yakushin

Date: 27 Jan 2011

Abstract: Coincident observations with gravitational wave (GW) detectors and other astronomical instruments are in the focus of the experiments with the network of LIGO, Virgo and GEO detectors. They will become a necessary part of the future GW astronomy as the next generation of advanced detectors comes online. The success of such joint observations directly depends on the source localization capabilities of the GW detectors. In this paper we present studies of the sky localization of transient sources with the future advanced detector networks and describe their fundamental properties. By reconstructing sky coordinates of ad hoc signals injected into simulated detector noise we study the accuracy of the source localization and its dependence on the strength of injected signals, waveforms and network configurations.

#### Jan 31, 2011

1101.5408 (/preprints)
2011-01-31, 13:31 

## [1101.5306] Gravitational and electromagnetic emission by magnetized coalescing binary systems

Authors: S. Capozziello, M. De Laurentis, I. De Martino, M. Formisano, D. Vernieri

Date: 27 Jan 2011

Abstract: We discuss the possibility to obtain an electromagnetic emission accompanying the gravitational waves emitted in the coalescence of a compact binary system. Motivated by the existence of black hole configurations with open magnetic field lines along the rotation axis, we consider a magnetic dipole in the system, the evolution of which leads to (i) electromagnetic radiation, and (ii) a contribution to the gravitational radiation, the luminosity of both being evaluated. Starting from the observations on magnetars, we impose upper limits for both the electromagnetic emission and the contribution of the magnetic dipole to the gravitational wave emission. Adopting this model for the evolution of neutron star binaries leading to short gamma ray bursts, we compare the correction originated by the electromagnetic field to the gravitational waves emission, finding that they are comparable for particular values of the magnetic field and of the orbital radius of the binary system. Finally we calculate the electromagnetic and gravitational wave energy outputs which result comparable for some values of magnetic field and radius.

#### Jan 28, 2011

1101.5306 (/preprints)
2011-01-28, 09:03 

## [1101.5169] The Binary Fraction of Low Mass White Dwarfs

Authors: Justin M. Brown (1), Mukremin Kilic (2), Warren R. Brown (2), Scott J. Kenyon (3) ((1) Franklin and Marshall College (2) Smithsonian Astrophysical Observatory)

Date: 26 Jan 2011

Abstract: We describe spectroscopic observations of 21 low-mass (<0.45 M_sun) white dwarfs (WDs) from the Palomar-Green Survey obtained over four years. We use both radial velocities and infrared photometry to identify binary systems, and find that the fraction of single, low-mass WDs is <30%. We discuss the potential formation channels for these single stars including binary mergers of lower-mass objects. However, binary mergers are not likely to explain the observed number of single low-mass WDs. Thus additional formation channels, such as enhanced mass loss due to winds or interactions with substellar companions, are likely.

#### Jan 28, 2011

1101.5169 (/preprints)
2011-01-28, 09:03 

## [1101.4997] Probing the size of extra dimension with gravitational wave astronomy

Authors: Kent Yagi, Norihiro Tanahashi, Takahiro Tanaka

Date: 26 Jan 2011

Abstract: In Randall-Sundrum II (RS-II) braneworld model, it has been conjectured according to the AdS/CFT correspondence that brane-localized black hole (BH) larger than the bulk AdS curvature scale $\ell$ cannot be static, and it is dual to a four dimensional BH emitting the Hawking radiation through some quantum fields. In this scenario, the number of the quantum field species is so large that this radiation changes the orbital evolution of a BH binary. We derived the correction to the gravitational waveform phase due to this effect and estimated the upper bounds on $\ell$ by performing Fisher analyses. We found that DECIGO/BBO can put a stronger constraint than the current table-top result by detecting gravitational waves from small mass BH/BH and BH/neutron star (NS) binaries. Furthermore, DECIGO/BBO is expected to detect 10$ˆ5$ BH/NS binaries per year. Taking this advantage, we found that DECIGO/BBO can actually measure $\ell$ down to $\ell=0.33 \mu$m for 5 year observation if we know that binaries are circular a priori. This is about 40 times smaller than the upper bound obtained from the table-top experiment. On the other hand, when we take eccentricities into binary parameters, the detection limit weakens to $\ell=1.5 \mu$m due to strong degeneracies between $\ell$ and eccentricities. We also derived the upper bound on $\ell$ from the expected detection number of extreme mass ratio inspirals (EMRIs) with LISA and BH/NS binaries with DECIGO/BBO, extending the discussion made recently by McWilliams. We found that these less robust constraints are weaker than the ones from phase differences.

#### Jan 26, 2011

1101.4997 (/preprints)
2011-01-26, 18:37 

## [1101.4939] Efficiently enclosing the compact binary parameter space by singular-value decomposition

Authors: Kipp Cannon, Chad Hanna, Drew Keppel

Date: 25 Jan 2011

Abstract: Gravitational-wave searches for the merger of compact binaries use matched-filtering as the method of detecting signals and estimating parameters. Such searches construct a fine mesh of filters covering a signal parameter space at high density. Previously it has been shown that singular value decomposition can reduce the effective number of filters required to search the data. Here we study how the basis provided by the singular value decomposition changes dimension as a function of template bank density. We will demonstrate that it is sufficient to use the basis provided by the singular value decomposition of a low density bank to accurately reconstruct arbitrary points within the boundaries of the template bank. Since this technique is purely numerical it may have applications to interpolating the space of numerical relativity waveforms.

#### Jan 26, 2011

1101.4939 (/preprints)
2011-01-26, 18:37 

## [1101.4669] Bounding the Time Delay between High-energy Neutrinos and Gravitational-wave Transients from Gamma-ray Bursts

Authors: Bruny Baret, Imre Bartos, Boutayeb Bouhou, Alessandra Corsi, Irene Di Palma, Corinne Donzaud, V&#xe9;ronique Van Elewyck, Chad Finley, Gareth Jones, Antoine Kouchner, Szabolcs M&#xe0;rka, Zsuzsa M&#xe0;rka, Luciano Moscoso, Eric Chassande-Mottin, Maria Alessandra Papa, Thierry Pradier, Peter Raffai, Jameson Rollins, Patrick Sutton

Date: 24 Jan 2011

Abstract: We derive a conservative coincidence time window for joint searches of gravita-tional-wave (GW) transients and high-energy neutrinos (HENs, with energies above 100GeV), emitted by gamma-ray bursts (GRBs). The last are among the most interesting astrophysical sources for coincident detections with current and near-future detectors. We take into account a broad range of emission mechanisms. We take the upper limit of GRB durations as the 95% quantile of the T90's of GRBs observed by BATSE, obtaining a GRB duration upper limit of ~150s. Using published results on high-energy (>100MeV) photon light curves for 8 GRBs detected by Fermi LAT, we verify that most high-energy photons are expected to be observed within the first ~150s of the GRB. Taking into account the breakout-time of the relativistic jet produced by the central engine, we allow GW and HEN emission to begin up to 100s before the onset of observable gamma photon production. Using published precursor time differences, we calculate a time upper bound for precursor activity, obtaining that 95% of precursors occur within ~250s prior to the onset of the GRB. Taking the above different processes into account, we arrive at a time window of tHEN - tGW ~ [-500s,+500s]. Considering the above processes, an upper bound can also be determined for the expected time window of GW and/or HEN signals coincident with a detected GRB, tGW - tGRB ~ tHEN - tGRB ~ [-350s,+150s].

#### Jan 26, 2011

1101.4669 (/preprints)
2011-01-26, 16:15 

## [1101.3990] Nano-Hertz Gravitational Waves Searches with Interferometric Pulsar Timing Experiments

Authors: Massimo Tinto

Date: 20 Jan 2011

Abstract: We estimate the sensitivity to nano-Hertz gravitational waves of pulsar timing experiments in which two highly-stable millisecond pulsars are tracked simultaneously with two neighboring radio telescopes that are referenced to the same time-keeping subsystem (i.e. "the clock"). By taking the difference of the two time-of-arrival residual data streams we can exactly cancel the clock noise in the combined data set, thereby enhancing the sensitivity to gravitational waves. We estimate that, in the band ($10ˆ{-9} - 10ˆ{-8}$) Hz, this "interferometric" pulsar timing technique can potentially improve the sensitivity to gravitational radiation by almost two orders of magnitude over that of single-telescopes. Interferometric pulsar timing experiments could be performed with neighboring pairs of antennas of the forthcoming large arraying projects.

#### Jan 26, 2011

1101.3990 (/preprints)
2011-01-26, 16:14 

## [1101.4684] Mergers of Supermassive Black Holes in Astrophysical Environments

Authors: Tanja Bode, Tamara Bogdanovic, Roland Haas, James Healy, Pablo Laguna, Deirdre Shoemaker

Date: 24 Jan 2011

Abstract: Modeling the late inspiral and merger of supermassive black holes is central to understanding accretion processes and the conditions under which electromagnetic emission accompanies gravitational waves. We use fully general relativistic, hydrodynamics simulations to investigate how electromagnetic signatures correlate with black hole spins, mass ratios, and the gaseous environment in this final phase of binary evolution. In all scenarios, we find some form of characteristic electromagnetic variability whose pattern depends on the spins and binary mass ratios. Binaries in hot accretion flows exhibit a flare followed by a sudden drop in luminosity associated with the plunge and merger, as well as quasi-periodic oscillations correlated with the gravitational waves during the inspiral. Conversely, circumbinary disk systems are characterized by a low luminosity of variable emission, suggesting challenging prospects for their detection.

#### Jan 26, 2011

1101.4684 (/preprints)
2011-01-26, 16:14 

## [1101.4650] Supermassive black holes do not correlate with dark matter halos of galaxies

Authors: John Kormendy, Ralf Bender

Date: 24 Jan 2011

Abstract: Supermassive black holes have been detected in all galaxies that contain bulge components when the galaxies observed were close enough so that the searches were feasible. Together with the observation that bigger black holes live in bigger bulges, this has led to the belief that black hole growth and bulge formation regulate each other. That is, black holes and bulges "coevolve". Therefore, reports of a similar correlation between black holes and the dark matter halos in which visible galaxies are embedded have profound implications. Dark matter is likely to be nonbaryonic, so these reports suggest that unknown, exotic physics controls black hole growth. Here we show - based in part on recent measurements of bulgeless galaxies - that there is almost no correlation between dark matter and parameters that measure black holes unless the galaxy also contains a bulge. We conclude that black holes do not correlate directly with dark matter. They do not correlate with galaxy disks, either. Therefore black holes coevolve only with bulges. This simplifies the puzzle of their coevolution by focusing attention on purely baryonic processes in the galaxy mergers that make bulges.

#### Jan 26, 2011

1101.4650 (/preprints)
2011-01-26, 16:14 

## [1101.3940] Detector configuration of DECIGO/BBO and identification of cosmological neutron-star binaries

Authors: Kent Yagi, Naoki Seto

Date: 20 Jan 2011

Abstract: The primary target for the planned space-borne gravitational wave interferometers DECIGO/BBO is a primordial gravitational wave background (PGWB). However there exist astrophysical foregrounds and among them, gravitational waves from neutron star (NS) binaries are the solid and strong component that must be identified and subtracted. In this paper, we discuss the geometry of detector configurations preferable for identifying the NS/NS binary signals. As a first step, we analytically estimate the minimum signal-to-noise ratios (SNRs) of the binaries for several static detector configurations that are characterized by adjustable geometrical parameters, and determine the optimal values for these parameters. Next we perform numerical simulations to take into account the effect of detector motions, and find reasonable agreements with the analytical results. We show that, with the standard network formed by 4 units of triangle detectors, the proposed BBO sensitivity would be sufficient in receiving gravitational waves from all the NS/NS binaries at $z\le 5$ with SNRs higher than 25. We also discuss the minimum sensitivity of DECIGO required for the foreground identification.

#### Jan 26, 2011

1101.3940 (/preprints)
2011-01-26, 16:14 

## [1101.3749] Transition from adiabatic inspiral to plunge into a spinning black hole

Authors: Michael Kesden

Date: 19 Jan 2011

Abstract: A test particle of mass mu on a bound geodesic of a Kerr black hole of mass M >> mu will slowly inspiral as gravitational radiation extracts energy and angular momentum from its orbit. This inspiral can be considered adiabatic when the orbital period is much shorter than the timescale on which energy is radiated, and quasi-circular when the radial velocity is much less than the azimuthal velocity. Although the inspiral always remains adiabatic provided mu << M, the quasi-circular approximation breaks down as the particle approaches the innermost stable circular orbit (ISCO). In this paper, we relax the quasi-circular approximation and solve the radial equation of motion explicitly near the ISCO. We use the requirement that the test particle's 4-velocity remain properly normalized to calculate a new contribution to the difference between its energy and angular momentum. This difference determines how a black hole's spin changes following a test-particle merger, and can be extrapolated to help predict the mass and spin of the final black hole produced in finite-mass-ratio black-hole mergers. Our new contribution is particularly important for nearly maximally spinning black holes, as it can affect whether a merger produces a naked singularity.

#### Jan 20, 2011

1101.3749 (/preprints)
2011-01-20, 09:24 

## [1101.3765] Reduced basis catalogs for gravitational wave templates

Authors: Scott E. Field, Chad R. Galley, Frank Herrmann, Jan S. Hesthaven, Evan Ochsner, Manuel Tiglio

Date: 19 Jan 2011

Abstract: We introduce a reduced basis approach as a new paradigm for modeling, representing and searching for gravitational waves. We construct waveform catalogs for non-spinning compact binary coalescences, and we find that for accuracies of $99\%$ and $99.999\%$ the method generates a factor of about $10-10ˆ5$ fewer templates than standard placement methods. The continuum of gravitational waves can be represented by a finite and comparatively compact basis. The method is robust under variations in the noise of detectors, implying that only a single catalog needs to be generated.

#### Jan 20, 2011

1101.3765 (/preprints)
2011-01-20, 09:23 

## [1101.3759] Pulsar Timing Sensitivity to Very-Low-Frequency Gravitational Waves

Authors: Fredrick A. Jenet, J.W. Armstrong, Massimo Tinto

Date: 19 Jan 2011

Abstract: At nanohertz frequencies gravitational waves (GWs) cause variations in time-of-arrival of pulsar signals potentially measurable via precision timing observations. Here we compute very-low-frequency GW sensitivity constrained by instrumental, propagation, and other noises fundamentally limiting pulsar timing observations. Reaching expected GW signal strengths will require estimation and removal of $\simeq$99% of time-of-arrival fluctuations caused by typical interstellar plasma turbulence and a reduction of white rms timing noise to $\sim$100 nsec or less. If these were achieved, single-pulsar signal-to-noise ratio (SNR) = 1 sensitivity is then limited by the best current terrestrial time standards at $h_{rms} \sim$2 $\times 10ˆ{-16}$ [f/(1 cycle/year)]$ˆ{-½}$ for $f < 3 \times 10ˆ{-8}$ Hz, where f is Fourier frequency and a bandwidth of 1 cycle/(10 years) is assumed. This sensitivity envelope may be optimistic in that it assumes negligible intrinsic pulsar rotational noise, perfect time transfer from time standard to observatory, and stable pulse profiles. Nonetheless it can be compared to predicted signal levels for a broadband astrophysical GW background from supermassive black hole binaries. Such a background is comparable to timekeeping-noise only for frequencies lower than about 1 cycle/(10 years), indicating that reliable detections will require substantial improvements in signal-to-noise ratio through pulsar array signal processing.

#### Jan 20, 2011

1101.3759 (/preprints)
2011-01-20, 09:23 

## [1101.3331] Beyond the geodesic approximation: conservative effects of the gravitational self-force in eccentric orbits around a Schwarzschild black hole

Authors: Leor Barack, Norichika Sago

Date: 17 Jan 2011

Abstract: We study conservative finite-mass corrections to the motion of a particle in a bound (eccentric) strong-field orbit around a Schwarzschild black hole. We assume the particle's mass $\mu$ is much smaller than the black hole mass $M$, and explore post-geodesic corrections of $O(\mu/M)$. Our analysis uses numerical data from a recently developed code that outputs the Lorenz-gauge gravitational self-force (GSF) acting on the particle along the eccentric geodesic. First, we calculate the $O(\mu/M)$ conservative correction to the periastron advance of the orbit, as a function of the (gauge dependent) semi-latus rectum and eccentricity. A gauge-invariant description of the GSF precession effect is made possible in the circular-orbit limit, where we express the correction to the periastron advance as a function of the invariant azimuthal frequency. We compare this relation with results from fully nonlinear numerical-relativistic simulations. In order to obtain a gauge-invariant measure of the GSF effect for fully eccentric orbits, we introduce a suitable generalization of Detweiler's circular-orbit "red shift" invariant. We compute the $O(\mu/M)$ conservative correction to this invariant, expressed as a function of the two invariant frequencies that parametrize the orbit. Our results are in good agreement with results from post-Newtonian calculations in the weak field regime, as we shall report elsewhere. The results of our study can inform the development of analytical models for the dynamics of strongly-gravitating binaries. They also provide an accurate benchmark for future numerical-relativistic simulations.

#### Jan 20, 2011

1101.3331 (/preprints)
2011-01-20, 09:23 

## [1101.3591] Measuring parameters of massive black hole binaries with partially aligned spins

Authors: Ryan N. Lang, Scott A. Hughes, Neil J. Cornish

Date: 19 Jan 2011

Abstract: The future space-based gravitational wave detector LISA will be able to measure parameters of coalescing massive black hole binaries, often to extremely high accuracy. Previous work has demonstrated that the black hole spins can have a strong impact on the accuracy of parameter measurement. Relativistic spin-induced precession modulates the waveform in a manner which can break degeneracies between parameters, in principle significantly improving how well they are measured. Recent studies have indicated, however, that spin precession may be weak for an important subset of astrophysical binary black holes: those in which the spins are aligned due to interactions with gas. In this paper, we examine how well a binary's parameters can be measured when its spins are partially aligned and compare results using waveforms that include higher post-Newtonian harmonics to those that are truncated at leading quadrupole order. We find that the weakened precession can substantially degrade parameter estimation, particularly for the "extrinsic" parameters sky position and distance. Absent higher harmonics, LISA typically localizes the sky position of a nearly aligned binary about an order of magnitude less accurately than one for which the spin orientations are random. Our knowledge of a source's sky position will thus be worst for the gas-rich systems which are most likely to produce electromagnetic counterparts. Fortunately, higher harmonics of the waveform can make up for this degradation. By including harmonics beyond the quadrupole in our waveform model, we find that the accuracy with which most of the binary's parameters are measured can be substantially improved. In some cases, the improvement is such that they are measured almost as well as when the binary spins are randomly aligned.

#### Jan 20, 2011

1101.3591 (/preprints)
2011-01-20, 09:19 

## [1101.3555] On the connection between Newtonian simulations and General Relativity

Authors: Nora Elisa Chisari, Matias Zaldarriaga

Date: 18 Jan 2011

Abstract: On large scales, comparable to the horizon, the observable clustering properties of galaxies are affected by various General Relativistic effects. To calculate these effects one needs to consistently solve for the metric, densities and velocities in a specific coordinate system or gauge. The method of choice for simulating large scale structure is numerical N-body simulations which are performed in the Newtonian limit. Even though one might worry that the use of the Newtonian approximation would make it impossible to use these simulations to compute properties on very large scales we show that the simulations are still solving the dynamics correctly even for long modes and give formulas to obtain the position of particles in the Newtonian gauge given the positions computed in the simulation. We also give formulas to convert from the output coordinates of N-body simulations to the observable coordinates of the particles.

#### Jan 20, 2011

1101.3555 (/preprints)
2011-01-20, 09:18 

## [1101.2901] Cataclysmic Variables: Eight Breakthroughs in Eight Years

Authors: Christian Knigge (University of Southampton)

Date: 14 Jan 2011

Abstract: The last few years have seen tremendous progress in our understanding of cataclysmic variable stars. As a result, we are finally developing a much clearer picture of their evolution as binary systems, the physics of the accretion processes powering them, and their relation to other compact accreting objects. In this review, I will highlight some of the most exciting recent breakthroughs. Several of these have opened up completely new avenues of research that will probably lead to additional major advances over the next decade.

#### Jan 18, 2011

1101.2901 (/preprints)
2011-01-18, 12:31 

## [1101.2762] The astrophysical gravitational wave stochastic background

Authors: Tania Regimbau

Date: 14 Jan 2011

Abstract: A gravitational wave stochastic background of astrophysical origin may have resulted from the superposition of a large number of unresolved sources since the beginning of stellar activity. Its detection would put very strong constrains on the physical properties of compact objects, the initial mass function or the star formation history. On the other hand, it could be a 'noise' that would mask the stochastic background of cosmological origin. We review the main astrophysical processes able to produce a stochastic background and discuss how it may differ from the primordial contribution by its statistical properties. Current detection methods are also presented.

#### Jan 18, 2011

1101.2762 (/preprints)
2011-01-18, 12:31 

## [1101.3096] Status and Prospects of Fermi LAT Pulsar Blind Searches

Authors: P. M. Saz Parkinson (for the Fermi-LAT Collaboration)

Date: 16 Jan 2011

Abstract: Blind Searches of Fermi Large Area Telescope (LAT) data have resulted in the discovery of 24 gamma-ray pulsars in the first year of survey operations, most of which remain undetected in radio, despite deep radio follow-up searches. I summarize the latest Fermi LAT blind search efforts and results, including the discovery of a new Geminga-like pulsar, PSR J0734-1559. Finally, I discuss some of the challenges faced in carrying out these searches into the future, as well as the prospects for finding additional pulsars among the large number of LAT unassociated sources.

#### Jan 18, 2011

1101.3096 (/preprints)
2011-01-18, 12:31 

## [1101.3242] GMSW as a detector of gravitational waves

Authors: Yu.G. Ignatyev

Date: 17 Jan 2011

Abstract: The effect of an excitation of a gravitational wave (GW) on shock waves in a highly magnetized plasma, GMSW, is studied as an effective means for the detection of GW radiated by neutron stars. It is shown that there is every reason to identify the giant impulses of the pulsar NP 0532 with GMSW.

#### Jan 18, 2011

1101.3242 (/preprints)
2011-01-18, 12:30 

## [1101.0811] Measuring the Spins of Accreting Black Holes

Authors: Jeffrey E. McClintock, Ramesh Narayan, Shane W. Davis, Lijun Gou, Akshay Kulkarni, Jerome A. Orosz, Robert F. Penna, Ronald A. Remillard, James F. Steiner

Date: 4 Jan 2011

Abstract: A typical galaxy is thought to contain tens of millions of stellar-mass black holes, the collapsed remnants of once massive stars, and a single nuclear supermassive black hole. Both classes of black holes accrete gas from their environments. The accreting gas forms a flattened orbiting structure known as an accretion disk. During the past several years, it has become possible to obtain measurements of the spins of the two classes of black holes by modeling the X-ray emission from their accretion disks. Two methods are employed, both of which depend upon identifying the inner radius of the accretion disk with the innermost stable circular orbit (ISCO), whose radius depends only on the mass and spin of the black hole. In the Fe K method, which applies to both classes of black holes, one models the profile of the relativistically-broadened iron line with a special focus on the gravitationally redshifted red wing of the line. In the continuum-fitting method, which has so far only been applied to stellar-mass black holes, one models the thermal X-ray continuum spectrum of the accretion disk. We discuss both methods, with a strong emphasis on the continuum-fitting method and its application to stellar-mass black holes. Spin results for eight stellar-mass black holes are summarized. These data are used to argue that the high spins of at least some of these black holes are natal, and that the presence or absence of relativistic jets in accreting black holes is not entirely determined by the spin of the black hole.

#### Jan 18, 2011

1101.0811 (/preprints)
2011-01-18, 12:30 

## [1101.2921] Non-Spinning Black Holes in Alternative Theories of Gravity

Authors: Nicolas Yunes, Leo C. Stein

Date: 14 Jan 2011

Abstract: We study two large classes of alternative theories, modifying the action through algebraic, quadratic curvature invariants coupled to scalar fields. We find one class that admits solutions that solve the vacuum Einstein equations and another that does not. In the latter, we find a deformation to the Schwarzschild metric that solves the modified field equations in the small coupling approximation. We calculate the event horizon shift, the innermost stable circular orbit shift, and corrections to gravitational waves, mapping them to the parametrized post-Einsteinian framework.

#### Jan 18, 2011

1101.2921 (/preprints)
2011-01-18, 12:29 

## [1101.2925] Effective source approach to self-force calculations

Authors: Ian Vega, Barry Wardell, Peter Diener

Date: 14 Jan 2011

Abstract: Numerical evaluation of the self-force on a point particle is made difficult by the use of delta functions as sources. Recent methods for self-force calculations avoid delta functions altogether, using instead a finite and extended "effective source" for a point particle. We provide a review of the general principles underlying this strategy, using the specific example of a scalar point charge moving in a black hole spacetime. We also report on two new developments: (i) the construction and evaluation of an effective source for a scalar charge moving along a generic orbit of an arbitrary spacetime, and (ii) the successful implementation of hyperboloidal slicing that significantly improves on previous treatments of boundary conditions used for effective-source-based self-force calculations. Finally, we identify some of the key issues related to the effective source approach that will need to be addressed by future work.

#### Jan 18, 2011

1101.2925 (/preprints)
2011-01-18, 12:29 

## [1101.1969] Electromagnetic extraction of energy from black hole-neutron star binaries

Authors: Sean T. McWilliams, Janna Levin

Date: 10 Jan 2011

Abstract: The coalescence of black hole-neutron star binaries is expected to be a principal source of gravitational waves for the next generation of detectors, Advanced LIGO and Advanced Virgo. Ideally, these and other gravitational wave sources would have a distinct electromagnetic counterpart, as significantly more information could be gained through two separate channels. In addition, since these detectors will probe distances with non-negligible redshift, a coincident observation of an electromagnetic counterpart to a gravitational wave signal would facilitate a novel measurement of dark energy [1]. For black hole masses not much larger than the neutron star mass, the tidal disruption and subsequent accretion of the neutron star by the black hole provides one avenue for generating an electromagnetic counterpart [2]. However, in this work, we demonstrate that, for all black hole-neutron star binaries observable by Advanced LIGO/Virgo, the interaction of the black hole with the magnetic field of the neutron star will drive a Poynting flux. This Poynting flux generates synchrotron/curvature radiation as the electron-positron plasma in the neutron star magnetosphere is accel- erated, and thermal radiation as the plasma is focused onto the neutron star magnetic poles, creating a "hot spot" on the neutron star surface. This novel effect will gener- ate copious luminosity, comparable to supernovae and active galactic nuclei, so that black hole-neutron star coalescences detectable with gravitational waves by Advanced LIGO/Virgo could also potentially be detectable electromagnetically.

#### Jan 15, 2011

1101.1969 (/preprints)
2011-01-15, 12:10 

## [1101.2324] Are Time-Domain Self-Force Calculations Contaminated by Jost Solutions?

Authors: Jose Luis Jaramillo (1 and 2), Carlos F. Sopuerta (3), Priscilla Canizares (3) ((1) AEI, (2) LUTH, (3) ICE, CSIC-IEEC)

Date: 12 Jan 2011

Abstract: The modeling of the gravitational-wave emission from extreme-mass-ratio inspirals is crucial for their detection and analysis with the future space-based observatory LISA. The inspiral can be described as the action of a local force, the self-force, determined by the gravitational perturbations created by the small object, described as a point mass, on the background geometry. The calculation of the self-force is a challenging task that requires the control of sources of error such as spurious modes or numerical noise. Here we address the question of the possible emergence of a persistent spurious solution in time-domain schemes, referred to as a Jost junk solution in the literature, that may contaminate self-force calculations. Previous studies suggested that Jost solutions are due to the use of zero initial data, which is inconsistent with the singular sources associated with the point mass. However, in this work we show that the specific origin is an inconsistency in the translation of the singular sources into jump conditions. More importantly, we identify the correct implementation of the sources at late times as the sufficient condition guaranteeing the absence of Jost junk solutions.

#### Jan 15, 2011

1101.2324 (/preprints)
2011-01-15, 12:09 

## [1101.2526] Tuning Time-Domain Pseudospectral Computations of the Self-Force on a Charged Scalar Particle

Authors: Priscilla Canizares, Carlos F. Sopuerta (ICE, CSIC-IEEC)

Date: 13 Jan 2011

Abstract: The computation of the self-force constitutes one of the main challenges for the construction of precise theoretical waveform templates in order to detect and analyze extreme-mass-ratio inspirals with the future space-based gravitational-wave observatory LISA. Since the number of templates required is quite high, it is important to develop fast algorithms both for the computation of the self-force and the production of waveforms. In this article we show how to tune a recent time-domain technique for the computation of the self-force, what we call the Particle without Particle scheme, in order to make it very precise and at the same time very efficient. We also extend this technique in order to allow for highly eccentric orbits.

#### Jan 15, 2011

1101.2526 (/preprints)
2011-01-15, 12:08 

## [1101.1673] Post-1-Newtonian tidal effects in the gravitational waveform from binary inspirals

Authors: Justin Vines, Tanja Hinderer, &#xc9;anna &#xc9;. Flanagan

Date: 9 Jan 2011

Abstract: The gravitational wave signal from an inspiralling binary neutron star system will contain detailed information about tidal coupling in the system, and thus, about the internal physics of the neutron stars. To extract this information will require highly accurate models for the gravitational waveform. We present here a calculation of the gravitational wave signal from a binary with quadrupolar tidal interactions which includes all post-1-Newtonian-order effects in both the conservative dynamics and wave generation. We consider stars with adiabatically induced quadrupoles moving in circular orbits, and work to linear in the stars' quadrupole moments. We find that post-1-Newtonian corrections increase the tidal signal by approximately 20% at gravitational wave frequencies of 400 Hz.

#### Jan 11, 2011

1101.1673 (/preprints)
2011-01-11, 13:40 

## [1101.1459] A coherent triggered search for single spin compact binary coalescences in gravitational wave data

Authors: Ian Harry, Stephen Fairhurst

Date: 7 Jan 2011

Abstract: In this paper we present a method for conducting a coherent search for single spin compact binary coalescences in gravitational wave data and compare this search to the existing coincidence method for single spin searches. We propose a method to characterize the regions of the parameter space where the single spin search, both coincident and coherent, will increase detection efficiency over the existing non-precessing search. We also show example results of the coherent search on a stretch of data from LIGO's fourth science run but note that a set of signal based vetoes will be needed before this search can be run to try to make detections.

#### Jan 09, 2011

1101.1459 (/preprints)
2011-01-09, 23:00 

## [1101.1391] Testing General Relativity using Bayesian model selection: Applications to observations of gravitational waves from compact binary systems

Authors: Walter Del Pozzo, John Veitch, Alberto Vecchio

Date: 7 Jan 2011

Abstract: Second generation interferometric gravitational wave detectors, such as Advanced LIGO and Advanced Virgo, are expected to begin operation by 2015. Such instruments plan to reach sensitivities that will offer the unique possibility to test General Relativity in the dynamical, strong field regime and investigate departures from its predictions, in particular using the signal from coalescing binary systems. We introduce a statistical framework based on Bayesian model selection in which the Bayes factor between two competing hypotheses measures which theory is favored by the data. Probability density functions of the model parameters are then used to quantify the inference on individual parameters. We also develop a method to combine the information coming from multiple independent observations of gravitational waves, and show how much stronger inference could be. As an introduction and illustration of this framework - and a practical numerical implementation through the Monte Carlo integration technique of nested sampling - we apply it to gravitational waves from the inspiral phase of coalescing binary systems as predicted by General Relativity and a very simple alternative theory in which the graviton has a non-zero mass. This method can trivially (and should) be extended to more realistic and physically motivated theories.

#### Jan 09, 2011

1101.1391 (/preprints)
2011-01-09, 23:00 

## [1012.5111] Forced motion near black holes

Authors: Jonathan R. Gair, Eanna E. Flanagan, Steve Drasco, Tanja Hinderer, Stanislav Babak

Date: 22 Dec 2010

Abstract: We present two methods for integrating forced geodesic equations in the Kerr spacetime, which can accommodate arbitrary forces. As a test case, we compute inspirals under a simple drag force, mimicking the presence of gas. We verify that both methods give the same results for this simple force. We find that drag generally causes eccentricity to increase throughout the inspiral. This is a relativistic effect qualitatively opposite to what is seen in gravitational-radiation-driven inspirals, and similar to what is observed in hydrodynamic simulations of gaseous binaries. We provide an analytic explanation by deriving the leading order relativistic correction to the Newtonian dynamics. If observed, an increasing eccentricity would provide clear evidence that the inspiral was occurring in a non-vacuum environment. Our two methods are especially useful for evolving orbits in the adiabatic regime. Both use the method of osculating orbits, in which each point on the orbit is characterized by the parameters of the geodesic with the same instantaneous position and velocity. Both methods describe the orbit in terms of the geodesic energy, axial angular momentum, Carter constant, azimuthal phase, and two angular variables that increase monotonically and are relativistic generalizations of the eccentric anomaly. The two methods differ in their treatment of the orbital phases and the representation of the force. In one method the geodesic phase and phase constant are evolved together as a single orbital phase parameter, and the force is expressed in terms of its components on the Kinnersley orthonormal tetrad. In the second method, the phase constants of the geodesic motion are evolved separately and the force is expressed in terms of its Boyer-Lindquist components. This second approach is a generalization of earlier work by Pound and Poisson for planar forces in a Schwarzschild background.

#### Jan 07, 2011

1012.5111 (/preprints)
2011-01-07, 22:18 

## [1101.0584] Composite gravitational-wave detection of compact binary coalescence

Authors: Kipp Cannon, Chad Hanna, Drew Keppel, Antony C. Searle

Date: 3 Jan 2011

Abstract: The detection of gravitational waves from compact binaries relies on a computationally burdensome processing of gravitational-wave detector data. The parameter space of compact-binary-coalescence gravitational waves is large and optimal detection strategies often require nearly redundant calculations. Previously, it has been shown that singular value decomposition of search filters removes redundancy. Here we will demonstrate the use of singular value decomposition for a composite detection statistic. This can greatly improve the prospects for a computationally feasible rapid detection scheme across a large compact binary parameter space.

#### Jan 07, 2011

1101.0584 (/preprints)
2011-01-07, 22:18 

## [1101.0063] Detection of Gravitational Waves through Observations of a Group of Pulsars

Authors: Alexander E. Rodin

Date: 30 Dec 2010

Abstract: We suggest a new approach to the detection of gravitational waves using observations of a group of millisecond pulsars. In contrast to the usual method, based on increasing the accuracy of the arrival times of pulses by excluding possible distorting factors, our method supposes that the additive phase noise that is inevitably present even in the most accurate observational data has various spectral components, which have characteristic amplitudes and begin to appear on different time scales. We use the "Caterpillar" (Singular Spectral Analysis, SSA) method to decompose the signal into its components. Our initial data are the residuals of the pulse arrival times for six millisecond pulsars. We constructed the angular correlation function for components of the decomposition of a given number, whose theoretical form for the case of an isotropic and homogeneous gravitational-wave background is known. The individual decomposition components show a statistically significant agreement with the theoretical expectations (correlation coefficient $\rho=0.92\pm 0.10$).

#### Jan 07, 2011

1101.0063 (/preprints)
2011-01-07, 22:18 

## [1011.2842] Testing gravity using the growth of large scale structure in the Universe

Authors: Elise Jennings, Carlton M. Baugh, Silvia Pascoli

Date: 12 Nov 2010

Abstract: Future galaxy surveys hope to distinguish between the dark energy and modified gravity scenarios for the accelerating expansion of the Universe using the distortion of clustering in redshift space. The aim is to model the form and size of the distortion to infer the rate at which large scale structure grows. We test this hypothesis and assess the performance of current theoretical models for the redshift space distortion using large volume N-body simulations of the gravitational instability process. We simulate competing cosmological models which have identical expansion histories - one is a quintessence dark energy model with a scalar field and the other is a modified gravity model with a time varying gravitational constant - and demonstrate that they do indeed produce different redshift space distortions. This is the first time this approach has been verified using a technique that can follow the growth of structure at the required level of accuracy. Our comparisons show that theoretical models for the redshift space distortion based on linear perturbation theory give a surprisingly poor description of the simulation results. Furthermore, the application of such models can give rise to catastrophic systematic errors leading to incorrect interpretation of the observations. We show that an improved model is able to extract the correct growth rate. Further enhancements to theoretical models of redshift space distortions, calibrated against simulations, are needed to fully exploit the forthcoming high precision clustering measurements.

#### Jan 07, 2011

1011.2842 (/preprints)
2011-01-07, 22:18 

## [1101.0996] The Carter Constant for Inclined Orbits About a Massive Kerr Black Hole: II. near-circular, near-polar orbits

Authors: P. G. Komorowski, S. R. Valluri, M. Houde

Date: 5 Jan 2011

Abstract: In an extreme mass ratio binary black hole system, a non-equatorial orbit will list (i.e. increase its angle of inclination, ${\iota}$) as it evolves in Kerr spacetime. The abutment, a set of evolving near-polar retrograde orbits for which the instantaneous Carter constant (${Q)}$ is at its maximum value (${Q}_{X}$), for given values of latus rectum (${\tilde{l}}$) and eccentricity (${e}$), has been introduced as a device by which the consistency of $dQ/dt$ with corresponding evolution equations for $d\tilde{l}% /dt$ and $de/dt$ might be tested, and as a means of elucidating second-order effects on the listing rate of the orbital angle of inclination, $\partial {% \iota}/\partial {t}$ (independently of a specific radiation back-reaction model). Our present work expands upon these two uses.

#### Jan 07, 2011

1101.0996 (/preprints)
2011-01-07, 22:18 

## [1101.0986] The evolution of white dwarfs with a varying gravitational constant

Authors: L. G. Althaus, A. H. Corsico, S. Torres, P. Loren-Aguilar, J. Isern, E. Garcia-Berro

Date: 5 Jan 2011

Abstract: Within the theoretical framework of some modern unification theories the constants of nature are functions of cosmological time. White dwarfs offer the possibility of testing a possible variation of G and, thus, to place constraints to these theories. We present full white dwarf evolutionary calculations in the case that G decreases with time. White dwarf evolution is computed in a self-consistent way, including the most up-to-date physical inputs, non-gray model atmospheres and a detailed core chemical composition that results from the calculation of the full evolution of progenitor stars. We find that the mechanical structure and the energy balance of white dwarfs are strongly modified by the presence of a varying G. In particular, for certain values of the rate of change of G, the evolution of cool white dwarfs is markedly affected. The impact of a varying G is more notorious in the case of more massive white dwarfs. In view of the recent results reporting that a very accurate white dwarf cooling age can be derived for the old and metal-rich open cluster NGC 6791, our study suggests that this cluster could be a potential target to constrain or detect a ypothetical secular variation of G.

#### Jan 07, 2011

1101.0986 (/preprints)
2011-01-07, 22:18 

## [1012.5968] From laboratory experiments to LISA Pathfinder: achieving LISA geodesic motion

Authors: F Antonucci, M Armano, H Audley, G Auger, M Benedetti, P Binetruy, C Boatella, J Bogenstahl, D Bortoluzzi, P Bosetti, N Brandt, M Caleno, A Cavalleri, M Cesa, M Chmeissani, G Ciani, A Conchillo, G Congedo, I Cristofolini, M Cruise, K Danzmann, F De Marchi, M Diaz-Aguilo, I Diepholz, G Dixon, R Dolesi, N Dunbar, J Fauste, L Ferraioli, D Fertin, W Fichter, E Fitzsimons, M Freschi, A Garc&#xed;a Marin, C Garc&#xed;a Marirrodriga, R Gerndt, L Gesa, D Giardini, F Gibert, C Grimani, A Grynagier, B Guillaume, F Guzm&#xe1;n, I Harrison, G Heinzel, M Hewitson, D Hollington, J Hough, D Hoyland, M Hueller, J Huesler, O Jeannin, O Jennrich, P Jetzer, B Johlander, C Killow, X Llamas, I Lloro, A Lobo, R Maarschalkerweerd, S Madden, D Mance, I Mateos, P W McNamara, J Mendest&#xec;, E Mitchell, A Monsky, D Nicolini, D Nicolodi, M Nofrarias, F Pedersen, M Perreur-Lloyd, A Perreca, E Plagnol, P Prat, G D Racca, B Rais, J Ramos-Castro, J Reiche, J A Romera Perez, D Robertson, H Rozemeijer, J Sanjuan, A Schleicher, M Schulte, D Shaul, L Stagnaro, S Strandmoe, F Steier, T J Sumner, A Taylor, D Texier, C Trenkel, D Tombolato, S Vitale, G Wanner, H Ward, S Waschke, P Wass, W J Weber, P Zweifel

Date: 29 Dec 2010

Abstract: This paper presents a quantitative assessment of the performance of the upcoming LISA Pathfinder geodesic explorer mission. The findings are based on the results of extensive ground testing and simulation campaigns using flight hardware and flight control and operations algorithms. The results show that, for the central experiment of measuring the stray differential acceleration between the LISA test masses, LISA Pathfinder will be able to verify the overall acceleration noise to within a factor two of the LISA requirement at 1 mHz and within a factor 10 at 0.1 mHz. We also discuss the key elements of the physical model of disturbances, coming from LISA Pathfinder and ground measurement, that will guarantee the LISA performance.

#### Jan 04, 2011

1012.5968 (/preprints)
2011-01-04, 13:34 

## [1012.5860] A Fast Frequency-Domain Algorithm for Gravitational Self-Force: I, Circular Orbits in Schwarzschild Spacetime

Authors: Sarp Akcay

Date: 29 Dec 2010

Abstract: Fast, reliable orbital evolutions of compact objects around massive black holes will be needed as input for gravitational wave search algorithms in the data stream generated by the planned Laser Interferometer Space Antenna (LISA). Currently, the state of the art is a time-domain code by [Phys. Rev. D{\bf 81}, 084021, (2010)] that computes the gravitational self-force on a point-particle in an eccentric orbit around a Schwarzschild black hole. Currently, time-domain codes take up to a few days to compute just one point in parameter space. In a series of articles, we advocate the use of a frequency-domain approach to the problem of gravitational self-force (GSF) with the ultimate goal of orbital evolution in mind. Here, we compute the GSF for a particle in a circular orbit in Schwarzschild spacetime. We solve the linearized Einstein equations for the metric perturbation in Lorenz gauge. Our frequency-domain code reproduces the time-domain results for the GSF up to $\sim 1000$ times faster for small orbital radii. In forthcoming companion papers, we will generalize our frequency-domain methods to include bound (eccentric) orbits in Schwarzschild and (eventually) Kerr spacetimes for computing the GSF, where we will employ the method of extended homogeneous solutions [Phys. Rev. D {\bf 78}, 084021 (2008)].

#### Jan 04, 2011

1012.5860 (/preprints)
2011-01-04, 13:32