**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.

1109.4839
(/preprints)

2011-09-30, 11:56
**[edit]**

**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.

1109.5128
(/preprints)

2011-09-30, 11:55
**[edit]**

**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.

1109.4978
(/preprints)

2011-09-30, 11:52
**[edit]**

**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.

1109.5685
(/preprints)

2011-09-30, 11:52
**[edit]**

**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.

1109.5384
(/preprints)

2011-09-30, 11:52
**[edit]**

**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.

1109.5642
(/preprints)

2011-09-30, 11:51
**[edit]**

**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.

1109.5177
(/preprints)

2011-09-30, 11:50
**[edit]**

**Authors**: R. O'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.

1109.5224
(/preprints)

2011-09-30, 11:50
**[edit]**

**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.

1109.6312
(/preprints)

2011-09-30, 11:49
**[edit]**

**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.

1109.6201
(/preprints)

2011-09-30, 11:48
**[edit]**

**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.

1109.6339
(/preprints)

2011-09-30, 11:47
**[edit]**

**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.

1109.6324
(/preprints)

2011-09-30, 11:45
**[edit]**

**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.

1109.6329
(/preprints)

2011-09-30, 11:45
**[edit]**

**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.

1109.6021
(/preprints)

2011-09-29, 15:57
**[edit]**

**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.

1109.4585
(/preprints)

2011-09-22, 17:13
**[edit]**

**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.

1109.4256
(/preprints)

2011-09-22, 16:03
**[edit]**

**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.

1109.3707
(/preprints)

2011-09-22, 16:02
**[edit]**

**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.

1109.4170
(/preprints)

2011-09-22, 16:02
**[edit]**

**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.

1109.3611
(/preprints)

2011-09-19, 10:31
**[edit]**

**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.

1109.3506
(/preprints)

2011-09-19, 10:31
**[edit]**

**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.

1109.3498
(/preprints)

2011-09-19, 10:30
**[edit]**

**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.

1109.3492
(/preprints)

2011-09-19, 10:29
**[edit]**

**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.

1109.3402
(/preprints)

2011-09-16, 10:23
**[edit]**

**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.

1109.2284
(/preprints)

2011-09-14, 13:46
**[edit]**

**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.

1109.2744
(/preprints)

2011-09-14, 13:45
**[edit]**

**Authors**: Brian D. Metzger, Edo Berger

**Date**: 30 Aug 2011

**Abstract**: The final inspiral of double neutron star and neutron star-black hole binaries are likely to be detected by advanced networks of ground-based gravitational wave (GW) interferometers. Maximizing the science returns from such a discovery will require the identification and localization of an electromagnetic (EM) counterpart. Here we critically evaluate and compare several possible counterparts, including short-duration gamma-ray bursts (SGRBs), "orphan" optical and radio afterglows, and ~day-long optical transients powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta ("kilonovae"). We assess the promise of each counterpart in terms of four "Cardinal Virtues": detectability, high fraction, identifiability, and positional accuracy. Taking into account the search strategy for typical error regions of ~10s degs sq., we conclude that SGRBs are the most useful to confirm the cosmic origin of a few GW events, and to test the association with NS mergers. However, for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events, kilonovae are instead preferred. Off-axis optical afterglows will be detectable for at most ~10% of all events, while radio afterglows are promising only for the unique combination of energetic relativistic ejecta in a high density medium, and even then will require hundreds of hours of EVLA time per event. Our main recommendations are:(i) an all-sky gamma-ray satellite is essential for temporal coincidence detections, and for GW searches of gamma-ray triggered events; (ii) LSST should adopt a 1-day cadence follow-up strategy, ideally with ~0.5 hr per pointing to cover GW error regions (the standard 4-day cadence and depth will severely limit the probability of a unique identification); and (iii) radio searches should only focus on the relativistic case, which requires observations for a few months.

1108.6056
(/preprints)

2011-09-09, 07:00
**[edit]**

**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.

1108.5161
(/preprints)

2011-09-09, 06:59
**[edit]**

**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.

1108.5618
(/preprints)

2011-09-09, 06:58
**[edit]**

**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.]

1108.5486
(/preprints)

2011-09-09, 06:57
**[edit]**

**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.

1109.0081
(/preprints)

2011-09-09, 06:55
**[edit]**

**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.

1109.0068
(/preprints)

2011-09-09, 06:55
**[edit]**

**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.

1109.0501
(/preprints)

2011-09-09, 06:54
**[edit]**

**Authors**: Christian Röver

**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.

1109.0442
(/preprints)

2011-09-09, 06:53
**[edit]**

**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.

1109.0928
(/preprints)

2011-09-09, 06:53
**[edit]**

**Authors**: Carlos F. Sopuerta, Nicolás 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.

1109.0572
(/preprints)

2011-09-09, 06:51
**[edit]**

**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.

1109.0542
(/preprints)

2011-09-09, 06:51
**[edit]**

**Authors**: R. O'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.

1109.1050
(/preprints)

2011-09-09, 06:50
**[edit]**

**Authors**: Han Wang, Jan Steinhoff, Jing Zeng, Gerhard Schäfer

**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.

1109.1182
(/preprints)

2011-09-09, 06:50
**[edit]**

**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.

1108.5175
(/preprints)

2011-09-09, 06:48
**[edit]**

**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.

1108.5588
(/preprints)

2011-09-09, 06:48
**[edit]**

**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.

1108.5174
(/preprints)

2011-09-09, 06:48
**[edit]**

© M. Vallisneri 2012 — last modified on 2010/01/29

*Tantum in modicis, quantum in maximis*