**Authors**: Lijing Shao, Norbert Wex, Michael Kramer

**Date**: 24 Sep 2012

**Abstract**: Preferred frame effects (PFEs) are predicted by a number of alternative gravity theories which include vector or additional tensor fields, besides the canonical metric tensor. In the framework of parametrized post-Newtonian (PPN) formalism, we investigate PFEs in the orbital dynamics of binary pulsars, characterized by the two strong-field PPN parameters, \alpha_1 and \alpha_2. In the limit of a small orbital eccentricity, \alpha_1 and \alpha_2 contributions decouple. By utilizing recent radio timing results and optical observations of PSRs J1012+5307 and J1738+0333, we obtained the best limits of \alpha_1 and \alpha_2 in the strong-field regime. The constraint on \alpha_1 also surpasses its counterpart in the weak-field regime.

1209.5171
(/preprints)

2012-09-28, 09:54
**[edit]**

**Authors**: A. Passamonti, E. Gaertig, K. Kokkotas

**Date**: 24 Sep 2012

**Abstract**: We study the dynamical evolution of the gravitational-wave driven instability of the f-mode in rapidly rotating relativistic stars. With an approach based on linear perturbation theory we describe the evolution of the mode amplitude and follow the trajectory of a newborn neutron star through its instability window. The influence on the f-mode instability of the magnetic field and the presence of an unstable r-mode is also considered. Two different configurations are studied in more detail; a standard N = 1 polytrope with a typical mass and radius and a more extreme polytropic N = 2/3 model which describes a supramassive neutron star. We study several evolutions with different initial rotation rates and temperature and determine the gravitational waves radiated during the instability. For reasonable values of the mode saturation amplitude, i.e. with a mode energy of about 1e6 Msun cˆ2, the gravitational-wave signal can be detected by the Einstein Telescope detector from the Virgo cluster. The magnetic field affects the evolution and then the detectability of the gravitational radiation when its strength is higher than 1e12 G, while the effects of an unstable r-mode become dominant when this mode reaches the maximum saturation value allowed by non-linear mode couplings. However, the relative saturation amplitude of the f- and r-modes must be known more accurately in order to provide a definitive answer to this issue. From the thermal evolution we find also that the heat generated by shear viscosity during the saturation phase completely balances the neutrino cooling and prevents the star from entering the regime of mutual friction. The evolution time of the instability is therefore longer and the star loses significantly larger amounts of angular momentum via gravitational waves.

1209.5308
(/preprints)

2012-09-28, 09:54
**[edit]**

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

**Date**: 25 Sep 2012

**Abstract**: An extreme-mass-ratio burst (EMRB) is a gravitational wave signal emitted when a compact object passes through periapsis on a highly eccentric orbit about a much more massive body, in our case a stellar mass object about the 4.31 \times 10ˆ6 M_sol massive black hole (MBH) in the Galactic Centre. We investigate how EMRBs could constrain the parameters of the Galaxy's MBH. EMRBs should be detectable if the periapsis is r_p < 65 r_g for a \mu = 10 M_sol orbiting object, where r_g = GM/cˆ2 is the gravitational radius. The signal-to-noise ratio \rho scales like log(\rho) = -2.7 log(r_p/r_g) + log(\mu/M_sol) + 4.9. For periapses smaller than ~ 10 r_g, EMRBs can be informative, providing good constraints on both the MBH's mass and spin.

1209.5731
(/preprints)

2012-09-28, 09:53
**[edit]**

**Authors**: Koutarou Kyutoku, Kunihito Ioka, Masaru Shibata

**Date**: 25 Sep 2012

**Abstract**: We propose a possibility of ultra-relativistic electromagnetic counterparts to gravitational waves from binary neutron star mergers at any viewing angle. Mechanisms include the merger-shock propagation accelerating a smaller fraction of the neutron star surface to larger Lorentz factor, with the outer parts carrying less energy, ~10ˆ{47} \Gammaˆ{-1} erg. This mechanism is difficult to resolve by current 3D numerical simulations, and depends on the neutron star equation of state. The outflows emit synchrotron flares for seconds to days by shocking the surrounding medium. Ultra-relativistic flares shine at early times and high energies, potentially detectable by current X-ray to radio instruments, such as Swift XRT and Pan-STARRS, and even in low ambient density ~10ˆ{-2} cmˆ{-3} by EVLA. The flares probe the merger position and time, and the merger types as black hole-neutron star outflows would be non/mildly-relativistic.

1209.5747
(/preprints)

2012-09-28, 09:52
**[edit]**

**Authors**: Matthew Adams, Neil Cornish, Tyson Littenberg

**Date**: 27 Sep 2012

**Abstract**: Theoretical studies in gravitational wave astronomy have mostly focused on the information that can be extracted from individual detections, such as the mass of a binary system and its location in space. Here we consider how the information from multiple detections can be used to constrain astrophysical population models. This seemingly simple problem is made challenging by the high dimensionality and high degree of correlation in the parameter spaces that describe the signals, and by the complexity of the astrophysical models, which can also depend on a large number of parameters, some of which might not be directly constrained by the observations. We present a method for constraining population models using a Hierarchical Bayesian modeling approach which simultaneously infers the source parameters and population model and provides the joint probability distributions for both. We illustrate this approach by considering the constraints that can be placed on population models for galactic white dwarf binaries using a future space based gravitational wave detector. We find that a mission that is able to resolve ~5000 of the shortest period binaries will be able to constrain the population model parameters, including the chirp mass distribution and a characteristic galaxy disk radius to within a few percent. This compares favorably to existing bounds, where electromagnetic observations of stars in the galaxy constrain disk radii to within 20%.

1209.6286
(/preprints)

2012-09-28, 09:48
**[edit]**

**Authors**: Alessandra Buonanno, Guillaume Faye, Tanja Hinderer

**Date**: 27 Sep 2012

**Abstract**: We calculate the gravitational waveform for spinning, precessing compact binary inspirals through second post-Newtonian order in the amplitude. When spins are collinear with the orbital angular momentum and the orbits are quasi-circular, we further provide explicit expressions for the gravitational-wave polarizations and the decomposition into spin-weighted spherical-harmonic modes. Knowledge of the second post-Newtonian spin terms in the waveform could be used to improve the physical content of analytical templates for data analysis of compact binary inspirals and for more accurate comparisons with numerical-relativity simulations.

1209.6349
(/preprints)

2012-09-28, 09:48
**[edit]**

**Authors**: George Pappas, Theocharis A. Apostolatos

**Date**: 27 Sep 2012

**Abstract**: We have tested the appropriateness of two-soliton analytic metric to describe the exterior of all types of neutron stars, no matter what their equation of state or rotation rate is. The particular analytic solution of the vaccuum Einstein equations proved quite adjustable to mimic the metric functions of all numerically constructed neutron-star models that we used as a testbed. The neutron-star models covered a wide range of stiffness, with regard to the equation of state of their interior, and all rotation rates up to the maximum possible rotation rate allowed for each such star. Apart of the metric functions themselves, we have compared the radius of the innermost stable circular orbit $R_{\rm{ISCO}}$, the orbital frequency $\Omega\equiv\frac{d\phi}{dt}$ of circular geodesics, and their epicyclic frequencies $\Omega_{\rho}, \Omega_z$, as well as the change of the energy of circular orbits per logarithmic change of orbital frequency $\Delta\tilde{E}$. All these quantities, calculated by means of the two-soliton analytic metric, fitted with good accuracy the corresponding numerical ones as in previous analogous comparisons (although previous attempts were restricted to neutron star models with either high or low rotation rates). We believe that this particular analytic solution could be considered as an analytic faithful representation of the gravitation field of any rotating neutron star with such accuracy, that one could explore the interior structure of a neutron star by using this space-time to interpret observations of astrophysical processes that take place around it.

1209.6148
(/preprints)

2012-09-28, 09:47
**[edit]**

**Authors**: Liang Dai, Marc Kamionkowski, Donghui Jeong

**Date**: 4 Sep 2012

**Abstract**: Most calculations in cosmological perturbation theorydecompose those perturbations into plane waves (Fourier modes). However, for some calculations, particularly those involving observations performed on a spherical sky, a decomposition into waves of fixed total angular momentum (TAM) may be more appropriate. Here we introduce TAM waves, solutions of fixed total angular momentum to the Helmholtz equation, for three-dimensional scalar, vector, and tensor fields. The vector TAM waves of given total angular momentum can be decomposed further into a set of three basis functions of fixed orbital angular momentum (OAM), a set of fixed helicity, or a basis consisting of a longitudinal (L) and two transverse (E and B) TAM waves. The symmetric traceless rank-2 tensor TAM waves can be similarly decomposed into a basis of fixed OAM or fixed helicity, or a basis that consists of a longitudinal (L), two vector (VE and VB, of opposite parity), and two tensor (TE and TB, of opposite parity) waves. We show how all of the vector and tensor TAM waves can be obtained by applying derivative operators to scalar TAM waves. This operator approach then allows one to decompose a vector field into three covariant scalar fields for the L, E, and B components and symmetric-traceless-tensor fields into five covariant scalar fields for the L, VE, VB, TE, and TB components. We provide projections of the vector and tensor TAM waves onto vector and tensor spherical harmonics. We provide calculational detail to facilitate the assimilation of this formalism into cosmological calculations. As an example, we calculate the power spectra of the deflection angle for gravitational lensing by density perturbations and by gravitational waves. We comment on an alternative approach to CMB fluctuations based on TAM waves. Our work may have applications elsewhere in field theory and in general relativity.

1209.0761
(/preprints)

2012-09-21, 10:53
**[edit]**

**Authors**: Paolo Pani, Vitor Cardoso, Leonardo Gualtieri, Emanuele Berti, Akihiro Ishibashi

**Date**: 4 Sep 2012

**Abstract**: We discuss a general method to study linear perturbations of slowly rotating black holes which is valid for any perturbation field, and particularly advantageous when the field equations are not separable. As an illustration of the method we investigate massive vector (Proca) perturbations in the Kerr metric, which do not appear to be separable in the standard Teukolsky formalism. Working in a perturbative scheme, we discuss two important effects induced by rotation: a Zeeman-like shift of nonaxisymmetric quasinormal modes and bound states with different azimuthal number m, and the coupling between axial and polar modes with different multipolar index l. We explicitly compute the perturbation equations up to second order in rotation, but in principle the method can be extended to any order. Working at first order in rotation we show that polar and axial Proca modes can be computed by solving two decoupled sets of equations, and we derive a single master equation describing axial perturbations of spin s=0 and s=+-1. By extending the calculation to second order we can study the superradiant regime of Proca perturbations in a self-consistent way. For the first time we show that Proca fields around Kerr black holes exhibit a superradiant instability, which is significantly stronger than for massive scalar fields. Because of this instability, astrophysical observations of spinning black holes provide the tightest upper limit on the mass of the photon: mv<4x10ˆ{-20} eV under our most conservative assumptions. Spin measurements for the largest black holes could reduce this bound to mv<10ˆ{-22} eV or lower.

1209.0773
(/preprints)

2012-09-21, 10:52
**[edit]**

**Authors**: Sarp Akcay, Leor Barack, Thibault Damour, Norichika Sago

**Date**: 5 Sep 2012

**Abstract**: We compute the conservative piece of the gravitational self-force (GSF) acting on a particle of mass $m_1$ as it moves along an (unstable) circular geodesic orbit between the innermost stable circular orbit (ISCO) and the light ring of a Schwarzschild black hole of mass $m_2\gg m_1$. More precisely, we construct the function $h_{uu}(x) \equiv h_{\mu\nu} uˆ{\mu} uˆ{\nu}$ (related to Detweiler's gauge-invariant "redshift" variable), where $h_{\mu\nu}$ is the regularized metric perturbation in the Lorenz gauge, $uˆ{\mu}$ is the four-velocity of $m_1$, and $x\equiv [Gcˆ{-3}(m_1+m_2)\Omega]ˆ{2/3}$ is an invariant coordinate constructed from the orbital frequency $\Omega$. In particular, we explore the behavior of $h_{uu}$ just outside the "light ring" at $x=1/3$, where the circular orbit becomes null. Using the recently discovered link between $h_{uu}$ and the piece $a(u)$, linear in the symmetric mass ratio $nu$, of the main radial potential $A(u,\nu)$ of the Effective One Body (EOB) formalism, we compute $a(u)$ over the entire domain $0<u<1/3$ (extending previous results for $u\leq 1/5$). We find that $a(u)$ {\it diverges} like $\approx 0.25 (1-3u)ˆ{-½}$ at the light-ring limit, explain the physical origin of this divergent behavior, and discuss its consequences for the EOB formalism. We construct accurate global analytic fits for $a(u)$, valid on the entire domain $0<u<1/3$ (and possibly beyond), and give accurate numerical estimates of the values of $a(u)$ and its first 3 derivatives at the ISCO. In previous work we used GSF data on slightly eccentric orbits to compute a certain linear combination of $a(u)$ and its first two derivatives, involving also the $O(\nu)$ piece $\bar d(u)$ of a second EOB radial potential ${\bar D}(u,\nu)$. Combining these results with our present global analytic representation of $a(u)$, we numerically compute $\bar d(u)$ on the interval $0<u\leq 1/6$.

1209.0964
(/preprints)

2012-09-21, 10:52
**[edit]**

**Authors**: Bruno Giacomazzo, Rosalba Perna

**Date**: 4 Sep 2012

**Abstract**: Neutron stars (NSs) in the astrophysical Universe are often surrounded by accretion disks. Accretion of matter onto a NS may increase its mass above the maximum value allowed by its equation of state, inducing its collapse to a black hole (BH). Here we study this process for the first time, in 3D, and in full general relativity. By considering three initial NS configurations, each with and without a surrounding disk (of mass ~7% M_{NS}), we investigate the effect of the accretion disk on the dynamics of the collapse and its imprint on both the gravitational wave (GW) and electromagnetic (EM) signals that can be emitted by these sources. We show in particular that, even if the GW signal is similar for the accretion induced collapse (AIC) and the collapse of a NS in vacuum (and detectable only for Galactic sources), the EM counterpart could allow to discriminate between these two types of events. In fact, our simulations show that, while the collapse of a NS in vacuum leaves no appreciable baryonic matter outside the event horizon, an AIC is followed by a phase of rapid accretion of the surviving disk onto the newly formed BH. The post-collapse accretion rates, on the order of ~10ˆ{-2} M_{sun} sˆ{-1}, make these events tantalizing candidates as engines of short Gamma-Ray Bursts.

1209.0783
(/preprints)

2012-09-21, 10:52
**[edit]**

**Authors**: E. Bon, P. Jovanović, P. Marziani, A. I. Shapovalova, N. Bon, V. Borka Jovanović, D. Borka, J. Sulentic, L. Č. Popović

**Date**: 20 Sep 2012

**Abstract**: One of the most intriguing scenarios proposed to explain how active galactic nuclei are triggered involves the existence of a supermassive binary black hole system in their cores. Here we present an observational evidence for the first spectroscopically resolved sub-parsec orbit of a such system in the core of Seyfert galaxy NGC 4151. Using a method similar to those typically applied for spectroscopic binary stars we obtained radial velocity curves of the supermassive binary system, from which we calculated orbital elements and made estimates about the masses of components. Our analysis shows that periodic variations in the light and radial velocity curves can be accounted for an eccentric, sub-parsec Keplerian orbit of a 15.9-year period. The flux maximum in the lightcurve correspond to the approaching phase of a secondary component towards the observer. According to the obtained results we speculate that the periodic variations in the observed H{\alpha} line shape and flux are due to shock waves generated by the supersonic motion of the components through the surrounding medium. Given the large observational effort needed to reveal this spectroscopically resolved binary orbital motion we suggest that many such systems may exist in similar objects even if they are hard to find. Detecting more of them will provide us with insight into black hole mass growth process.

1209.4524
(/preprints)

2012-09-21, 10:30
**[edit]**

**Authors**: Lijing Shao, Norbert Wex

**Date**: 20 Sep 2012

**Abstract**: In the post-Newtonian parametrization of semi-conservative gravity theories, local Lorentz invariance (LLI) violation is characterized by two parameters, alpha_1 and alpha_2. In binary pulsars the isotropic violation of LLI in the gravitational sector leads to characteristic preferred frame effects (PFEs) in the orbital dynamics, if the barycenter of the binary is moving relative to the preferred frame with a velocity w. For small-eccentricity binaries, the effects induced by alpha_1 and alpha_2 decouple, and can therefore be tested independently. We use recent timing results of two compact pulsar-white dwarf binaries with known 3D velocity, PSRs J1012+5307 and J1738+0333, to constrain PFEs for strongly self-gravitating bodies. We derive a limit |alpha_2| < 1.8e-4 (95% CL), which is the most constraining limit for strongly self-gravitating systems up to now. Concerning alpha_1, we propose a new, robust method to constrain this parameter. Our most conservative result, alpha_1 = -0.4ˆ{+3.7}_{-3.1} e-5 (95% CL) from PSR J1738+0333, constitutes a significant improvement compared to current most stringent limits obtained both in Solar system and binary pulsar tests. We also derive corresponding limits for alpha_1 and alpha_2 for a preferred frame that is at rest with respect to our Galaxy, and preferred frames that locally co-move with the rotation of our Galaxy. (Abridged)

1209.4503
(/preprints)

2012-09-21, 10:29
**[edit]**

**Authors**: Hee-Suk Cho, Evan Ochsner, Richard O'Shaughnessy, Chunglee Kim, Chang-Hwan Lee

**Date**: 20 Sep 2012

**Abstract**: Inspiralling black hole-neutron star (BH-NS) binaries emit a complicated gravitational wave signature, produced by multiple harmonics sourced by their strong local gravitational field and further modulated by the orbital plane's precession. Some features of this complex signal are easily accessible to ground-based interferometers (e.g., the rate of change of frequency); others less so (e.g., the polarization content); and others unavailable (e.g., features of the signal out of band). For this reason, an ambiguity function (a diagnostic of dissimilarity) between two such signals varies on many parameter scales and ranges. In this paper, we present a method for computing an approximate, effective Fisher matrix from variations in the ambiguity function on physically pertinent scales which depend on the relevant signal to noise ratio. As a concrete example, we explore how higher harmonics improve parameter measurement accuracy. As previous studies suggest, for our fiducial BH-NS binaries and for plausible signal amplitudes, we see that higher harmonics at best marginally improve our ability to measure parameters. For non-precessing binaries, these Fisher matrices separate into intrinsic (mass, spin) and extrinsic (geometrical) parameters; higher harmonics principally improve our knowledge about the line of sight. For the precessing binaries, the extra information provided by higher harmonics is distributed across several parameters. We provide concrete estimates for measurement accuracy, using coordinates adapted to the precession cone in the detector's sensitive band.

1209.4494
(/preprints)

2012-09-21, 10:29
**[edit]**

**Authors**: Nicholas Stone, Abraham Loeb, Edo Berger

**Date**: 18 Sep 2012

**Abstract**: The precise origin of short gamma ray bursts (SGRBs) remains an important open question in relativistic astrophysics. Increasingly, observational evidence suggests the merger of a binary compact object system as the source for most SGRBs, but it is currently unclear how to distinguish observationally between a binary neutron star progenitor and a black hole-neutron star progenitor. We suggest the quasi-periodic signal of jet precession as an observational signature of SGRBs originating in mixed binary systems, and quantify both the fraction of mixed binaries capable of producing SGRBs, and the distributions of precession amplitudes and periods. The difficulty inherent in disrupting a neutron star outside the horizon of a stellar-mass black hole biases the jet precession signal towards low amplitude and high frequency. Precession periods of ~ 0.01-0.1 s and disk-BH spin misalignments ~10 degrees are generally expected, although sufficiently high viscosity may prevent the accumulation of multiple precession periods during the SGRB. The precessing jet will naturally cover a larger solid angle in the sky than would standard SGRB jets, enhancing observability for both prompt emission and optical afterglows.

1209.4097
(/preprints)

2012-09-20, 09:28
**[edit]**

**Authors**: Eanna E. Flanagan, Scott A. Hughes, Uchupol Ruangsri

**Date**: 20 Aug 2012

**Abstract**: The inspiral of a stellar mass ($1 - 100\,M_\odot$) compact body into a massive ($10ˆ5 - 10ˆ7\,M_\odot$) black hole has been a focus of much effort, both for the promise of such systems as astrophysical sources of gravitational waves, and because they are a clean limit of the general relativistic two-body problem. Our understanding of this problem has advanced significantly in recent years, with much progress in modeling the "self force" arising from the small body's interaction with its own spacetime deformation. Recent work has shown that this self interaction is especially interesting when the frequencies associated with the orbit's $\theta$ and $r$ motions are in an integer ratio: $\Omega_\theta/\Omega_r = \beta_\theta/\beta_r$, with $\beta_\theta$ and $\beta_r$ both integers. In this paper, we show that key aspects of the self interaction for such "resonant" orbits can be understood with a relatively simple Teukolsky-equation-based calculation of gravitational-wave fluxes. We show that fluxes from resonant orbits depend on the relative phase of radial and angular motions. The purpose of this paper is to illustrate in simple terms how this phase dependence arises using tools that are good for strong-field orbits, and to present a first study of how strongly the fluxes vary as a function of this phase and other orbital parameters. Future work will use the full dissipative self force to examine resonant and near resonant strong-field effects in greater depth.

1208.3906
(/preprints)

2012-09-19, 18:07
**[edit]**

**Authors**: Roman Gold, Bernd Bruegmann

**Date**: 18 Sep 2012

**Abstract**: We perform a parameter study of non-spinning, equal and unequal mass black hole binaries on generic, eccentric orbits in numerical relativity. The linear momentum considered ranges from that of a circular orbit to ten times that value. We discuss the different manifestations of zoom-whirl behavior in the hyperbolic and the elliptic regime. The hyperbolic data set applies to dynamical capture scenarios (e.g. in globular clusters). Evolutions in the elliptic regime correspond to possible end states of supermassive black hole binaries. We spot zoom-whirl behavior for eccentricities as low as $e\sim0.5$, i.e. within the expected range of eccentricities in massive black hole binaries from galaxy mergers and binaries near galactic centers. The resulting gravitational waveforms reveal a rich structure, which will effectively break degeneracies in parameter space improving parameter estimation.

1209.4085
(/preprints)

2012-09-19, 18:07
**[edit]**

**Authors**: Clifford M. Will

**Date**: 20 Aug 2012

**Abstract**: We obtain approximate analytic expressions for the critical value of the total angular momentum of a non-relativistic test particle moving in the Kerr geometry, such that it will be captured by the black hole. The expressions apply to arbitrary orbital inclinations, and are accurate over the entire range of angular momentum for the Kerr black hole. The expressions can be easily implemented in N-body simulations of the evolution of star clusters around massive galactic black holes, where such captures play an important role.

1208.3931
(/preprints)

2012-09-19, 18:06
**[edit]**

**Authors**: Craig Hogan

**Date**: 17 Aug 2012

**Abstract**: All existing experimental results are currently interpreted using classical geometry. However, there are theoretical reasons to suspect that at a deeper level, geometry emerges as an approximate macroscopic behavior of a quantum system at the Planck scale. If directions in emergent quantum geometry do not commute, new quantum-geometrical degrees of freedom can produce detectable macroscopic deviations from classicality: spatially coherent, transverse position indeterminacy between any pair of world lines, with a displacement amplitude much larger than the Planck length. Positions of separate bodies are entangled with each other, and undergo quantum-geometrical fluctuations that are not describable as metric fluctuations or gravitational waves. These fluctuations can either be cleanly identified or ruled out using interferometers. A Planck-precision test of the classical coherence of space-time on a laboratory scale is now underway at Fermilab.

1208.3703
(/preprints)

2012-09-19, 18:06
**[edit]**

**Authors**: Alberto Sesana, Nicola Sartore, Bernadetta Devecchi, Andrea Possenti

**Date**: 21 Aug 2012

**Abstract**: Intermediate mass black holes (IMBHs) are among the most elusive objects in contemporary astrophysics. Both theoretical and observational evidence of their existence is subject of debate. Conversely, both theory and observations confirm the presence of a large population of millisecond pulsars (MSPs) with low mass companions residing in globular cluster (GC) centers. If IMBHs are common in GC centers as well, then dynamical interactions will inevitably break up many of these binaries, causing the ejection of several fast MSPs in the Galactic halo. Such population of fast halo MSPs, hard to produce with 'standard' MSP generation mechanisms, would provide a strong, albeit indirect, evidence of the presence of a substantial population of IMBHs in GCs. In this paper we study in detail the dynamical formation and evolution of such fast MSPs population, highlighting the relevant observational properties and assessing detection prospects with forthcoming radio surveys.

1208.4365
(/preprints)

2012-09-19, 18:05
**[edit]**

**Authors**: Kazuhiro Hayama, Atsushi Nishizawa

**Date**: 22 Aug 2012

**Abstract**: The observation of gravitational waves with a global network of interferometric detectors such as advanced LIGO, advanced Virgo, and KAGRA will make it possible to probe into the nature of space-time structure. Besides Einstein's general theory of relativity, there are several theories of gravitation that passed experimental tests so far. The gravitational-wave observation provides a new experimental test of alternative theories of gravity because a gravitational wave may have at most six independent modes of polarization, of which properties and number of modes are dependent on theories of gravity. This paper proposes a method to reconstruct the independent modes of polarization in time-series data of an advanced detector network. Since the method does not rely on any specific model, it gives model-independent test of alternative theories of gravity.

1208.4596
(/preprints)

2012-09-19, 18:04
**[edit]**

**Authors**: Domènec Espriu, Daniel Puigdomènech

**Date**: 17 Sep 2012

**Abstract**: We have considered the propagation of gravitational waves (GW) in de Sitter space time and how a non-zero value of the cosmological constant might affect their detection in pulsar timing arrays (PTA). If \Lambda{} is different from zero waves are non-linear in Friedmann-Robertson-Walker coordinates and although the amount of non-linearity is very small it gives noticeable effects for GW originating in extragalactic sources such as spiraling black hole binaries. The results indicate that the timing residuals induced by gravitational waves from such sources in PTA would show a peculiar angular dependence with a marked enhancement around a particular value of the angle subtended by the source and the pulsars, depending mainly on the actual value of the cosmological constant and the distance to the source. The position of the peak could represent a gauge of the value of \Lambda. The enhancement that the new effect brings about could facilitate the first direct detection of gravitational waves while representing a local measurement of \Lambda.

1209.3724
(/preprints)

2012-09-19, 18:03
**[edit]**

**Authors**: R. O'Shaughnessy (1), L. London (2), J. Healy (2), D. Shoemaker (2) ((1) University of Wisconsin-Milwaukee, (2) Center for Relativistic Astrophysics, Georgia Tech)

**Date**: 17 Sep 2012

**Abstract**: The short gravitational wave signal from the merger of compact binaries encodes a surprising amount of information about the strong-field dynamics of merger into frequencies accessible to ground-based interferometers. In this paper we describe a previously-unknown "precession" of the peak emission direction with time, both before and after the merger, about the total angular momentum direction. We demonstrate the gravitational wave polarization encodes the orientation of this direction to the line of sight. We argue the effects of polarization can be estimated nonparametrically, directly from the gravitational wave signal as seen along one line of sight, as a slowly-varying feature on top of a rapidly-varying carrier. After merger, our results can be interpreted as a coherent excitation of quasinormal modes of different angular orders, a superposition which naturally "precesses" and modulates the line-of-sight amplitude. Recent analytic calculations have arrived at a similar geometric interpretation. We suspect the line-of-sight polarization content will be a convenient observable with which to define new high-precision tests of general relativity using gravitational waves. Additionally, as the nonlinear merger process seeds the initial coherent perturbation, we speculate the amplitude of this effect provides a new probe of the strong-field dynamics during merger. To demonstrate the ubiquity of the effects we describe, we summarize the post-merger evolution of 104 generic precessing binary mergers. Finally, we provide estimates for the detectable impacts of precession on the waveforms from high-mass sources. These expressions may identify new precessing binary parameters whose waveforms are dissimilar from the existing sample.

1209.3712
(/preprints)

2012-09-19, 18:02
**[edit]**

**Authors**: John F. Donoghue

**Date**: 16 Sep 2012

**Abstract**: This is a pedagogical introduction to the treatment of quantum general relativity as an effective field theory. It starts with an overview of the methods of effective field theory and includes an explicit example. Quantum general relativity matches this framework and I discuss gravitational examples as well as the limits of the effective field theory. I also discuss the insights from effective field theory on the gravitational effects on running couplings in the perturbative regime.

1209.3511
(/preprints)

2012-09-19, 18:02
**[edit]**

**Authors**: Lorenzo De Vittori, Philippe Jetzer, Antoine Klein

**Date**: 18 Sep 2012

**Abstract**: Unbound interacting compact binaries emit gravitational radiation in a wide frequency range. Since short burst-like signals are expected in future detectors, such as LISA or advanced LIGO, it is interesting to study their energy spectrum and the position of the frequency peak. Here we derive them for a system of massive objects interacting on hyperbolic orbits within the quadrupole approximation, following the work of Capozziello et al. In particular, we focus on the derivation of an analytic formula for the energy spectrum of the emitted waves. Within numerical approximation our formula is in agreement with the two known limiting cases: for the eccentricity {\epsilon} = 1, the parabolic case, whose spectrum was computed by Berry and Gair, and the large {\epsilon} limit with the formula given by Turner.

1209.3986
(/preprints)

2012-09-19, 18:01
**[edit]**

**Authors**: Jean-Francois Dufaux

**Date**: 18 Sep 2012

**Abstract**: We review cosmological backgrounds of gravitational waves with a particular attention to the scientific potential of the eLISA/NGO mission. After an overview of cosmological backgrounds and detectors, we consider different cosmological sources that could lead to an observable signal. We then study the backgrounds produced by first-order phase transitions and networks of cosmic strings, assessing the prospects for their detection.

1209.4024
(/preprints)

2012-09-19, 18:01
**[edit]**

**Authors**: William E. East, Frans Pretorius

**Date**: 27 Aug 2012

**Abstract**: We study dynamical capture, binary neutron star mergers as may arise in dense stellar regions such as globular clusters. Using general-relativistic hydrodynamics we find that these mergers can result in the prompt collapse to a black hole or in the formation of a hypermassive neutron star, depending not only on the neutron star equation of state, but also on impact parameter. We also find that these mergers can produce accretion disks of up to a tenth of a solar mass, and unbound ejected material of up to a few percent of a solar mass. We comment on the gravitational radiation and electromagnetic transients that these sources may produce.

1208.5279
(/preprints)

2012-09-17, 13:45
**[edit]**

**Authors**: J. J. Hermes, Mukremin Kilic, Warren R. Brown, D. E. Winget, Carlos Allende Prieto, A. Gianninas, Anjum S. Mukadam, Antonio Cabrera-Lavers, Scott J. Kenyon

**Date**: 24 Aug 2012

**Abstract**: We report the detection of orbital decay in the 12.75-min, detached binary white dwarf (WD) SDSS J065133.338+284423.37 (hereafter J0651). Our photometric observations over a 13-month baseline constrain the orbital period to 765.206543(55) s and indicate the orbit is decreasing as a rate of (-9.8 +/- 2.8) x 10ˆ(-12) s/s (or -0.31 +/- 0.09 ms/yr). We revise the system parameters based on our new photometric and spectroscopic observations: J0651 contains two WDs with M1 = 0.26 +/- 0.04 Msun and M2 = 0.50 +/- 0.04 Msun. General relativity predicts orbital decay due to gravitational wave radiation of (-8.2 +/- 1.7) x 10ˆ(-12) s/s (or -0.26 +/- 0.05 ms/yr). Our observed rate of orbital decay is consistent with this expectation. J0651 is currently the second-loudest gravitational wave source known in the milli-Hertz range and the loudest non-interacting binary, which makes it an excellent verification source for future missions aimed at directly detecting gravitational waves. Our work establishes the feasibility of monitoring this system's orbital period decay at optical wavelengths.

1208.5051
(/preprints)

2012-09-17, 13:45
**[edit]**

**Authors**: László Á. Gergely, Peter L. Biermann

**Date**: 26 Aug 2012

**Abstract**: We prove that merging supermassive black holes (SMBHs) typically have neither equal masses, nor is their mass ratio too extreme. The majority of such mergers fall into the mass ratio range of 1:30 to 1:3, implying a spin flip during the inspiral. We also present a simple expression for the final spin $\chi_{f}$ of the emerging SMBH, as function of the mass ratio, initial spin magnitudes, and orientation of the spins with respect to the orbital plane and each other. This formula approximates well more cumbersome expressions obtained from the fit with numerical simulations. By integrating over all equally likely orientations for precessing mergers we determine a lower approximant to the final spin distribution as function of the mass ratio alone. By folding this with the derived mass ratio dependent merger rate we derive a lower bound to the typical final spin value after mergers. We repeat the procedure deriving an upper bound for the typical spin in the case when the spins are aligned to the orbital angular momentum, such that there is no precession in the system. Both slopes of $\chi_{f}$ as function of the initial spins being smaller than one lead to two attractors at $\chi_{f}ˆ{prec}=0.2$ and $\chi_{f}ˆ{align}=0.45$, respectively. Real mergers, biased toward partial alignment by interactions with the environment (accretion, host galaxy, etc.) would generate a typical final spin lying between these two limiting values. These are the typical values of the spin after the merger, starting from which the spin can built up by further gaseous accretion.

1208.5251
(/preprints)

2012-09-17, 13:45
**[edit]**

**Authors**: Nathan K. Johnson-McDaniel, Benjamin J. Owen

**Date**: 26 Aug 2012

**Abstract**: We present a method for calculating the maximum elastic quadrupolar deformations of relativistic stars, generalizing the previous Newtonian, Cowling approximation integral given by [G. Ushomirsky et al., Mon. Not. R. Astron. Soc. 319, 902 (2000)]. (We also present a method for Newtonian gravity with no Cowling approximation.) We apply these methods to the m = 2 quadrupoles most relevant for gravitational radiation in three cases: crustal deformations, deformations of crystalline cores of hadron-quark hybrid stars, and deformations of entirely crystalline color superconducting quark stars. In all cases, we find suppressions of the quadrupole due to relativity compared to the Newtonian Cowling approximation, particularly for compact stars. For the crust these suppressions are up to a factor ~6, for hybrid stars they are up to ~4, and for solid quark stars they are at most ~2, with slight enhancements instead for low mass stars. We also explore ranges of masses and equations of state more than in previous work, and find that for some parameters the maximum quadrupoles can still be very large. Even with the relativistic suppressions, we find that 1.4 solar mass stars can sustain crustal quadrupoles of a few times 10ˆ39 g cmˆ2 for the SLy equation of state or close to 10ˆ40 g cmˆ2 for equations of state that produce less compact stars. Solid quark stars of 1.4 solar masses can sustain quadrupoles of around 10ˆ44 g cmˆ2. Hybrid stars typically do not have solid cores at 1.4 solar masses, but the most massive ones (~2 solar masses) can sustain quadrupoles of a few times 10ˆ41 g cmˆ2 for typical microphysical parameters and a few times 10ˆ42 g cmˆ2 for extreme ones. All of these quadrupoles assume a breaking strain of 0.1 and can be divided by 10ˆ45 g cmˆ2 to yield the fiducial "ellipticities" quoted elsewhere.

1208.5227
(/preprints)

2012-09-17, 13:45
**[edit]**

**Authors**: Hans-Jürgen Schmidt

**Date**: 26 Aug 2012

**Abstract**: We classify the existent Birkhoff-type theorems into four classes: First, in field theory, the theorem states the absence of helicity 0- and spin 0-parts of the gravitational field. Second, in relativistic astrophysics, it is the statement that the gravitational far-field of a spherically symmetric star carries, apart from its mass, no information about the star; therefore, a radially oscillating star has a static gravitational far-field. Third, in mathematical physics, Birkhoff's theorem reads: up to singular exceptions of measure zero, the spherically symmetric solutions of Einstein's vacuum field equation with Lambda = 0 can be expressed by the Schwarzschild metric; for Lambda unequal 0, it is the Schwarzschild-de Sitter metric instead. Fourth, in differential geometry, any statement of the type: every member of a family of pseudo-Riemannian space-times has more isometries than expected from the original metric ansatz, carries the name Birkhoff-type theorem. Within the fourth of these classes we present some new results with further values of dimension and signature of the related spaces; including them are some counterexamples: families of space-times where no Birkhoff-type theorem is valid. These counterexamples further confirm the conjecture, that the Birkhoff-type theorems have their origin in the property, that the two eigenvalues of the Ricci tensor of two-dimensional pseudo-Riemannian spaces always coincide, a property not having an analogy in higher dimensions. Hence, Birkhoff-type theorems exist only for those physical situations which are reducible to two dimensions.

1208.5237
(/preprints)

2012-09-17, 13:44
**[edit]**

**Authors**: Kent Yagi, Nicolas Yunes, Takahiro Tanaka

**Date**: 25 Aug 2012

**Abstract**: Dynamical Chern-Simons gravity cannot be strongly constrained with current experiments because it reduces to General Relativity in the weak-field limit. This theory, however, introduces modifications in the non-linear, dynamical regime, and thus, it could be greatly constrained with gravitational waves from the late inspiral of black hole binaries. We complete the first self-consistent calculation of such gravitational waves in this theory. We find that future gravitational wave detectors could place constraints that are 6 orders of magnitude stronger than current ones.

1208.5102
(/preprints)

2012-09-17, 13:44
**[edit]**

**Authors**: A. Emir Gumrukcuoglu, Sachiko Kuroyanagi, Chunshan Lin, Shinji Mukohyama, Norihiro Tanahashi

**Date**: 29 Aug 2012

**Abstract**: We discuss the detectability of gravitational waves with a time dependent mass contribution, by means of the stochastic gravitational wave observations. Such a mass term typically arises in the cosmological solutions of massive gravity theories. We conduct the analysis based on a general quadratic action, and thus the results apply universally to any massive gravity theories in which modification of general relativity appears primarily in the tensor modes. The primary manifestation of the modification in the gravitational wave spectrum is a sharp peak. The position and height of the peak carry information on the present value of the mass term, as well as the duration of the inflationary stage. We also discuss the detectability of such a gravitational wave signal using the future-planned gravitational wave observatories.

1208.5975
(/preprints)

2012-09-17, 13:43
**[edit]**

**Authors**: Anıl Zenginoğlu, Gaurav Khanna, Lior M. Burko

**Date**: 29 Aug 2012

**Abstract**: The numerical investigation of wave propagation in the asymptotic domain of Kerr spacetime has only recently been possible thanks to the construction of suitable hyperboloidal coordinates. The asymptotics revealed a puzzle in the decay rates of scalar fields: the late-time rates seemed to depend on whether finite distance observers are in the strong field domain or far away from the rotating black hole. In this paper we study late-time decay rates using a horizon-penetrating, hyperboloidal slicing with transmitting layers attached to a compact domain in Boyer--Lindquist coordinates. The technical construction of transmitting layers for Kerr spacetime should be useful in future studies of wave propagation. We discuss splitting of local decay rates in certain projected modes and in the full field. The splitting in the full field rates is explained by competition between projected modes. For the splitting in certain projected modes we argue that, asymptotically in time, the strong field rates are valid at all finite distances, but at any given late time, there are three domains with different local decay rates whose boundaries move during evolution.

1208.5839
(/preprints)

2012-09-17, 13:42
**[edit]**

**Authors**: Francesco Pannarale

**Date**: 29 Aug 2012

**Abstract**: We present a model for determining the dimensionless spin parameter and mass of the black hole remnant of black hole-neutron star mergers with parallel orbital angular momentum and initial black hole spin. This approach is based on the Buonanno, Kidder, and Lehner method for binary black holes and it is successfully tested against the results of numerical-relativity simulations: the dimensionless spin parameter is predicted with absolute error $\lesssim 0.02$, whereas the relative error on the final mass is $\lesssim 2$%, its distribution being pronouncedly peaked at 1%. Our approach and the fit to the torus remnant mass reported in Foucart (2012) thus constitute an easy-to-use analytical model that accurately describes the remnant of BH-NS mergers. We investigate the space of parameters consisting of the binary mass ratio, the initial black hole spin, and the neutron star mass and equation of state. We provide indirect support to the cosmic censorship conjecture for black hole remnants of black hole-neutron star mergers. We show that the presence of a neutron star affects the quasi-normal mode frequency of the black hole remnant, thus suggesting that the ringdown epoch of the gravitational wave signal may virtually be used to (1) distinguish binary black hole from black hole-neutron star mergers and to (2) constrain the neutron star equation of state.

1208.5869
(/preprints)

2012-09-17, 13:42
**[edit]**

**Authors**: Louis Gallouin, Hiroyuki Nakano, Nicolas Yunes, Manuela Campanelli

**Date**: 31 Aug 2012

**Abstract**: We construct a closed-form, fully analytical 4-metric that approximately represents the spacetime evolution of non-precessing, spinning black hole binaries from infinite separations up to a few orbits prior to merger. We employ the technique of asymptotic matching to join a perturbed Kerr metric in the neighborhood of each spinning black hole to a near-zone, post-Newtonian metric farther out. The latter is already naturally matched to a far-zone, post-Minkowskian metric that accounts for full temporal retardation. The result is a 4-metric that is approximately valid everywhere in space and in a small bundle of spatial hypersurfaces. We here restrict our attention to quasi- circular orbits, but the method is valid for any orbital motion or physical scenario, provided an overlapping region of validity or buffer zone exists. A simple extension of such a metric will allow for future studies of the accretion disk and jet dynamics around spinning back hole binaries.

1208.6489
(/preprints)

2012-09-17, 13:41
**[edit]**

**Authors**: David Merritt, Eugene Vasiliev

**Date**: 30 Aug 2012

**Abstract**: The spin angular momentum S of a supermassive black hole (SBH) precesses due to torques from orbiting stars, and the stellar orbits precess due to dragging of inertial frames by the spinning hole. We solve the coupled post-Newtonian equations describing the joint evolution of S and the stellar angular momenta Lj, j = 1…N in spherical, rotating nuclear star clusters. In the absence of gravitational interactions between the stars, two evolutionary modes are found: (1) nearly uniform precession of S about the total angular momentum vector of the system; (2) damped precession, leading, in less than one precessional period, to alignment of S with the angular momentum of the rotating cluster. Beyond a certain distance from the SBH, the time scale for angular momentum changes due to gravitational encounters between the stars is shorter than spin-orbit precession times. We present a model, based on the Ornstein-Uhlenbeck equation, for the stochastic evolution of star clusters due to gravitational encounters and use it to evaluate the evolution of S in nuclei where changes in the Lj are due to frame dragging close to the SBH and to encounters farther out. Long-term evolution in this case is well described as uniform precession of the SBH about the cluster's rotational axis, with an increasingly important stochastic contribution when SBH masses are small. Spin precessional periods are predicted to be strongly dependent on nuclear properties, but typical values are 10-100 Myr for low-mass SBHs in dense nuclei, 100 Myr - 10 Gyr for intermediate mass SBHs, and > 10 Gyr for the most massive SBHs. We compare the evolution of SBH spins in stellar nuclei to the case of torquing by an inclined, gaseous accretion disk.

1208.6274
(/preprints)

2012-09-17, 13:40
**[edit]**

**Authors**: Kipp Cannon, Chad Hanna, Drew Keppel

**Date**: 4 Sep 2012

**Abstract**: Coalescing compact binary systems consisting of neutron stars and/or black holes should be detectable with upcoming advanced gravitational-wave detectors such as LIGO, Virgo, GEO and {KAGRA}. Gravitational-wave experiments to date have been riddled with non-Gaussian, non-stationary noise that makes it challenging to ascertain the significance of an event. A popular method to estimate significance is to time shift the events collected between detectors in order to establish a false coincidence rate. Here we propose a method for estimating the false alarm probability of events using variables commonly available to search candidates that does not rely on explicitly time shifting the events while still capturing the non-Gaussianity of the data. We present a method for establishing a statistical detection of events in the case where several silver-plated (3--5$\sigma$) events exist but not necessarily any gold-plated ($>5\sigma$) events. We use LIGO data and a simulated, realistic, blind signal population to test our method.

1209.0718
(/preprints)

2012-09-17, 13:39
**[edit]**

**Authors**: Mathieu Beau (STP-DIAS)

**Date**: 4 Sep 2012

**Abstract**: We propose a vectorial field theory (G) based on a so-called \textit{gravitational induction principle} saying that a density current of accelerated particles generate a vectorial field $G_\mu$. The aim of this article is to give a first approach of this theory. We fist construct a Lagrangian $L_G=-mcˆ2 G_\mu(x)\ddot{x}ˆ\mu$ (where $m$ is the mass of the particle, $\ddot{x}ˆ\mu$ its acceleration) analogous to the electrodynamic Lagrangian $L_{ED}=-q A_\mu(x)\dot{x}ˆ\mu$ (where $q$ is the charge of the particle, $\dot{x}ˆ\mu$ its velocity) and then we give the analogous field equations based on the induction principle. Using these field equations, we establish new kind of gravitomagnetic equations (GEM-G) valid for small velocities, analogous to the gravitomagnetic field equation (GEM) proved by the general theory of the relativity (RG). Then, we will propose a first approach about the description of the vectorial field theory (G) in the framework of the general relativity theory (RG), we will give the field equations and we will discuss the conceptual consequences of a theory of gravitation with both different fields.

1209.0611
(/preprints)

2012-09-17, 13:39
**[edit]**

**Authors**: A. Fienga

**Date**: 4 Sep 2012

**Abstract**: We review here the tests of fundamental physics based on the dynamics of solar system objects.

1209.0635
(/preprints)

2012-09-17, 13:39
**[edit]**

**Authors**: Paolo Pani, Vitor Cardoso, Leonardo Gualtieri, Emanuele Berti, Akihiro Ishibashi

**Date**: 3 Sep 2012

**Abstract**: Generic extensions of the standard model predict the existence of ultralight bosonic degrees of freedom. Several ongoing experiments are aimed at detecting these particles or constraining their mass range. Here we show that massive vector fields around rotating black holes can give rise to a strong superradiant instability which extracts angular momentum from the hole. The observation of supermassive spinning black holes imposes limits on this mechanism. We show that current supermassive black hole spin estimates provide the tightest upper limits on the mass of the photon (mv<4x10ˆ{-20} eV according to our most conservative estimate), and that spin measurements for the largest known supermassive black holes could further lower this bound to mv<10ˆ{-22} eV. Our analysis relies on a novel framework to study perturbations of rotating Kerr black holes in the slow-rotation regime, that we developed up to second order in rotation, and that can be extended to other spacetime metrics and other theories.

1209.0465
(/preprints)

2012-09-17, 13:38
**[edit]**

**Authors**: Xing-Jiang Zhu, Eric J. Howell, David G. Blair, Zong-Hong Zhu

**Date**: 4 Sep 2012

**Abstract**: This paper reports a comprehensive study on the gravitational wave (GW) background from compact binary coalescences. We consider in our calculations newly available observation-based neutron star and black hole mass distributions and complete analytical waveforms that include post-Newtonian amplitude corrections. Our results show that: (i) post-Newtonian effects cause a small reduction in the GW background signal; (ii) below 100 Hz the background depends primarily on the local coalescence rate $r_0$ and the average chirp mass and is independent of the chirp mass distribution; (iii) the effects of cosmic star formation rates and delay times between the formation and merger of binaries are linear below 100 Hz and can be represented by a single parameter within a factor of ~ 2; (iv) a simple power law model of the energy density parameter $\Omega_{GW}(f) ~ fˆ{2/3}$ up to 50-100 Hz is sufficient to be used as a search template for ground-based interferometers. In terms of the detection prospects of the background signal, we show that: (i) detection (a signal-to-noise ratio of 3) within one year of observation by the Advanced LIGO detectors (H1-L1) requires a coalescence rate of $r_0 = 3 (0.2) Mpcˆ{-3} Myrˆ{-1}$ for binary neutron stars (binary black holes); (ii) this limit on $r_0$ could be reduced 3-fold for two co-located detectors, whereas the currently proposed worldwide network of advanced instruments gives only ~ 30% improvement in detectability; (iii) the improved sensitivity of the planned Einstein Telescope allows not only confident detection of the background but also the high frequency components of the spectrum to be measured. Finally we show that the residual noise arising from sub-threshold binary neutron star merger events will be a challenging issue for terrestrial searches of primordial GWs from the early Universe.

1209.0595
(/preprints)

2012-09-17, 13:38
**[edit]**

**Authors**: Leslie Wade, Xavier Siemens, David Kaplan, Benjamin Knispel, Bruce Allen

**Date**: 13 Sep 2012

**Abstract**: We consider a simulated population of isolated Galactic neutron stars. The rotational frequency of each neutron star evolves through a combination of electromagnetic and gravitational wave emission. The magnetic field strength dictates the dipolar emission, and the ellipticity (a measure of a neutron star's deformation) dictates the gravitational wave emission. Through both analytic and numerical means, we assess the detectability of the Galactic neutron star population and bound the magnetic field strength and ellipticity parameter space of Galactic neutron stars with or without a direct gravitational wave detection. While our simulated population is primitive, this work establishes a framework by which future efforts can be conducted.

1209.2971
(/preprints)

2012-09-17, 13:37
**[edit]**

**Authors**: Luke Zoltan Kelley, Ilya Mandel, Enrico Ramirez-Ruiz

**Date**: 13 Sep 2012

**Abstract**: The detection of an electromagnetic transient which may originate from a binary neutron star merger can increase the probability that a given segment of data from the LIGO-Virgo ground-based gravitational-wave detector network contains a signal from a binary coalescence. Additional information contained in the electromagnetic signal, such as the sky location or distance to the source, can help rule out false alarms, and thus lower the necessary threshold for a detection. Here, we develop a framework for determining how much sensitivity is added to a gravitational-wave search by triggering on an electromagnetic transient. We apply this framework to a variety of relevant electromagnetic transients, from short GRBs to signatures of r-process heating to optical and radio orphan afterglows. We compute the expected rates of multi-messenger observations in the Advanced detector era, and find that searches triggered on short GRBs — with current high-energy instruments, such as Fermi — and nucleosynthetic ‘kilonovae’ — with future optical surveys, like LSST — can boost the number of multi-messenger detections by 15% and 40%, respectively, for a binary neutron star progenitor model. Short GRB triggers offer precise merger timing, but suffer from detection rates decreased by beaming and the high a priori probability that the source is outside the LIGO-Virgo sensitive volume. Isotropic kilonovae, on the other hand, could be commonly observed within the LIGO-Virgo sensitive volume with an instrument roughly an order of magnitude more sensitive than current optical surveys. We propose that the most productive strategy for making multi-messenger gravitational-wave observations is using triggers from future deep, optical all-sky surveys, with characteristics comparable to LSST, which could make as many as ten such coincident observations a year.

1209.3027
(/preprints)

2012-09-17, 13:36
**[edit]**

**Authors**: Tim Johannsen (Waterloo, Perimeter, Arizona)

**Date**: 13 Sep 2012

**Abstract**: General relativity has been tested by many experiments, which, however, almost exclusively probe weak spacetime curvatures. In this thesis, I create two frameworks for testing general relativity in the strong-field regime with observations of black holes in the electromagnetic spectrum using current or near-future instruments. In the first part of this thesis, I design tests of the no-hair theorem, which uniquely characterizes the nature of black holes in general relativity in terms of their masses and spins and which states that these compact objects are described by the Kerr metric. I investigate a quasi-Kerr metric and construct a Kerr-like spacetime, both of which contain an independent parameter in addition to mass and spin. If the no-hair theorem is correct, then any deviation from the Kerr metric has to be zero. I show that already moderate changes of the deviation parameters in either metric lead to significant modifications of the observed signals. I apply this framework to the imaging of supermassive black holes using very-long baseline interferometry as well as to the quasi-periodic variability and relativistically broadened iron lines observed in both galactic and supermassive black holes. In the second part of this thesis, I devise a method to test the predicted evaporation of black holes in Randall-Sundrum-type braneworld gravity through the orbital evolution of black-hole X-ray binaries and obtain constraints on the size of the extra dimension from A0620-00 and XTE J1118+480.

1209.3024
(/preprints)

2012-09-17, 13:35
**[edit]**

**Authors**: Keith Riles

**Date**: 4 Sep 2012

**Abstract**: Gravitational wave science should transform in this decade from a study of what has not been seen to a full-fledged field of astronomy in which detected signals reveal the nature of cataclysmic events and exotic objects. The LIGO Scientific Collaboration and Virgo Collaboration have recently completed joint data runs of unprecedented sensitivities to gravitational waves. So far, no gravitational radiation has been seen (although data mining continues). It seems likely that the first detection will come from 2nd-generation LIGO and Virgo interferometers now being installed. These new detectors are expected to detect ~40 coalescences of neutron star binary systems per year at full sensitivity. At the same time, research and development continues on 3rd-generation underground interferometers and on space-based interferometers. In parallel there is a vigorous effort in the radio pulsar community to detect ~several-nHz gravitational waves via the timing residuals from an array of pulsars at different locations in the sky. As the dawn of gravitational wave astronomy nears, this review, intended primarily for interested particle and nuclear physicists, describes what we have learned to date and the prospects for direct discovery of gravitational waves.

1209.0667
(/preprints)

2012-09-15, 23:32
**[edit]**

**Authors**: Krzysztof Belczynski, Tomasz Bulik, Ilya Mandel, B.S. Sathyaprakash, Andrzej Zdziarski, Joanna Mikolajewska

**Date**: 12 Sep 2012

**Abstract**: There are no known double black hole (BH-BH) or black hole-neutron star (BH-NS) systems. We argue that Cyg X-3 is a very likely BH-BH or BH-NS progenitor. This Galactic X-ray binary consists of a compact object, wind-fed by a Wolf-Rayet (WR) type companion. Based on a comprehensive analysis of observational data, it was recently argued that Cyg X-3 harbors a 2-4.5 Msun BH and a 7.5-14.2 Msun WR companion. We find that the fate of such a binary leads to the prompt (<1 Myr) formation of a close BH-BH system for the high end of the allowed WR mass (M_WR>13 Msun). For the low- to mid-mass range of the WR star (M_WR=7-10 Msun) Cyg X-3 is most likely (probability 70%) disrupted when WR ends up as a supernova. However, with smaller probability, it may form a wide (15%) or a close (15%) BH-NS system. The advanced LIGO/VIRGO detection rate for mergers of BH-BH systems from the Cyg X-3 formation channel is 10 per year, while it drops down to 0.1 per year for BH-NS systems. If Cyg X-3 in fact hosts a low mass BH and massive WR star, it lends additional support for the existence of BH-BH/BH-NS systems.

1209.2658
(/preprints)

2012-09-12, 19:28
**[edit]**

**Authors**: Priscilla Canizares (1), Jonathan R. Gair (1), Carlos F. Sopuerta (2) ((1) IoA, Cambridge, (2) ICE, CSIC-IEEC)

**Date**: 12 Sep 2012

**Abstract**: General Relativity (GR) describes gravitation well at the energy scales which we have so far been able to achieve or detect. However, we do not know whether GR is behind the physics governing stronger gravitational field regimes, such as near neutron stars or massive black-holes (MBHs). Gravitational-wave (GW) astronomy is a promising tool to test and validate GR and/or potential alternative theories of gravity. The information that a GW waveform carries not only will allow us to map the strong gravitational field of its source, but also determine the theory of gravity ruling its dynamics. In this work, we explore the extent to which we could distinguish between GR and other theories of gravity through the detection of low-frequency GWs from extreme-mass-ratio inspirals (EMRIs) and, in particular, we focus on dynamical Chern-Simons modified gravity (DCSMG). To that end, we develop a framework that enables us, for the first time, to perform a parameter estimation analysis for EMRIs in DCSMG. Our model is described by a 15-dimensional parameter space, that includes the Chern-Simons (CS) parameter which characterises the deviation between the two theories, and our analysis is based on Fisher information matrix techniques together with a (maximum-mismatch) criterion to assess the validity of our results. In our analysis, we study a 5-dimensional parameter space, finding that a GW detector like the Laser Interferometer Space Antenna (LISA) or eLISA (evolved LISA) should be able to discriminate between GR and DCSMG with fractional errors below 5%, and hence place bounds four orders of magnitude better than current Solar System bounds.

1209.2534
(/preprints)

2012-09-12, 19:27
**[edit]**

**Authors**: Márton Tápai, Zoltán Keresztes, László Árpád Gergely

**Date**: 8 Sep 2012

**Abstract**: We derive spin-dominated waveforms (SDW) for binary systems composed of spinning black holes with unequal masses (less than 1:30). Such systems could be formed by an astrophysical black hole with a smaller black hole or a neutron star companion; and typically arise for supermassive black hole encounters. SDW characterize the last stages of the inspiral, when the larger spin dominates over the orbital angular momentum (while the spin of the smaller companion can be neglected). They emerge as a double expansion in the post-Newtonian parameter $\varepsilon$ and the ratio $\xi $ of the orbital angular momentum and dominant spin. The SDW amplitudes are presented to ($\varepsilonˆ{3/2},\xi$) orders, while the phase of the gravitational waves to ($\varepsilonˆ{2},\xi$) orders (omitting the highest order mixed terms). To this accuracy the amplitude includes the (leading order) spin-orbit contributions, while the phase the (leading order) spin-orbit, self-spin and mass quadrupole-monopole contributions. While the SDW hold for any mass ratio smaller than 1:30, lower bounds for the mass ratios are derived from the best sensitivity frequency range expected for Advanced LIGO (giving 1:140), the Einstein Telescope ($7\times 10ˆ{-4}$), the LAGRANGE ($7\times 10ˆ{-7}$) and LISA missions ($7\times 10ˆ{-9}$), respectively.

1209.1722
(/preprints)

2012-09-10, 22:19
**[edit]**

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

*Tantum in modicis, quantum in maximis*