[astro-ph/0608600] Gravitational Recoil from Binary Black Hole Mergers:the Close-Limit Approximation

Authors: Carlos F. Sopuerta, Nicolas Yunes, Pablo Laguna (Penn State)

Date: Mon, 28 Aug 2006

Abstract: [abridged] The coalescence of a binary black hole system is one of the main sources of gravitational waves that present and future detectors will study. Apart from the energy and angular momentum that these waves carry, for unequal-mass binaries there is also a net flux of linear momentum that implies a recoil velocity of the resulting final black hole in the opposite direction. We present a computation of the recoil velocity based on the close-limit approximation scheme, which gives excellent results for head-on and grazing collisions of black holes when compared to full numerical relativistic calculations. We obtain a maximum recoil velocity of ~ 57 km/s for a symmetric mass ratio eta = M_1 M_2/(M_1+M_2)ˆ2 ~ 0.19 and an initial proper separation of 4 M, where M is the total ADM mass of the system. This separation is the maximum at which the close-limit approximation is expected to provide accurate results. If we supplement this estimate with PN calculations up to the innermost stable circular orbit, we obtain a lower bound for the recoil velocity, with a maximum around 80 km/s. This is a lower bound because it neglects the initial merger phase. We can however obtain a rough estimate by using PN methods or the close-limit approximation. Since both methods are known to overestimate the amount of radiation, we obtain in this way an upper bound for the recoil with maxima in the range of 214-240 km/s. We also provide non-linear fits to these estimated upper and lower bounds. These estimates are subject to uncertainties related to issues such as the choice of initial data and higher effects in perturbation theory. Nonetheless, our estimates are consistent with previous results in the literature and suggest a narrower range of possible recoil velocities.

Aug 29, 2006

0608600 (/preprints/astro-ph)
2006-08-29, 17:52 

[gr-qc/0608121] Constraining Torsion with Gravity Probe B

Authors: Yi Mao (MIT), Max Tegmark (MIT), Alan Guth (MIT), Serkan Cabi (MIT)

Date: Tue, 29 Aug 2006

Abstract: It is well-entrenched folklore that torsion gravity theories predict observationally negligible torsion in the solar system, since torsion (if it exists) couples only to the intrinsic spin of elementary particles, not to rotational angular momentum. We argue that this assumption has a logical loophole which can and should be tested experimentally. We give an explicit counterexample where a rotating body generates a torsion field in Weitzenbock spacetime with a Hayashi-Shirafuji Lagrangian. More generally, in the spirit of action=reaction, if a rotating mass like a planet can generate torsion, then a gyroscope should also feel torsion.
Using symmetry arguments, we show that to lowest order, the torsion field around a uniformly rotating spherical mass is determined by seven dimensionless parameters. These parameters effectively generalize the PPN formalism and provide a concrete framework for further testing GR. We construct a parametrized Lagrangian that includes both standard torsion-free GR and Hayashi-Shirafuji maximal torsion gravity as special cases. We demonstrate that classic solar system tests rule out the latter and constrain two observable parameters. We show that Gravity Probe B (GPB) is an ideal experiment for further constraining torsion theories, and work out the most general torsion-induced precession of its gyroscope in terms of our torsion parameters.

Aug 29, 2006

0608121 (/preprints/gr-qc)
2006-08-29, 17:51 

[gr-qc/0604115] Strategies for observing extreme mass ratio inspirals

Authors: Steve Drasco

Date: Sat, 26 Aug 2006

Abstract: I review the status of research, conducted by a variety of independent groups, aimed at the eventual observation of Extreme Mass Ratio Inspirals (EMRIs) with gravitational wave detectors. EMRIs are binary systems in which one of the objects is much more massive than the other, and which are in a state of dynamical evolution that is dominated by the effects of gravitational radiation. Although these systems are highly relativistic, with the smaller object moving relative to the larger at nearly light-speed, they are well described by perturbative calculations which exploit the mass ratio as a natural small parameter. I review the use of such approximations to generate waveforms needed by data analysis algorithms for observation. I also briefly review the status of developing the data analysis algorithms themselves. Although this article is almost entirely a review of previous work, it includes (as an appendix) a new analytical estimate for the time over which the influence of radiation on the binary itself is observationally negligible.

Aug 28, 2006

0604115 (/preprints/gr-qc)
2006-08-28, 18:08 

[gr-qc/0608114] When is Enough Good Enough in Gravitational Wave Source Modeling?

Authors: Louis J. Rubbo

Date: Mon, 28 Aug 2006

Abstract: A typical approach to developing an analysis algorithm for analyzing gravitational wave data is to assume a particular waveform and use its characteristics to formulate a detection criteria. Once a detection has been made, the algorithm uses those same characteristics to tease out parameter estimates from a given data set. While an obvious starting point, such an approach is initiated by assuming a single, correct model for the waveform regardless of the signal strength, observation length, noise, etc. This paper introduces the method of Bayesian model selection as a way to select the most plausible waveform model from a set of models given the data and prior information. The discussion is done in the scientific context for the proposed Laser Interferometer Space Antenna.

Aug 28, 2006

0608114 (/preprints/gr-qc)
2006-08-28, 18:07 

[astro-ph/0608460] Astrophysics of extreme mass ratio inspiral sources

Authors: Clovis Hopman (Leiden Observatory)

Date: Tue, 22 Aug 2006

Abstract: Compact remnants on orbits with peri-apses close to the Schwarzschild radius of a massive black hole (MBH) lose orbital energy by emitting gravitational waves (GWs) and spiral in. Scattering with other stars allows successful inspiral of such extreme mass ratio inspiral sources (EMRIs) only within small distances, a < few \times 0.01 pc from the MBH. The event rate of EMRIs is therefore dominated by the stellar dynamics and content in the inner few \times 0.01 pc. I discuss the relevant dynamical aspects and resulting estimated event rates of EMRIs. Subjects considered include the loss-cone treatment of inspiral sources; mass segregation; resonant relaxation; and alternative routes to EMRI formation such as tidal binary disruptions, stellar formation in disks and tidal capture of massive main sequence stars. The EMRI event rate is estimated to be of order few \times 10ˆ2/Gyr per MBH, giving excellent prospects for observation by LISA.

Aug 28, 2006

0608460 (/preprints/astro-ph)
2006-08-28, 18:07 

[astro-ph/0512212] Prospects of LIGO for constraining inclination of merging compact binaries associated with three-dimensionally localized short-hard GRBs

Authors: Naoki Seto

Date: Thu, 24 Aug 2006

Abstract: We study prospects of a method to constrain the inclination of a coalescing compact binary by detecting its gravitational waves associated with a three-dimensionally localized (direction and distance) short-hard gamma-ray burst. We take advantage of a synergy of these two observations, and our method can be applied even with a single interferometer. For a nearly face-on binary the inclination angle $I$ can be constrained in the range 1-1/SNR < cosI \le 1 (SNR: the signal to noise ratio of gravitational wave detection), provided that the error of the distance estimation is negligible. This method would help us to study properties of the short-hard bursts, including potentially collimated jet-like structures as indicated by recent observation.

Aug 27, 2006

0512212 (/preprints/astro-ph)
2006-08-27, 22:40 

[gr-qc/0608112] Slice & Dice: Identifying and Removing Bright Galactic Binaries from LISA Data

Authors: Louis J. Rubbo, Neil J. Cornish, Ronald W. Hellings

Date: Fri, 25 Aug 2006

Abstract: Here we describe a hierarchal and iterative data analysis algorithm used for searching, characterizing, and removing bright, monochromatic binaries from the Laser Interferometer Space Antenna (LISA) data streams. The algorithm uses the F-statistic to provide an initial solution for individual bright sources, followed by an iterative least squares fitting for all the bright sources. Using the above algorithm, referred to as Slice & Dice, we demonstrate the removal of multiple, correlated galactic binaries from simulated LISA data. Initial results indicate that Slice & Dice may be a useful tool for analyzing the forthcoming LISA data.

Aug 27, 2006

0608112 (/preprints/gr-qc)
2006-08-27, 22:40 

[gr-qc/0608091] A numerical study of the quasinormal mode excitation of Kerr black holes

Authors: Ernst Nils Dorband, Emanuele Berti, Peter Diener, Erik Schnetter, Manuel Tiglio

Date: Fri, 18 Aug 2006

Abstract: We present numerical results from three-dimensional evolutions of scalar perturbations of Kerr black holes. Our simulations make use of a high-order accurate multi-block code which naturally allows for fixed adaptivity and smooth inner (excision) and outer boundaries. We focus on the quasinormal ringing phase, presenting a systematic method for extraction of the quasinormal mode frequencies and amplitudes and comparing our results against perturbation theory.
The amplitude of each mode depends exponentially on the starting time of the quasinormal regime, which is not defined unambiguously. We show that this time-shift problem can be circumvented by looking at appropriately chosen relative mode amplitudes. From our simulations we extract the quasinormal frequencies and the relative and absolute amplitudes of corotating and counterrotating modes (including overtones in the corotating case). We study the dependence of these amplitudes on the shape of the initial perturbation, the angular dependence of the mode and the black hole spin, comparing against results from perturbation theory in the so-called asymptotic approximation. We also compare the quasinormal frequencies from our numerical simulations with predictions from perturbation theory, finding excellent agreement. Finally we study under what conditions the relative amplitude between given pairs of modes gets maximally excited and present a quantitative analysis of rotational mode-mode coupling. The main conclusions and techniques of our analysis are quite general and, as such, should be of interest in the study of ringdown gravitational waves produced by astrophysical gravitational wave sources.

Aug 22, 2006

0608091 (/preprints/gr-qc)
2006-08-22, 18:53 

[gr-qc/0608103] Detecting gravitational waves from inspiraling binaries with a network of detectors : coherent versus coincident strategies

Authors: Himan Mukhopadhyay, Norichica Sago, Hideyuki Tagoshi, Sanjeev Dhurandhar, Hirotaka Takahashi, Nobuyuki Kanda

Date: Tue, 22 Aug 2006

Abstract: We compare two strategies of multi-detector detection of compact binary inspiral signals, namely, the coincidence and the coherent. For simplicity we consider here two identical detectors having the same power spectral density of noise, that of initial LIGO, located in the same place and having the same orientation. We consider the cases of independent noise as well as that of correlated noise. The coincident strategy involves separately making two candidate event lists, one for each detector, and from these choosing those pairs of events from the two lists which lie within a suitable parameter window, which then are called as coincidence detections. The coherent strategy on the other hand involves combining the data phase coherently, so as to obtain a single network statistic which is then compared with a single threshold. Here we attempt to shed light on the question as to which strategy is better. We compare the performances of the two methods by plotting the Receiver Operating Characteristics (ROC) for the two strategies. Several of the results are obtained analytically in order to gain insight. Further we perform numerical simulations in order to determine certain parameters in the analytic formulae and thus obtain the final complete results. We consider here several cases from the relatively simple to the astrophysically more relevant in order to establish our results. The bottom line is that the coherent strategy although more computationally expensive in general than the coincidence strategy, is superior to the coincidence strategy - considerably less false dismissal probability for the same false alarm probability in the viable false alarm regime.

Aug 22, 2006

0608103 (/preprints/gr-qc)
2006-08-22, 18:52 

[astro-ph/0606076] The Spin of the Near-Extreme Kerr Black Hole GRS 1915 105

Authors: Jeffrey E. McClintock, Rebecca Shafee, Ramesh Narayan, Ronald A. Remillard, Shane W. Davis, Li-Xin Li

Date: Wed, 16 Aug 2006

Abstract: Based on a spectral analysis of the X-ray continuum that employs a fully relativistic accretion-disk model, we conclude that the compact primary of the binary X-ray source GRS 1915+105 is a rapidly-rotating Kerr black hole. We find a lower limit on the dimensionless spin parameter of a* greater than 0.98. Our result is robust in the sense that it is independent of the details of the data analysis and insensitive to the uncertainties in the mass and distance of the black hole. Furthermore, our accretion-disk model includes an advanced treatment of spectral hardening. Our data selection relies on a rigorous and quantitative definition of the thermal state of black hole binaries, which we used to screen all of the available RXTE and ASCA data for the thermal state of GRS 1915+105. In addition, we focus on those data for which the accretion disk luminosity is less than 30% of the Eddington luminosity. We argue that these low-luminosity data are most appropriate for the thin alpha-disk model that we employ. We assume that there is zero torque at the inner edge of the disk, as is likely when the disk is thin, although we show that the presence of a significant torque does not affect our results. Our model and the model of the relativistic jets observed for this source constrain the distance and black hole mass and could thus be tested by determining a VLBA parallax distance and improving the measurement of the mass function. Finally, we comment on the significance of our results for relativistic-jet and core-collapse models, and for the detection of gravitational waves.

Aug 16, 2006

0606076 (/preprints/astro-ph)
2006-08-16, 22:16 

[astro-ph/0608275] Spin-orbit interactions in black-hole binaries

Authors: M. Campanelli, C. O. Lousto, Y. Zlochower

Date: Sun, 13 Aug 2006

Abstract: We perform numerical simulations of black-hole binaries to study the exchange of spin and orbital angular momentum during the last, highly nonlinear, stages of the coalescence process. To calculate the transfer of angular momentum from orbital to spin, we start with a configuration of initially non-spinning holes in a quasicircular orbit. We find that each individual black hole horizon acquires a non-vanishing spin and that this spin is two orders of magnitude smaller than what is needed to tidally lock the binary into a corotation state. We also study the converse transfer from spin into orbital motion. In this case, we start the simulations with parallel, highly-spinning non-boosted black holes. As the collision proceeds, the system acquires a non-head-on orbital motion, due to spin-orbit coupling, that leads to the radiation of angular momentum. We are able to accurately measure the energy and angular momentum losses and model their dependence on the initial spins.

Aug 15, 2006

0608275 (/preprints/astro-ph)
2006-08-15, 17:52 

[gr-qc/0608062] Measuring coalescing massive binary black holes with gravitational waves: The impact of spin-induced precession

Authors: Ryan N. Lang, Scott A. Hughes

Date: Fri, 11 Aug 2006

Abstract: The coalescence of massive black holes generates gravitational waves (GWs) that will be measurable by space-based detectors such as LISA to large redshifts. The spins of a binary's black holes have an important impact on its waveform. Specifically, geodetic and gravitomagnetic effects cause the spins to precess; this precession then modulates the waveform, adding periodic structure which encodes useful information about the binary's members. Following pioneering work by Vecchio, we examine the impact upon GW measurements of including these precession-induced modulations in the waveform model. We find that the additional periodicity due to spin precession breaks degeneracies among certain parameters, greatly improving the accuracy with which they may be measured. In particular, mass measurements are improved tremendously, by one to several orders of magnitude. Localization of the source on the sky is also improved, though not as much -- low redshift systems can be localized to an ellipse which is roughly $10- {a few} \times 10$ arcminutes in the long direction and a factor of 2 -- 5 smaller in the short direction. Though not a drastic improvement relative to analyses which neglect spin precession, even modest gains in source localization will greatly facilitate searches for electromagnetic counterparts to GW events. Determination of distance to the source is likewise improved: We find that relative error in measured luminosity distance is commonly $\sim 0.1-0.4%$ at $z \sim 1$. Finally, with the inclusion of precession, we find that the magnitude of the spins themselves can typically be determined for low redshift systems with an accuracy of about $0.1-10 %$, depending on the spin value, allowing accurate surveys of mass and spin evolution over cosmic time.

Aug 13, 2006

0608062 (/preprints/gr-qc)
2006-08-13, 21:41 

[astro-ph/0607657] Constraint on the Post-Newtonian Parameter gamma on Galactic Size Scales

Authors: Adam S. Bolton (1), Saul Rappaport (2), Scott Burles (2) ((1) CfA, (2) MIT)

Date: Sun, 30 Jul 2006

Abstract: We constrain the post-Newtonian gravity parameter gamma on kiloparsec scales by comparing the masses of 15 elliptical lensing galaxies from the Sloan Lens ACS Survey as determined in two independent ways. The first method assumes only that Newtonian gravity is correct and is independent of gamma, while the second uses gravitational lensing which depends on gamma. More specifically, we combine Einstein radii and radial surface-brightness gradient measurements of the lens galaxies with empirical distributions for the mass concentration and velocity anisotropy of elliptical galaxies in the local universe to predict gamma-dependent probability distributions for the lens-galaxy velocity dispersions. By comparing with observed velocity dispersions, we derive a maximum-likelihood value of gamma = 0.98 +/- 0.07 (68% confidence). This result is in excellent agreement with the prediction of general relativity that has previously been verified to this accuracy only on solar-system length scales.

Aug 03, 2006

0607657 (/preprints/astro-ph)
2006-08-03, 13:09