[gr-qc/0601091] The last orbit of binary black holes

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

Date: Mon, 23 Jan 2006

Abstract: We have used our new technique for fully numerical evolutions of orbiting black-hole binaries without excision to model the last orbit and merger of an equal-mass black-hole system. We track the trajectories of the individual apparent horizons and find that the binary completed approximately one and a third orbits before forming a common horizon. Upon calculating the complete gravitational radiation waveform, horizon mass, and spin, we find that the binary radiated 3.2% of its mass and 24% of its angular momentum. The early part of the waveform, after a relatively short initial burst of spurious radiation, is oscillatory with increasing amplitude and frequency, as expected from orbital motion. The waveform then transitions to a typical ‘plunge’ waveform; i.e. a rapid rise in amplitude followed by quasinormal ringing. The plunge part of the waveform is remarkably similar to the waveform from the previously studied ‘ISCO’ configuration. We anticipate that the plunge waveform, when starting from quasicircular orbits, has a generic shape that is essentially independent of the initial separation of the binary.

abs pdf

Jan 24, 2006

0601091 (/preprints/gr-qc)
2006-01-24, 08:59 [edit]

[gr-qc/0601076] Variability of signal to noise ratio and the network analysis of gravitational wave burst signals

Authors: Soumya D. Mohanty, Malik Rakhmanov, Sergei Klimenko, Guenakh Mitselmakher

Date: Wed, 18 Jan 2006

Abstract: The detection and estimation of gravitational wave burst signals, with {\em a priori} unknown polarization waveforms, requires the use of data from a network of detectors. For determining how the data from such a network should be combined, approaches based on the maximum likelihood principle have proven to be useful. The most straightforward among these uses the global maximum of the likelihood over the space of all waveforms as both the detection statistic and signal estimator. However, in the case of burst signals, a physically counterintuitive situation results: for two aligned detectors the statistic includes the cross-correlation of the detector outputs, as expected, but this term disappears even for an infinitesimal misalignment. This {\em two detector paradox} arises from the inclusion of improbable waveforms in the solution space of maximization. Such waveforms produce widely different responses in detectors that are closely aligned. We show that by penalizing waveforms that exhibit large signal-to-noise ratio (snr) variability, as the corresponding source is moved on the sky, a physically motivated restriction is obtained that (i) resolves the two detector paradox and (ii) leads to a better performing statistic than the global maximum of the likelihood. Waveforms with high snr variability turn out to be precisely the ones that are improbable in the sense mentioned above. The coherent network analysis method thus obtained can be applied to any network, irrespective of the number or the mutual alignment of detectors.

abs pdf

Jan 19, 2006

0601076 (/preprints/gr-qc)
2006-01-19, 09:35 [edit]

[astro-ph/0601275] Short GRB and binary black hole standard sirens as a probe of dark energy

Authors: Neal Dalal (CITA), Daniel E. Holz (LANL and U. Chicago), Scott A. Hughes (MIT), Bhuvnesh Jain (U. Penn.)

Date: Fri, 13 Jan 2006

Abstract: Observations of the gravitational radiation from well-localized, inspiraling compact object binaries can measure absolute source distances with high accuracy. When coupled with an independent determination of redshift through an electromagnetic counterpart, these standard sirens can provide an excellent probe of the expansion history of the Universe and the dark energy. Short gamma-ray bursts, if produced by merging neutron star binaries, would be standard sirens with known redshifts detectable by ground-based GW networks such as LIGO-II, Virgo, and AIGO. Depending upon the collimation of these GRBs, a single year of observation of their gravitational waves can measure the Hubble constant to about 2%. When combined with measurement of the absolute distance to the last scattering surface of the cosmic microwave background, this determines the dark energy equation of state parameter w to 9%. Similarly, supermassive binary black hole inspirals will be standard sirens detectable by LISA. Depending upon the precise redshift distribution, 100 sources could measure w at the 4% level.

abs pdf

Jan 17, 2006

0601275 (/preprints/astro-ph)
2006-01-17, 10:59 [edit]

[astro-ph/0601271] Nonlinear perturbations for dissipative and interacting relativistic fluids

Authors: David Langlois, Filippo Vernizzi

Date: Thu, 12 Jan 2006

Abstract: We develop a covariant formalism to study nonlinear perturbations of dissipative and interacting relativistic fluids. We derive nonlinear evolution equations for various covectors defined as linear combinations of the spatial gradients of the local number of e-folds and of some scalar quantities characterizing the fluid, such as the energy density or the particle number density. For interacting fluids we decompose perturbations into adiabatic and entropy components and derive their coupled evolution equations, recovering and extending the results obtained in the context of the linear theory. For non-dissipative and noninteracting fluids, these evolution equations reduce to the conservation equations that we have obtained in recent works. We also illustrate geometrically the meaning of the covectors that we have introduced.

abs pdf

Jan 16, 2006

0601271 (/preprints/astro-ph)
2006-01-16, 14:27 [edit]

[astro-ph/0601161] Resonant relaxation near a massive black hole: the stellar distribution and gravitational wave sources

Authors: Clovis Hopman, Tal Alexander (Weizmann)

Date: Mon, 9 Jan 2006

Abstract: Resonant relaxation (RR) of orbital angular momenta occurs near massive black holes (MBHs) where the stellar orbits are nearly Keplerian and so do not precess significantly. The resulting coherent torques efficiently change the magnitude of the angular momenta and rotate the orbital inclination in all directions. As a result, many of the tightly bound stars very near the MBH are rapidly destroyed by falling into the MBH on low-angular momentum orbits, while the orbits of the remaining stars are efficiently randomized. We solve numerically the Fokker-Planck equation in energy for the steady state distribution of a single mass population with a RR sink term. We find that the steady state current of stars, which sustains the accelerated drainage close to the MBH, can be up to ~10 times larger than that due to non-coherent 2-body relaxation alone. RR mostly affects tightly bound stars, and so it increases only moderately the total tidal disruption rate, which is dominated by stars originating from less bound orbits farther away. We show that the event rate of gravitational wave (GW) emission from inspiraling stars, originating much closer to the MBH, is dominated by RR dynamics. The GW event rate depends on the uncertain efficiency of RR. The efficiency indicated by the few available simulations implies rates ~10 times higher than those predicted by 2-body relaxation, which would improve the prospects of detecting such events by future GW detectors, such as LISA. However, a higher, but still plausible RR efficiency can lead to the drainage of all tightly bound stars and strong suppression of GW events from inspiraling stars. We apply our results to the Galactic MBH, and show that the observed dynamical properties of stars there are consistent with RR.

abs pdf

Jan 12, 2006

0601161 (/preprints/astro-ph)
2006-01-12, 09:01 [edit]

[gr-qc/0510054] "Swimming" versus "swinging" in spacetime

Authors: Eduardo Gueron, Clovis A. S. Maia, George E. A. Matsas

Date: Tue, 10 Jan 2006

Abstract: Wisdom has recently unveiled a new relativistic effect, called ‘spacetime swimming’, where quasi-rigid free bodies in curved spacetimes can "speed up", "slow down" or "deviate" their falls by performing "local" cyclic shape deformations. We show here that for fast enough cycles this effect dominates over a non-relativistic related one, named here ‘space swinging’, where the fall is altered through "nonlocal" cyclic deformations in Newtonian gravitational fields. We expect, therefore, to clarify the distinction between both effects leaving no room to controversy. Moreover, the leading contribution to the swimming effect predicted by Wisdom is enriched with a higher order term and the whole result is generalized to be applicable in cases where the tripod is in large red-shift regions.

abs pdf

Jan 11, 2006

0510054 (/preprints/gr-qc)
2006-01-11, 08:14 [edit]

[gr-qc/0601036] Darwin Meets Einstein: LISA Data Analysis Using Genetic Algorithms

Authors: Jeff Crowder, Neil J. Cornish, Lucas Reddinger

Date: Tue, 10 Jan 2006

Abstract: This work presents the first application of the method of Genetic Algorithms (GAs) to data analysis for the Laser Interferometer Space Antenna (LISA). In the low frequency regime of the LISA band there are expected to be tens of thousands galactic binary systems that will be emitting gravitational waves detectable by LISA. The challenge of parameter extraction of such a large number of sources in the LISA data stream requires a search method that can efficiently explore the large parameter spaces involved. As signals of many of these sources will overlap, a global search method is desired. GAs represent such a global search method for parameter extraction of multiple overlapping sources in the LISA data stream. We find that GAs are able to correctly extract source parameters for overlapping sources. Several optimizations of a basic GA are presented with results derived from applications of the GA searches to simulated LISA data.

abs pdf

Jan 11, 2006

0601036 (/preprints/gr-qc)
2006-01-11, 08:13 [edit]

[gr-qc/0601029] Gravitomagnetic resonant excitation of Rossby modes in coalescing neutron star binaries

Authors: Éanna É. Flanagan, Étienne Racine

Date: Sat, 7 Jan 2006

Abstract: In coalescing neutron star binaries, r-modes in one of the stars can be resonantly excited by the gravitomagnetic tidal field of its companion. This post-Newtonian gravitomagnetic driving of these modes dominates over the Newtonian tidal driving previously computed by Ho and Lai. To leading order in the tidal expansion parameter R/r (where R is the radius of the neutron star and r is the orbital separation), only the l=2, |m|= 1 and |m| = 2 r-modes are excited. The tidal work done on the star through this driving has an effect on the evolution of the inspiral and on the phasing of the emitted gravitational wave signal. For a neutron star of mass M, radius R, spin frequency f_spin, modeled as a Gamma =2 polytrope, with a companion also of mass M, the gravitational wave phase shift for the m=2 mode is (0.1radians)(R/10km)ˆ4(M/1.4M_sun)ˆ{-10/3}(f_spin/100Hz)ˆ{2/3} for optimal spin orientation. For canonical neutron star parameters this phase shift will likely not be detectable by gravitational wave detectors such as LIGO, but if the neutron star radius is larger it may be detectable if the signal-to-noise ratio is moderately large. For neutron star - black hole binaries, the effect is smaller; the phase shift scales as companion mass to the -4/3 power for large companion masses. The net energy transfer from the orbit into the star is negative corresponding to a slowing down of the inspiral. This occurs because the interaction reduces the spin of the star, and occurs only for modes which satisfy the Chandrasekhar-Friedman-Schutz instability criterion.

abs pdf

Jan 09, 2006

0601029 (/preprints/gr-qc)
2006-01-09, 23:30 [edit]

[gr-qc/0512160] On gravitational-wave spectroscopy of massive black holes with the space interferometer LISA

Authors: Emanuele Berti, Vitor Cardoso, Clifford M. Will

Date: Thu, 29 Dec 2005

Abstract: Newly formed black holes are expected to emit characteristic radiation in the form of quasi-normal modes, called ringdown waves, with discrete frequencies. LISA should be able to detect the ringdown waves emitted by oscillating supermassive black holes throughout the observable Universe. We develop a multi-mode formalism, applicable to any interferometric detectors, for detecting ringdown signals, for estimating black hole parameters from those signals, and for testing the no-hair theorem of general relativity. Focusing on LISA, we use current models of its sensitivity to compute the expected signal-to-noise ratio for ringdown events, the relative parameter estimation accuracy, and the resolvability of different modes. We also discuss the extent to which uncertainties on physical parameters, such as the black hole spin and the energy emitted in each mode, will affect our ability to do black hole spectroscopy.

abs pdf

Jan 03, 2006

0512160 (/preprints/gr-qc)
2006-01-03, 09:42 [edit]

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