arXiv subjects

B. S. Sathyaprakash

Publications and source records attributed to B. S. Sathyaprakash.

At least 91 records · Page 5Linked to original sources

arXiv2012

Cyg X-3: a Galactic double black hole or black hole-neutron star progenitor

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.

astro-ph.HE↗

arXiv2012

Searching for gravitational waves from binary coalescence

We describe the implementation of a search for gravitational waves from compact binary coalescences in LIGO and Virgo data. This all-sky, all-time, multi-detector search for binary coalescence has been used to search data taken in recent LIGO and Virgo runs. The search is built around a matched filter analysis of the data, augmented by numerous signal consistency tests designed to distinguish artifacts of non-Gaussian detector noise from potential detections. We demonstrate the search performance using Gaussian noise and data from the fifth LIGO science run and demonstrate that the signal consistency tests are capable of mitigating the effect of non-Gaussian noise and providing a sensitivity comparable to that achieved in Gaussian noise.

gr-qc↗

arXiv2012

Bayesian model selection for testing the no-hair theorem with black hole ringdowns

General relativity predicts that a black hole that results from the merger of two compact stars (either black holes or neutron stars) is initially highly deformed but soon settles down to a quiescent state by emitting a superposition of quasi-normal modes (QNMs). The QNMs are damped sinusoids with characteristic frequencies and decay times that depend only on the mass and spin of the black hole and no other parameter - a statement of the no-hair theorem. In this paper we have examined the extent to which QNMs could be used to test the no-hair theorem with future ground- and space-based gravitational-wave detectors. We model departures from general relativity (GR) by introducing extra parameters which change the mode frequencies or decay times from their general relativistic values. With the aid of numerical simulations and Bayesian model selection, we assess the extent to which the presence of such a parameter could be inferred, and its value estimated. We find that it is harder to decipher the departure of decay times from their GR value than it is with the mode frequencies. Einstein Telescope (ET, a third generation ground-based detector) could detect departures of <1% in the frequency of the dominant QNM mode of a 500 Msun black hole, out to a maximum range of 4 Gpc. In contrast, the New Gravitational Observatory (NGO, an ESA space mission to detect gravitational waves) can detect departures of ~ 0.1% in a 10^8 Msun black hole to a luminosity distance of 30 Gpc (z = 3.5).

gr-qc↗

arXiv2012

Black-hole hair loss: learning about binary progenitors from ringdown signals

Perturbed Kerr black holes emit gravitational radiation, which (for the practical purposes of gravitational-wave astronomy) consists of a superposition of damped sinusoids termed quasi-normal modes. The frequencies and time-constants of the modes depend only on the mass and spin of the black hole - a consequence of the no-hair theorem. It has been proposed that a measurement of two or more quasi-normal modes could be used to confirm that the source is a black hole and to test if general relativity continues to hold in ultra-strong gravitational fields. In this paper we propose a practical approach to testing general relativity with quasi-normal modes. We will also argue that the relative amplitudes of the various quasi-normal modes encode important information about the origin of the perturbation that caused them. This helps in inferring the nature of the perturbation from an observation of the emitted quasi-normal modes. In particular, we will show that the relative amplitudes of the different quasi-normal modes emitted in the process of the merger of a pair of nonspinning black holes can be used to measure the component masses of the progenitor binary.

gr-qc↗

arXiv2010

Parametrized tests of post-Newtonian theory using Advanced LIGO and Einstein Telescope

General relativity has very specific predictions for the gravitational waveforms from inspiralling compact binaries obtained using the post-Newtonian (PN) approximation. We investigate the extent to which the measurement of the PN coefficients, possible with the second generation gravitationalwave detectors such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and the third generation gravitational-wave detectors such as the Einstein Telescope (ET), could be used to test post-Newtonian theory and to put bounds on a subclass of parametrized-post-Einstein theories which differ from general relativity in a parametrized sense. We demonstrate this possibility by employing the best inspiralling waveform model for nonspinning compact binaries which is 3.5PN accurate in phase and 3PN in amplitude. Within the class of theories considered, Advanced LIGO can test the theory at 1.5PN and thus the leading tail term. Future observations of stellar mass black hole binaries by ET can test the consistency between the various PN coefficients in the gravitational-wave phasing over the mass range of 11-44 Msun. The choice of the lower frequency cut off is important for testing post-Newtonian theory using the ET. The bias in the test arising from the assumption of nonspinning binaries is indicated.

gr-qc↗

arXiv2010

Weak lensing effects in the measurement of the dark energy equation of state with LISA

The Laser Interferometer Space Antenna's (LISA's) observation of supermassive binary black holes (SMBBH) could provide a new tool for precision cosmography. Inclusion of sub-dominant signal harmonics in the inspiral signal allows for high-accuracy sky localization, dramatically improving the chances of finding the host galaxy and obtaining its redshift. A SMBBH merger can potentially have component masses from a wide range ($10^5 - 10^8\,\Ms$) over which parameter accuracies vary considerably. We perform an in-depth study in order to understand (i) what fraction of possible SMBBH mergers allow for sky localization, depending on the parameters of the source, and (ii) how accurately $w$ can be measured when the host galaxy can be identified. We also investigate how accuracies on all parameters improve when a knowledge of the sky position can be folded into the estimation of errors. We find that $w$ can be measured to within a few percent in most cases, if the only error in measuring the luminosity distance is due to LISA's instrumental noise and the confusion background from Galactic binaries. However, weak lensing-induced errors will severely degrade the accuracy with which $w$ can be obtained, emphasizing that methods to mitigate weak lensing effects would be required to take advantage of LISA's full potential.

gr-qc↗

arXiv2010

A tapering window for time-domain templates and simulated signals in the detection of gravitational waves from coalescing compact binaries

Inspiral signals from binary black holes, in particular those with masses in the range $10M_\odot \lsim M \lsim 1000 M_\odot,$ may last for only a few cycles within a detector's most sensitive frequency band. The spectrum of a square-windowed time-domain signal could contain unwanted power that can cause problems in gravitational wave data analysis, particularly when the waveforms are of short duration. There may be leakage of power into frequency bins where no such power is expected, causing an excess of false alarms. We present a method of tapering the time-domain waveforms that significantly reduces unwanted leakage of power, leading to a spectrum that agrees very well with that of a long duration signal. Our tapered window also decreases the false alarms caused by instrumental and environmental transients that are picked up by templates with spurious signal power. The suppression of background is an important goal in noise-dominated searches and can lead to an improvement in the detection efficiency of the search algorithms.

gr-qc↗

arXiv2009

A stochastic template placement algorithm for gravitational wave data analysis

This paper presents an algorithm for constructing matched-filter template banks in an arbitrary parameter space. The method places templates at random, then removes those which are "too close" together. The properties and optimality of stochastic template banks generated in this manner are investigated for some simple models. The effectiveness of these template banks for gravitational wave searches for binary inspiral waveforms is also examined. The properties of a stochastic template bank are then compared to the deterministically placed template banks that are currently used in gravitational wave data analysis.

gr-qc↗

arXiv2009

Comparison of post-Newtonian templates for compact binary inspiral signals in gravitational-wave detectors

The two-body dynamics in general relativity has been solved perturbatively using the post-Newtonian (PN) approximation. The evolution of the orbital phase and the emitted gravitational radiation are now known to a rather high order up to O(v^8), v being the characteristic velocity of the binary. The orbital evolution, however, cannot be specified uniquely due to the inherent freedom in the choice of parameter used in the PN expansion as well as the method pursued in solving the relevant differential equations. The goal of this paper is to determine the (dis)agreement between different PN waveform families in the context of initial and advanced gravitational-wave detectors. The waveforms employed in our analysis are those that are currently used by Initial LIGO/Virgo, that is the time-domain PN models TaylorT1, TaylorT2, TaylorT3, TaylorT4 and TaylorEt, the effective one-body (EOB) model, and the Fourier-domain representation TaylorF2. We examine the overlaps of these models with one another and with the prototype effective one-body model (calibrated to numerical relativity simulations, as currently used by initial LIGO) for a number of different binaries at 2PN, 3PN and 3.5PN orders to quantify their differences and to help us decide whether there exist preferred families that are the most appropriate as search templates. We conclude that as long as the total mass remains less than a certain upper limit M_crit, all template families at 3.5PN order (except TaylorT3 and TaylorEt) are equally good for the purpose of detection. The value of M_crit is found to be ~ 12M_Sun for Initial, Enhanced and Advanced LIGO. From a purely computational point of view we recommend that 3.5PN TaylorF2 be used below Mcrit and EOB calibrated to numerical relativity simulations be used for total binary mass M > Mcrit.

gr-qc↗

arXiv2009

Cosmography with the Einstein Telescope

Einstein Telescope (ET) is a 3rd generation gravitational-wave (GW) detector that is currently undergoing a design study. ET can detect millions of compact binary mergers up to redshifts 2-8. A small fraction of mergers might be observed in coincidence as gamma-ray bursts, helping to measure both the luminosity distance and red-shift to the source. By fitting these measured values to a cosmological model, it should be possible to accurately infer the dark energy equation-of-state, dark matter and dark energy density parameters. ET could, therefore, herald a new era in cosmology.

astro-ph.CO↗

arXiv2009

The Mock LISA Data Challenges: from Challenge 1B to Challenge 3

The Mock LISA Data Challenges are a programme to demonstrate and encourage the development of LISA data-analysis capabilities, tools and techniques. At the time of this workshop, three rounds of challenges had been completed, and the next was about to start. In this article we provide a critical analysis of entries to the latest completed round, Challenge 1B. The entries confirm the consolidation of a range of data-analysis techniques for Galactic and massive--black-hole binaries, and they include the first convincing examples of detection and parameter estimation of extreme--mass-ratio inspiral sources. In this article we also introduce the next round, Challenge 3. Its data sets feature more realistic waveform models (e.g., Galactic binaries may now chirp, and massive--black-hole binaries may precess due to spin interactions), as well as new source classes (bursts from cosmic strings, isotropic stochastic backgrounds) and more complicated nonsymmetric instrument noise.

gr-qc↗

arXiv2009

Template banks to search for compact binaries with spinning components in gravitational wave data

Gravitational waves from coalescing compact binaries are one of the most promising sources for detectors such as LIGO, Virgo and GEO600. If the components of the binary posess significant angular momentum (spin), as is likely to be the case if one component is a black hole, spin-induced precession of a binary's orbital plane causes modulation of the gravitational-wave amplitude and phase. If the templates used in a matched-filter search do not accurately model these effects then the sensitivity, and hence the detection rate, will be reduced. We investigate the ability of several search pipelines to detect gravitational waves from compact binaries with spin. We use the post-Newtonian approximation to model the inspiral phase of the signal and construct two new template banks using the phenomenological waveforms of Buonanno, Chen and Vallisneri. We compare the performance of these template banks to that of banks constructed using the stationary phase approximation to the non-spinning post-Newtonian inspiral waveform currently used by LIGO and Virgo in the search for compact binary coalescence. We find that, at the same false alarm rate, a search pipeline using phenomenological templates is no more effective than a pipeline which uses non-spinning templates. We recommend the continued use of the non-spinning stationary phase template bank until the false alarm rate associated with templates which include spin effects can be substantially reduced.

gr-qc↗

arXiv2009

Physics, Astrophysics and Cosmology with Gravitational Waves

Gravitational wave detectors are already operating at interesting sensitivity levels, and they have an upgrade path that should result in secure detections by 2014. We review the physics of gravitational waves, how they interact with detectors (bars and interferometers), and how these detectors operate. We study the most likely sources of gravitational waves and review the data analysis methods that are used to extract their signals from detector noise. Then we consider the consequences of gravitational wave detections and observations for physics, astrophysics, and cosmology.

gr-qc↗

arXiv2009

Searching for binary coalescences with inspiral templates: Detection and parameter estimation

There has been remarkable progress in numerical relativity recently. This has led to the generation of gravitational waveform signals covering what has been traditionally termed the three phases of the coalescence of a compact binary - the inspiral, merger and ringdown. In this paper, we examine the usefulness of inspiral only templates for both detection and parameter estimation of the full coalescence waveforms generated by numerical relativity simulations. To this end, we deploy as search templates waveforms based on the effective one-body waveforms terminated at the light-ring as well as standard post-Newtonian waveforms. We find that both of these are good for detection of signals. Parameter estimation is good at low masses, but degrades as the mass of the binary system increases.

gr-qc↗

arXiv2008

A hierarchical search for gravitational waves from supermassive black hole binary mergers

We present a method to search for gravitational waves from coalescing supermassive binary black holes in LISA data. The search utilizes the $\mathcal{F}$-statistic to maximize over, and determine the values of, the extrinsic parameters of the binary system. The intrinsic parameters are searched over hierarchically using stochastically generated multi-dimensional template banks to recover the masses and sky locations of the binary. We present the results of this method applied to the mock LISA data Challenge 1B data set.

gr-qc↗

arXiv2008

A geometric algorithm for efficient coincident detection of gravitational waves

Data from a network of gravitational wave detectors can be analyzed in coincidence to increase detection confidence and reduce non-stationarity of the background. We propose and explore a geometric algorithm to combine the data from a network of detectors. The algorithm makes optimal use of the variances and covariances that exist amongst the different parameters of a signal in a coincident detection of events. The new algorithm essentially associates with each trigger ellipsoidal regions in parameter space defined by the covariance matrix. Triggers from different detectors are deemed to be in coincidence if their ellipsoids have a non-zero overlap. Compared to an algorithm that uses uncorrelated windows separately for each of the signal parameters, the new algorithm greatly reduces the background rate thereby increasing detection efficiency at a given false alarm rate.

gr-qc↗

arXiv2007

Higher signal harmonics, LISA's angular resolution, and dark energy

It is generally believed that the angular resolution of the Laser Interferometer Space Antenna (LISA) for binary supermassive black holes (SMBH) will not be good enough to identify the host galaxy or galaxy cluster. This conclusion, based on using only the dominant harmonic of the binary SMBH signal, changes substantially when higher signal harmonics are included in assessing the parameter estimation problem. We show that in a subset of the source parameter space the angular resolution increases by more than a factor of 10, thereby making it possible for LISA to identify the host galaxy/galaxy cluster. Thus, LISA's observation of certain binary SMBH coalescence events could constrain the dark energy equation of state to within a few percent, comparable to the level expected from other dark energy missions.

astro-ph↗

arXiv2007

Higher harmonics increase LISA's mass reach for supermassive black holes

Current expectations on the signal to noise ratios and masses of supermassive black holes which the Laser Interferometer Space Antenna (LISA) can observe are based on using in matched filtering only the dominant harmonic of the inspiral waveform at twice the orbital frequency. Other harmonics will affect the signal-to-noise ratio of systems currently believed to be observable by LISA. More significantly, inclusion of other harmonics in our matched filters would mean that more massive systems that were previously thought to be {\it not} visible in LISA should be detectable with reasonable SNRs. Our estimates show that we should be able to significantly increase the mass reach of LISA and observe the more commonly occurring supermassive black holes of masses $\sim 10^8M_\odot.$ More specifically, with the inclusion of all known harmonics LISA will be able to observe even supermassive black hole coalescences with total mass $\sim 10^8 M_\odot (10^9M_\odot)$ (and mass-ratio 0.1) for a low frequency cut-off of $10^{-4}{\rm Hz}$ $(10^{-5}{\rm Hz})$ with an SNR up to $\sim 60$ $(\sim 30)$ at a distance of 3 Gpc. This is important from the astrophysical viewpoint since observational evidence for the existence of black holes in this mass range is quite strong and binaries containing such supermassive black holes will be inaccessible to LISA if one uses as detection templates only the dominant harmonic.

gr-qc↗