arXiv subjects

Parameswaran Ajith

Publications and source records attributed to Parameswaran Ajith.

At least 37 records · Page 2Linked to original sources

arXiv2022

Improved early-warning estimates of luminosity distance and orbital inclination of compact binary mergers using higher modes of gravitational radiation

The pre-merger (early-warning) gravitational-wave (GW) detection and localization of a compact binary merger would enable astronomers to capture potential electromagnetic (EM) emissions around the time of the merger, thus shedding light on the complex physics of the merger. While early detection and sky localization are of primary importance to the multimessenger follow-up of the event, improved estimates of luminosity distance and orbital inclination could also provide insights on the observability of the EM emission. In this work, we demonstrate that the inclusion of higher modes of gravitational radiation, which vibrate at higher multiples of the orbital frequency than the dominant mode, would significantly improve the earlywarning estimates of the luminosity distance and orbital inclination of the binary. This will help astronomers to better determine their follow-up strategy. Focusing on future observing runs of the ground-based GW detector network [O5 run of LIGOVirgo-KAGRA, Voyager, and third-generation (3G) detectors], we show that for a range of masses spanning the neutron-star black-hole binaries that are potentially EM-bright, the inclusion of higher modes improve the luminosity distance estimates by a factor of ~ 1 - 1.5 (1.1 - 2) [1.1 - 5] for the O5 (Voyager) [3G] observing scenario, 45 (45) [300] seconds before the merger for the sources located at 100 Mpc. There are significant improvements in orbital inclination estimates as well. We also investigate these improvements with varying sky-location and polarization angle. Combining the luminosity distance uncertainties with localization skyarea estimates, we find that the number of galaxies within localization volume is reduced by a factor of ~ 1 - 2.5 (1.2 - 4) [1.2 - 10] with the inclusion of higher modes at early-warning time of 45 (45) [300] seconds in O5 (Voyager) [3G].

astro-ph.HE↗

arXiv2021

Rapid Identification of Strongly Lensed Gravitational-Wave Events with Machine Learning

A small fraction of the gravitational-wave (GW) signals that will be detected by second and third generation detectors are expected to be strongly lensed by galaxies and clusters, producing multiple observable copies. While optimal Bayesian model selection methods are developed to identify lensed signals, processing tens of thousands (billions) of possible pairs of events detected with second (third) generation detectors is both computationally intensive and time consuming. To mitigate this problem, we propose to use machine learning to rapidly rule out a vast majority of candidate lensed pairs. As a proof of principle, we simulate non-spinning binary black hole events added to Gaussian noise, and train the machine on their time-frequency maps (Q-transforms) and localisation skymaps (using Bayestar), both of which can be generated in seconds. We show that the trained machine is able to accurately identify lensed pairs with efficiencies comparable to existing Bayesian methods.

gr-qc↗

arXiv2021

Constraints on the time variation of the gravitational constant using gravitational-wave observations of binary neutron stars

We propose a method to constrain the variation of the gravitational constant $G$ with cosmic time using gravitational-wave (GW) observations of merging binary neutron stars. The method essentially relies on the fact that the maximum and minimum allowed masses of neutron stars at a particular cosmic epoch has a simple dependence on the value of $G$ at that epoch. GWs carry an imprint of the value of $G$ at the time of the merger. Thus, if the value of $G$ at merger is significantly different from its current value, the masses of the neutron stars inferred from the GW observations will be inconsistent with the theoretically allowed range. This enables us to place bounds on the variation of $G$ between the merger epoch and the present epoch. Using the observation of the binary neutron star system GW170817, we constrain the fractional difference in $G$ between the merger and the current epoch to be in the range $-1 \lesssim ΔG/G \lesssim 8$. Assuming a monotonic variation in $G$, this corresponds to a bound on the average rate of change of $-7 \times 10^{-9}~\mathrm{yr}^{-1} \le \dot{G}/G \le 5 \times 10^{-8}~\mathrm{yr}^{-1}$ between these epochs. Future observations will put tight constraints on the deviation of $G$ over vast cosmological epochs not probed by other observations.

gr-qc↗

arXiv2021

Search for the stochastic gravitational-wave background induced by primordial curvature perturbations in LIGO's second observing run

Primordial density perturbations in the radiation-dominated era of the early Universe are expected to generate stochastic gravitational waves (GWs) due to nonlinear mode coupling. In this \emph{Letter}, we report on a search for such a stochastic GW background in the data of the two LIGO detectors during their second observing run (O2). We focus on the primordial perturbations in the range of comoving wavenumbers $10^{16}-10^{18}~{\rm Mpc}^{-1}$ for which the stochastic background falls within the detectors' sensitivity band. We do not find any conclusive evidence of this stochastic signal in the data, and thus place the very first GW-based constraints on the amplitude of the power spectrum at these scales. We assume a lognormal shape for the power spectrum and Gaussian statistics for the primordial perturbations, and vary the width of the power spectrum to cover both narrow and broad spectra. Derived upper limits ($95\%$) on the amplitude of the power spectrum are $0.01-0.1$. As a byproduct, we are able to infer upper limits on the fraction of the Universe's mass in ultralight primordial black holes ($M_\mathrm{PBH} \simeq 10^{-20}-10^{-19}M_{\odot}$) at their formation time to be $\lesssim 10^{-25}$.

gr-qc↗

arXiv2021

Improved early warning of compact binary mergers using higher modes of gravitational radiation: A population study

A gravitational-wave (GW) early-warning of a compact-binary coalescence event, with a sufficiently tight localisation skymap, would allow telescopes to point in the direction of the potential electromagnetic counterpart before its onset. This will enable astronomers to extract valuable information of the complex astrophysical phenomena triggered around the time of the merger. Use of higher-modes of gravitational radiation, in addition to the dominant mode typically used in templated real-time searches, was recently shown to produce significant improvements in early-warning times and skyarea localisations for a range of asymmetric-mass binaries. In this work, we perform a large-scale study to assess the benefits of this method for a population of compact binary merger observations. In particular, we inject 100,000 such signals in Gaussian noise, with component masses $m_1 \in \left[1, 60 \right] M_{\odot}$ and $m_2 \in \left [1, 3 \right] M_{\odot}$. We consider three scenarios involving ground-based detectors: the fifth (O5) observing run of the Advanced LIGO-Virgo-KAGRA network, its projected Voyager upgrade, as well as a proposed third generation (3G) network. We find that for fixed early warning times of $20-60$ seconds, the inclusion of the higher modes can provide localisation improvements of a factor of $\gtrsim 2$ for up to $\sim 60\%$ ($70 \%$) of the neutron star-black hole systems in the O5 (Voyager) scenario. Considering only those neutron star-black hole systems which can produce potential electromagnetic counterparts, such improvements in the localisation can be expected for $\sim 5-35\%$ $(20-50\%)$ binaries in O5 (Voyager), although the localisation areas themselves depend on the distances. For the 3G scenario, a significant fraction of the events have time gains of a minute to several minutes, assuming fiducial target localisation areas of 100 to 1000 sq. deg.

astro-ph.HE↗

arXiv2020

Constraining black hole mimickers with gravitational wave observations

LIGO and Virgo have recently observed a number of gravitational wave (GW) signals that are fully consistent with being emitted by binary black holes described by general relativity. However, there are theoretical proposals of exotic objects that can be massive and compact enough to be easily confused with black holes. Nevertheless, these objects differ from black holes in having nonzero tidal deformabilities, which can allow one to distinguish binaries containing such objects from binary black holes using GW observations. Using full Bayesian parameter estimation, we investigate the possibility of constraining the parameter space of such "black hole mimickers" with upcoming GW observations. Employing perfect fluid stars with a polytropic equation of state as a simple model that can encompass a variety of possible black hole mimickers, we show how the observed masses and tidal deformabilities of a binary constrain the equation of state. We also show how such constraints can be used to rule out some simple models of boson stars.

gr-qc↗

arXiv2020

Testing the nature of gravitational-wave polarizations using strongly lensed signals

Gravitational-wave (GW) observations by a network of ground-based laser interferometric detectors allow us to probe the nature of GW polarizations. This would be an interesting test of general relativity (GR), since GR predicts only two polarization modes while there are theories of gravity that predict up to six polarization modes. The ability of GW observations to probe the nature of polarizations is limited by the available number of linearly independent detectors in the network. (To extract all polarization modes, there should be at least as many detectors as the polarization modes.) Strong gravitational lensing of GWs offers a possibility to significantly increase the effective number of detectors in the network. Due to strong lensing (e.g., by galaxies), multiple copies of the same signal can be observed with time delays of several minutes to weeks. Owing to the rotation of the earth, observation of the multiple copies of the same GW signal would allow the network to measure different combinations of the same polarizations. This effectively multiplies the number of detectors in the network. Focusing on strongly lensed signals from binary black hole mergers that produce two observable "images", using Bayesian model selection and assuming simple polarization models, we show that our ability to distinguish between polarization models is significantly improved.

gr-qc↗

arXiv2020

Prospects for probing ultralight primordial black holes using the stochastic gravitational-wave background induced by primordial curvature perturbations

Ultralight primordial black holes (PBHs) with masses $\lesssim 10^{15}$g and subatomic Schwarzschild radii, produced in the early Universe, are expected to have evaporated by the current cosmic age due to Hawking radiation. Based on this assumption, a number of constraints on the abundance of ultralight PBHs have been made. However, Hawking radiation has thus far not been verified experimentally. It would, therefore, be of interest if constraints on ultralight PBHs could be placed independent of the assumption of Hawking-radiation. In this paper, we explore the possibility of probing these PBHs, within a narrow mass range, using gravitational-wave (GW) data from the two LIGO detectors. The idea is that large primordial curvature perturbations that result in the formation of PBHs, would also generate GWs through non-linear mode couplings. These induced GWs would produce a stochastic background. Specifically, we focus our attention on PBHs of mass range $\sim 10^{13} - 10^{15}$g for which the induced stochastic GW background peak falls in the sensitivity band of LIGO. We find that, for both narrow and broad Gaussian PBH mass distributions, the corresponding GW background would be detectable using presently available LIGO data, provided we neglect the existing constraints on the abundance of PBHs, which are based on Hawking radiation. Furthermore, we find that these stochastic backgrounds would be detectable in LIGO's third observing run, even after considering the existing constraints on PBH abundance. A non-detection should enable us to constrain the amplitude of primordial curvature perturbations as well as the abundance of ultralight PBHs. We estimate that by the end of the third observing run, assuming non-detection, we should be able to place constraints that are orders of magnitude better than currently existing ones.

astro-ph.CO↗

arXiv2020

Of Harbingers and Higher Modes: Improved gravitational-wave early-warning of compact binary mergers

A crucial component to maximizing the science gain from the multi-messenger follow-up of gravitational-wave (GW) signals from compact binary mergers is the prompt discovery of the electromagnetic counterpart. Ideally, the GW detection and localization must be reported early enough to allow for telescopes to slew to the location of the GW-event before the onset of the counterpart. However, the time available for early warning is limited by the short duration spent by the dominant ($\ell = m = 2$) mode within the detector's frequency band. Nevertheless, we show that, including higher modes - which enter the detector's sensitivity band well before the dominant mode - in GW searches, can enable us to significantly improve the early warning ability for compact binaries with asymmetric masses (such as neutron-star-black-hole binaries). We investigate the reduction in the localization sky-area when the $\ell = m = 3$ and $\ell = m = 4$ modes are included in addition to the dominant mode, considering typical slew-times of electromagnetic telescopes ($30-60$ sec). We find that, in LIGO's projected "O5" ("Voyager") network with five GW detectors, some of the neutron-star-black-hole mergers, located at a distance of $40$ Mpc, can be localized to a few hundred sq. deg. $\sim 45$ sec prior to the merger, corresponding to a reduction-factor of $3-4$ ($5-6$) in sky-area. For a third-generation network, we get gains of up to 1.5 minutes in early warning times for a localization area of $100$ sq. deg., even when the source is placed at $100$ Mpc.

astro-ph.HE↗

arXiv2020

Can Kilonova Light curves be Standardized?

Binary neutron star mergers have been recently confirmed to be the progenitors of the optical transients kilonovae (KNe). KNe are powered by the radioactive decay of neutron-rich elements (r-process elements) which are believed to be the product of disruption of neutron stars during their merger. KNe exhibit interesting parallels with type Ia supernovae (SNe), whose light curves show specific correlations which allow them to be used as standardizable candles. In this paper, we investigate the possibility of the KN light curves exhibiting similar correlations. While a satisfactory answer to this question can only be provided by future KN observations, employing theoretical models we explore whether there is any ground for harboring such expectations. Using semi-analytic models of KN light curves in conjunction with results from numerical relativity simulations of binary neutron star mergers, we obtain the maximum bolometric luminosity ($L_{\mathrm{Bol}}^{\mathrm{max}}$) and decline in luminosity ($ΔL_{\mathrm{Bol}}$) for a simulated population of mergers. We find that theoretical light curves of KNe show remarkable correlations despite the complex physics governing their behavior. This presents a possibility of future observations to uncover such correlations in the observed light curves, eventually allowing observers to standardize these light curves and to use them for local distance measurements.

astro-ph.SR↗

arXiv2019

Testing the"no-hair" nature of binary black holes using the consistency of multipolar gravitational radiation

Gravitational-wave (GW) observations of binary black holes offer the best probes of the relativistic, strong-field regime of gravity. Gravitational radiation, in the leading order is quadrupolar. However, non-quadrupole (higher-order) modes make appreciable contribution to the radiation from binary black holes with large mass ratios and misaligned spins. The multipolar structure of the radiation is fully determined by the intrinsic parameters (masses and spin angular momenta of the companion black holes) of a binary in quasi-circular orbit. Following our previous work \cite{Dhanpal:2018ufk}, we develop multiple ways of testing the consistency of the observed GW signal with the expected multipolar structure of radiation from binary black holes in general relativity. We call this a "no-hair" test of binary black holes as this is similar to testing the "no-hair" theorem for isolated black holes through mutual consistency of the quasi-normal mode spectrum. We use Bayesian inference on simulated GW signals that are consistent/inconsistent with binary black holes in GR to demonstrate the power of the proposed tests. We also make estimate systematic errors arising as a result of neglecting companion spins.

gr-qc↗

arXiv2019

Including mode mixing in a higher-multipole model for gravitational waveforms from nonspinning black-hole binaries

As gravitational-wave (GW) observations of binary black holes are becoming a precision tool for physics and astronomy, several subdominant effects in the GW signals need to be accurately modeled. Previous studies have shown that neglecting subdominant modes in the GW templates causes an unacceptable loss in detection efficiency and large systematic errors in the estimated parameters for binaries with large mass ratios. Our recent work [Mehta et al., Phys. Rev. D 96, 124010 (2017)] constructed a phenomenological gravitational waveform family for nonspinning black-hole binaries that includes subdominant spherical harmonic modes $(\ell = 2, m = \pm 1)$, $(\ell = 3, m = \pm 3)$, and $(\ell = 4, m = \pm 4)$ in addition to the dominant quadrupole mode, $(\ell = 2, m=\pm 2)$. In this article, we construct analytical models for the ($\ell = 3, m = \pm 2$) and ($\ell = 4, m = \pm 3$) modes and include them in the existing waveform family. Accurate modeling of these modes is complicated by the mixing of multiple spheroidal harmonic modes. We develop a method for accurately modeling the effect of mode mixing, thus producing an analytical waveform family that has faithfulness >99.6%.

gr-qc↗

arXiv2018

Effectual gravitational-wave template banks for coalescing compact binaries using a hybrid placement algorithm

Recent discoveries of gravitational wave (GW) signals from astrophysical compact binary systems of neutron stars and black holes have firmly established them as prime sources for advanced GW detectors. Theoretical templates of expected signals from such systems have been used to filter the detector data using the \emph{matched filtering} technique. An efficient grid over the parameter space at a fixed minimal match has a direct impact on improving the computational efficiency of these searches. We present the construction of three-dimensional template banks (in component masses and an effective spin parameter) by incorporating several new optimizations to the hybrid geometric-random template placement algorithm that we proposed recently. These optimizations allow us to create more efficient template banks in future compact binary searches by shrinking the hybrid banks by $\sim 34\%$ in comparison to the basic algorithm. Such optimized banks are also found to be $\sim 22\%$ smaller than the optimized stochastic bank constructed over a nominal range of parameters. We also construct an explicit hybrid template bank with parameters identical to the `uber-bank' used in the recently-concluded CBC searches in the second observation run of the Advanced LIGO and Virgo detectors. We demonstrate a reduction of more than 53,000 templates over the stochastic template bank at a near-identical coverage as determined by fitting factor studies. A computationally efficient technique for semi-numerical calculation of the parameter space metric, applicable for aligned-spin waveform family, is also outlined. The resulting hybrid template banks can be generated much faster in comparison to the stochastic banks, and are ready to be used in the upcoming observation runs of Advanced LIGO and Virgo.

gr-qc↗

arXiv2018

Identifying strongly lensed gravitational wave signals from binary black hole mergers

Based on the rate of gravitational-wave (GW) detections by Advanced LIGO and Virgo, we expect these detectors to observe hundreds of binary black hole mergers as they achieve their design sensitivities (within a few years). A small fraction of them can undergo strong gravitational lensing by intervening galaxies, resulting in multiple images of the same signal. To a very good approximation, the lensing magnifies/de-magnifies these GW signals without affecting their frequency profiles. We develop a Bayesian inference technique to identify pairs of strongly lensed images among hundreds of binary black hole events and demonstrate its performance using simulated GW observations.

gr-qc↗

arXiv2018

Testing general relativity using gravitational wave signals from the inspiral, merger and ringdown of binary black holes

Advanced LIGO's recent observations of gravitational waves (GWs) from merging binary black holes have opened up a unique laboratory to test general relativity (GR) in the highly relativistic regime. One of the tests used to establish the consistency of the first LIGO event with a binary black hole merger predicted by GR was the inspiral-merger-ringdown consistency test. This involves inferring the mass and spin of the remnant black hole from the inspiral (low-frequency) part of the observed signal and checking for the consistency of the inferred parameters with the same estimated from the post-inspiral (high-frequency) part of the signal. Based on the observed rate of binary black hole mergers, we expect the advanced GW observatories to observe hundreds of binary black hole mergers every year when operating at their design sensitivities, most of them with modest signal to noise ratios (SNRs). Anticipating such observations, this paper shows how constraints from a large number of events with modest SNRs can be combined to produce strong constraints on deviations from GR. Using kludge modified GR waveforms, we demonstrate how this test could identify certain types of deviations from GR if such deviations are present in the signal waveforms. We also study the robustness of this test against reasonable variations of a variety of different analysis parameters.

gr-qc↗

arXiv2018

A "no-hair" test for binary black holes

One of the consequences of the black-hole "no-hair" theorem in general relativity (GR) is that gravitational radiation (quasi-normal modes) from a perturbed Kerr black hole is uniquely determined by its mass and spin. Thus, the spectrum of quasi-normal mode frequencies have to be all consistent with the same value of the mass and spin. Similarly, the gravitational radiation from a coalescing binary black hole system is uniquely determined by a small number of parameters (masses and spins of the black holes and orbital parameters). Thus, consistency between different spherical harmonic modes of the radiation is a powerful test that the observed system is a binary black hole predicted by GR. We formulate such a test, develop a Bayesian implementation, demonstrate its performance on simulated data and investigate the possibility of performing such a test using previous and upcoming gravitational wave observations.

gr-qc↗

arXiv2017

Effects of nonquadrupole modes in the detection and parameter estimation of black hole binaries with nonprecessing spins

We study the effect of nonquadrupolar modes in the detection and parameter estimation of gravitational waves (GWs) from black hole binaries with nonprecessing spins, using Advanced LIGO. We evaluate the loss of the signal-to-noise ratio (SNR) and the systematic errors in the estimated parameters when a quadrupole-mode template family is used to detect GW signals with all the relevant modes. Target signals including nonquadrupole modes are constructed by matching numerical-relativity simulations of nonprecessing black hole binaries describing the late inspiral, merger and ringdown with post-Newtonian/effective-one-body waveforms describing the early inspiral. We find that neglecting nonquadrupole modes will, in general, cause unacceptable loss in the detection rate and unacceptably large systematic errors in the estimated parameters, for the case of massive binaries with large mass ratios. For a given mass ratio, neglecting subdominant modes will result in a larger loss in the detection rate for binaries with aligned spins. For binaries with antialigned spins, quadrupole-mode templates are more effectual in detection, at the cost of introducing a larger systematic bias in the parameter estimation. We provide a summary of the regions in the parameter space where neglecting nonquadrupole modes will cause an unacceptable loss of detection rates and unacceptably large systematic biases in the estimated parameters.

gr-qc↗

arXiv2017

Accurate inspiral-merger-ringdown gravitational waveforms for non-spinning black-hole binaries including the effect of subdominant modes

We present an analytical waveform family describing gravitational waves (GWs) from the inspiral, merger and ringdown of non-spinning black-hole binaries including the effect of several non-quadrupole modes [($\ell = 2, m = \pm 1), (\ell = 3, m = \pm 3), (\ell = 4, m = \pm 4)$ apart from $(\ell = 2, m=\pm2)$]. We first construct spin-weighted spherical harmonics modes of hybrid waveforms by matching numerical-relativity simulations (with mass ratio $1-10$) describing the late inspiral, merger and ringdown of the binary with post-Newtonian/effective-one-body waveforms describing the early inspiral. An analytical waveform family is constructed in frequency domain by modeling the Fourier transform of the hybrid waveforms making use of analytical functions inspired by perturbative calculations. The resulting highly accurate, ready-to-use waveforms are highly faithful (unfaithfulness $\simeq 10^{-4} - 10^{-2}$) for observation of GWs from non-spinning black hole binaries and are extremely inexpensive to generate.

gr-qc↗