arXiv subjects

Lalit Pathak

Publications and source records attributed to Lalit Pathak.

9 recordsLinked to original sources

arXiv2026

On the Presence of a Tertiary Compact Object in GW190814

Gravitational waves from merging compact binaries are sensitive to line-of-sight acceleration (LOSA) induced by a massive companion in their vicinity. Interestingly, the leading-order contributions of LOSA and residual orbital eccentricity to the Fourier phase of the inspiral waveform exhibit similar frequency dependence, raising the possibility that a small eccentricity could mimic LOSA effects in transient GW events such as GW190814. We perform Bayesian inference using the IMRPhenomXPHM waveform family as the baseline LIGO-Virgo-KAGRA waveform model, augmented with leading-order LOSA and residual eccentricity corrections while using 32 seconds of data associated with GW190814. For a LOSA-only analysis, we find no evidence for a non-zero LOSA effect in GW190814, with a Bayes factor relative to the baseline model of approximately 0.22, consistent with the findings of Hendriks et al. and in tension with the claim by Yang et al., who employed only 4 seconds of GW190814 data. In a joint analysis that includes both leading-order LOSA and eccentricity effects, we obtain informative posteriors for both parameters, with representative values a/c approximately -2.8 x 10^{-3} s^{-1} and e_0 approximately 0.11. However, the corresponding Bayes factor relative to the baseline model is approximately 0.64, suggesting that the 32-second data do not provide significant evidence for either LOSA or residual eccentricity in GW190814. Further, our Bayesian runs reveal a strong correlation between the LOSA and eccentricity parameters, indicating a significant degeneracy in their imprint on the inspiral phase. This finding is consistent with our theoretical arguments and most likely explains the non-zero parameter estimates obtained in the joint analysis.

gr-qc↗

arXiv2026

Blinded Mock Data Challenge: Is the Spectral Siren Technique Robust for Measuring the Hubble Constant?

The measurement of the Hubble constant from gravitational wave (GW) sources is one of the independent avenues to shed light on the Hubble tension, which is associated with about an $8\%$ mismatch in the value of the Hubble constant inferred from low-redshift and high-redshift cosmological probes. Such a key measurement is expected from GW sources as it is a direct measurement of the Hubble constant using the luminosity distance without the need for any luminosity distance calibration. However, such a measurement relies strongly on the reliability of the independent inference of the source redshift of the GW source. As a result, it becomes pertinent to gauge the accuracy and precision of techniques in understanding their reliability in inferring redshifts of GW sources. In this work, we show the requirement of the spectral siren technique in knowing the mass distribution of BBHs across cosmic redshifts in order to make a reliable inference of the Hubble constant. We show by a blinded mock data challenge analysis the criticality in capturing the underlying metallicity dependence of the BBH mass distribution and its interplay with time-delay distribution for a robust inference of the Hubble constant using the spectral siren technique. In order to have a reliable measurement of the Hubble constant at the level required to resolve the Hubble tension in the future, the mass distribution of the BBHs needs to be independently inferred at all relevant redshifts with an accuracy less than the statistical uncertainty. Otherwise, a mismatch of the true model and the underlying assumption made in the analysis can lead to a best-fit model for the wrong value of both BBH population parameters as well as the Hubble constant.

astro-ph.CO↗

arXiv2025

A meshfree approach for rapid reconstruction of compact binary sources detected in the network of international gravitational wave detectors

This thesis explores parameter estimation methods for rapidly reconstructing compact binary sources generating gravitational waves. It employs numerical linear algebra and meshfree approximation techniques to expedite waveform generation and likelihood evaluation, crucial for estimating binary parameters like masses, spins, distance, and sky location. The thesis demonstrates the effectiveness of these methods through simulations and real GW events, highlighting their potential for multimessenger astronomy and future gravitational wave observatories.

gr-qc↗

arXiv2025

Rapid parameter estimation with the full symphony of compact binary mergers using meshfree approximation

We present a fast Bayesian inference framework to address the growing computational cost of gravitational-wave parameter estimation. The increased cost is driven by improved broadband detector sensitivity, particularly at low frequencies due to advances in detector commissioning, resulting in longer in-band signals and a higher detection rate. Waveform models now incorporate features like higher-order modes, further increasing the complexity of standard inference methods. Our framework employs meshfree likelihood interpolation with radial basis functions to accelerate Bayesian inference using the IMRPhenomXHM waveform model that incorporates higher modes of the gravitational-wave signal. In the initial start-up stage, interpolation nodes are placed within a constant-match metric ellipsoid in the intrinsic parameter space. During sampling, likelihood is evaluated directly using the precomputed interpolants, bypassing the costly steps of on-the-fly waveform generation and overlap-integral computation. We improve efficiency by sampling in a rotated parameter space aligned with the eigenbasis of the metric ellipsoid, where parameters are uncorrelated by construction. This speeds up sampler convergence. This method yields unbiased parameter recovery when applied to 100 simulated neutron-star-black-hole signals (NSBH) in LIGO-Virgo data, while reducing computational cost by up to an order of magnitude for the longest-duration signal. The meshfree framework equally applies to symmetric compact binary systems dominated by the quadrupole mode, supporting parameter estimation across a broad range of sources. Applied to a simulated NSBH signal in Einstein Telescope data, where the effects of Earth's rotation are neglected for simplicity, our method achieves an O(10^4) speed-up, demonstrating its potential use in the third-generation (3G) era.

gr-qc↗

arXiv2024

Fast and faithful interpolation of numerical relativity surrogate waveforms using meshfree approximation

Several theoretical waveform models have been developed over the years to capture the gravitational wave emission from the dynamical evolution of compact binary systems of neutron stars and black holes. As ground-based detectors improve their sensitivity at low frequencies, the real-time computation of these waveforms can become computationally expensive, exacerbating the steep cost of rapidly reconstructing source parameters using Bayesian methods. This paper describes an efficient numerical algorithm for generating high-fidelity interpolated compact binary waveforms at an arbitrary point in the signal manifold by leveraging computational linear algebra techniques such as singular value decomposition and meshfree approximation. The results are presented for the time-domain \texttt{NRHybSur3dq8} inspiral-merger-ringdown (IMR) waveform model that is fine tuned to numerical relativity simulations and parameterized by the two component-masses and two aligned spins. For demonstration, we target a specific region of the intrinsic parameter space inspired by the previously inferred parameters of the \texttt{GW200311\_115853} event -- a binary black hole system whose merger was recorded by the network of advanced-LIGO and Virgo detectors during the third observation run. We show that the meshfree interpolated waveforms can be evaluated in $\sim 2.3$ ms, which is about $\times 38$ faster than its brute-force (frequency-domain tapered) implementation in the \textsc{PyCBC} software package at a median accuracy of $\sim \mathcal{O}(10^{-5})$. The algorithm is computationally efficient and scales favourably with an increasing number of dimensions of the parameter space. This technique may find use in rapid parameter estimation and source reconstruction studies.

gr-qc↗

arXiv2023

Pinpointing coalescing binary neutron star sources with the IGWN, including LIGO-Aundha

LIGO-Aundha (A1), the Indian gravitational wave detector, is expected to join the IGWN and begin operations in the early 2030s. We study the impact of A1 on the accuracy of determining the direction of incoming transient signals from coalescing BNS sources with moderately high SNRs. It is conceivable that A1's sensitivity, effective bandwidth, and duty cycle will improve incrementally through multiple detector commissioning rounds to achieve the desired `LIGO-A+' design sensitivity. For this purpose, we examine A1 under two distinct noise PSDs. One mirrors the conditions during the O4 run of the LIGO Hanford and Livingston detectors, simulating an early commissioning stage, while the other represents the A+ design sensitivity. We consider various duty cycles of A1 at the sensitivities mentioned above for a comprehensive analysis. We show that even at the O4 sensitivity with a modest $20\%$ duty cycle, A1's addition to the IGWN leads to a $15\%$ reduction in median sky-localization errors ($\Delta \Omega_{90\%}$) to $5.6$~sq.~deg. At its design sensitivity and $80\%$ duty cycle, this error shrinks further to $2.4$~sq.~deg, with 84\% sources localized within a nominal error box of $10$~sq.~deg! Even in the worst-case scenario, where signals are sub-threshold in A1, we demonstrate its critical role in reducing the localization uncertainties of the BNS source. Our results are obtained from a large Bayesian PE study using simulated signals injected in a heterogeneous network of detectors using the recently developed meshfree approximation aided rapid Bayesian inference pipeline. We consider a seismic cut-off frequency of 10 Hz for all the detectors. We also present hypothetical improvements in sky localization for a few GWTC-like events injected in real data and demonstrate A1's role in resolving the degeneracy between the luminosity distance and inclination angle parameters.

gr-qc↗

arXiv2023

Prompt sky localization of compact binary sources using a meshfree approximation

The number of gravitational wave signals from the merger of compact binary systems detected in the network of advanced LIGO and Virgo detectors is expected to increase considerably in the upcoming science runs. Once a confident detection is made, it is crucial to reconstruct the source's properties rapidly, particularly the sky position and chirp mass, to follow up on these transient sources with telescopes operating at different electromagnetic bands for multi-messenger astronomy. In this context, we present a rapid parameter estimation (PE) method aided by mesh-free approximations to accurately reconstruct properties of compact binary sources from data gathered by a network of gravitational wave detectors. This approach builds upon our previous algorithm [L. Pathak et al., Fast likelihood evaluation using meshfree approximations for reconstructing compact binary sources, https://journals.aps.org/prd/abstract/10.1103/PhysRevD.108.064055, Phys. Rev. D 108, 064055 (2023)] to expedite the evaluation of the likelihood function and extend it to enable coherent network PE in a ten-dimensional parameter space, including sky position and polarization angle. Additionally, we propose an optimized interpolation node placement strategy during the start-up stage to enhance the accuracy of the marginalized posterior distributions. With this updated method, we can estimate the properties of binary neutron star (BNS) sources in approximately 2.4~(2.7) min for the \TaylorF~(\texttt{IMRPhenomD}) signal model by utilizing 64 CPU cores on a shared memory architecture. Furthermore, our approach can be integrated into existing parameter estimation pipelines, providing a valuable tool for the broader scientific community. We also highlight some areas for improvements to this algorithm in the future, which includes overcoming the limitations due to narrow prior bounds.

gr-qc↗

arXiv2023

Does the speed of gravitational waves depend on the source velocity?

The second postulate of special relativity states that the speed of light in vacuum is independent of the emitter's motion. The test of this postulate so far remains unexplored for gravitational radiation. We analyze data from the LIGO-Virgo detectors to test this postulate within the ambit of a model where the speed of the emitted GWs ($c'$) from a binary depends on a characteristic velocity $\tilde{v}$ proportional to that of the reduced one-body system as $c' = c + k\, \tilde{v}$, where $k$ is a constant. We have estimated the upper bound on the 90\% credible interval over $k$ to be ${k \leq 8.3 \times {10}^{-18}}$, which is several orders of magnitude more stringent compared to previous bounds obtained from electromagnetic observations. The Bayes' factor supports the second postulate with a strong evidence that the data is consistent with the null hypothesis $k = 0$, upholding the principle of relativity for gravitational interactions.

gr-qc↗

arXiv2022

Fast likelihood evaluation using meshfree approximations for reconstructing compact binary sources

Several rapid parameter estimation methods have recently been advanced to deal with the computational challenges of the problem of Bayesian inference of the properties of compact binary sources detected in the upcoming science runs of the terrestrial network of gravitational wave detectors. Some of these methods are well-optimized to reconstruct gravitational wave signals in nearly real-time necessary for multi-messenger astronomy. In this context, this work presents a new, computationally efficient algorithm for fast evaluation of the likelihood function using a combination of numerical linear algebra and mesh-free interpolation methods. The proposed method can rapidly evaluate the likelihood function at any arbitrary point of the sample space at a negligible loss of accuracy and is an alternative to the grid-based parameter estimation schemes. We obtain posterior samples over model parameters for a canonical binary neutron star system by interfacing our fast likelihood evaluation method with the nested sampling algorithm. The marginalized posterior distributions obtained from these samples are statistically identical to those obtained by brute force calculations. We find that such Bayesian posteriors can be determined within a few minutes of detecting such transient compact binary sources, thereby improving the chances of their prompt follow-up observations with telescopes at different wavelengths. It may be possible to apply the blueprint of the meshfree technique presented in this study to Bayesian inference problems in other domains.

gr-qc↗