arXiv subjects

Gopalkrishna Prabhu

Publications and source records attributed to Gopalkrishna Prabhu.

5 recordsLinked to original sources

arXiv2026

Probing magnetic fields of compact objects with continuous gravitational waves

Spinning, deformed compact objects such as neutron stars are canonical sources of continuous gravitational waves. These objects may be born with magnetic fields that can strongly influence their spin evolution and, consequently, their gravitational wave detectability. Employing a Bayesian framework, we use for the first time, the non detection of continuous gravitational waves in the LIGO Virgo KAGRA (LVK) third observing run (O3), using reported amplitude upper limits from various LVK and Einstein@Home searches, to place population level constraints on the birth magnetic field distribution of Galactic compact objects. To this end, we simulate compact object populations whose spin evolution is governed by gravitational wave emission and magnetic dipole radiation. We explore multiple ellipticity models and magnetic field decay timescales, and find that the lower limit on the birth magnetic field distribution hyperparameter $B_0$ is constrained to lie in the range $10^{9.6}\mathrm{G} \lesssim B_0\,\lesssim 10^{13.3}\mathrm{G}$. Furthermore, we reinterpret the number constraints on the total population of Galactic compact objects, reported previously by \citet{Prabhu_2024}, as upper limits on the number of compact objects with a population averaged magnetic field, presented as a function of ellipticity and gravitational wave frequency for all searches considered here.

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arXiv2025

Probing the spin of compact objects with gravitational microlensing of gravitational waves

Propagating gravitational waves (GWs) can encounter a massive object (lens) whose gravitational radius is comparable to the wavelength of the GWs (wave-optics regime). The resulting `microlensed' signal contains imprints about the properties of the lens. In this work, we compute the GW waveforms microlensed by a rotating compact object in weak-field gravity. Using these waveforms, for the first time, we assess how well the parameters of the rotating lens can be inferred from GW lensing observations. We find that if we allow for naked singular solutions within general relativity or beyond, which in principle can have spins that are not bounded to be extremal, our method can be used to extract the rotating lens parameters using observations of microlensed GWs with future detectors. As a result, we find that the lens parameters for such lenses are well recovered within $90\%$ confidence for signal-to-noise ratio (SNR) 50 and especially well for SNR$=100$ with Einstein Telescope.

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arXiv2025

Towards model-independent identification of lensed gravitational waves using Kramers-Kronig relation

Observations of microlensed gravitational waves (GWs) emanated by compact binary coalescences (CBCs) are essential for studying the mass density distribution in the universe, including black holes and dark matter halos. However, no confident detection of microlensed GWs have been reported to date. There are two important challenges in the identification of microlensed GWs. The first is that the source waveform and lens structure models are not known a-priori. The second is that certain classes of unlensed GWs could mimic microlensed GWs, resulting in undesirable false alarms. In this work, we propose to use the Kramers-Kronig relation for gravitational lensing systems. We argue that such systems are essentially linear response systems obeying causality, where KK relation must hold. The power of this method lies in the fact that microlensed GWs, regardless of the lens structure, must obey KK relation, while unlensed GW events are not in general expected to obey it. This, in principle, allows us to identify microlensed GWs while dismissing microlensing mimickers. We provide the first important steps towards a methodology that exploits KK relation, and test its usefulness under idealized conditions.

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arXiv2025

Surrogate modeling of gravitational waves microlensed by spherically symmetric potentials

The anticipated observation of the gravitational microlensing of gravitational waves (GWs) promises to shed light on a host of astrophysical and cosmological questions. However, extracting the parameters of the lens from the modulated GWs requires accurate modeling of the lensing amplification factor, accounting for wave-optics effects. Analytic solutions to the lens equation have not been found to date, except for a handful of simplistic lens models. While numerical solutions to this equation have been developed, the time and computational resources required to evaluate the amplification factor numerically make large-scale parameter estimation of the lens (and source) parameters prohibitive. On the other hand, surrogate modeling of GWs has proven to be a powerful tool to accurately, and rapidly, produce GW templates at arbitrary points in parameter space, interpolating from a finite set of available waveforms at discrete parameter values. In this work, we demonstrate that surrogate modeling can also effectively be applied to the evaluation of the time-domain microlensing amplification factor $\widetilde{F}(t)$. We show this by constructing $\widetilde{F}(t)$ for two lens models, viz. point-mass lens, and singular isothermal sphere, which notably includes logarithmic divergence behaviour. We find both surrogates reproduce the original lens models accurately, with mismatches $\lesssim 5 \times 10^{-4}$ across a range of plausible microlensed binary black hole sources observed by the Einstein Telescope. This surrogate is between 5 and $10^3$ times faster than the underlying lensing models, and can be evaluated in about 100 ms. The accuracy and efficiency attained by our surrogate models will enable practical parameter estimation analyses of microlensed GWs.

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arXiv2024

Constraining the abundance of spinning deformed Galactic compact objects with continuous gravitational waves

Galactic spinning compact objects (COs) with non-zero ellipticity are expected to be sources of continuous gravitational waves (CGWs). Certain classes of hypothetical COs, such as neutron stars with quark cores (hybrid stars), and quark stars, are thought to be capable of sustaining large ellipticities from theoretical considerations. Such exotic COs (eCOs) with large ellipticities should produce CGWs detectable by the current LIGO-Virgo-Kagra GW detector network. Since no detections for CGWs, from searches in LIGO-Virgo data, have so far been reported, we place constraints on the abundance of highly elliptical eCOs in our Galaxy. We formulate a Bayesian framework to place upper limits on the number count $N_{tot}$ of highly deformed Galactic eCOs. We divide our constraints into two classes: an "agnostic" set of upper limits on $N_{tot}$ evaluated on a CGW frequency and ellipticity grid that depend only on the choice of spatial distribution of COs; and a model-dependent set that additionally assumes prior information on the distribution of frequencies. We find that COs with ellipticities $ε\gtrsim 10^{-5}$ have abundance upper limits at $90\%$ confidence, of $N_{tot}^{90\%} \lesssim 100$, and those with $ε\gtrsim 10^{-6}$ have $N_{tot}^{90\%} \lesssim 10^4$. We additionally place upper-limits on the ellipticity of Galactic COs informed by our choices of spatial distributions, given different abundances $N_{tot}$.

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