arXiv subjects

Prayush Kumar

Publications and source records attributed to Prayush Kumar.

At least 19 recordsLinked to original sources

arXiv2026

3d Summation-by-Parts scheme for Linear Wave Equations on Hyperboloidal Slices

We derive a fully 3-dimensional Summation-By-Parts scheme for a class of linear wave equations on hyperboloidal slices that meet future null infinity on a Minkowski background. The scheme is derived in spherical polar coordinates, with a major strength being that it is provably stable and allows having grid points at the origin and on the $z$-axis, despite coordinate singularities, and at infinity, by introducing compactification followed by rescaling. Reducing it to the standard Cauchy problem, or on finite spacelike slices with an outer boundary, will follow a similar procedure. Interesting relations are obtained between the rescaling and compactification factors that simplify the equations, and the conditions on constraint addition terms are discovered to maintain symmetric hyperbolicity. Numerical implementation is achieved using finite-difference methods at second-order accuracy, which can be generalized to higher-order or spectral accuracies as well. Dissipation operators are given a more abstract treatment, which makes it possible to define them everywhere in the domain, including at the boundary points, in curvilinear coordinates, such that they satisfy the dissipative property (DP) in our energy norms. These generalizations reduce to the well-known Kreiss-Oliger dissipation operators whenever defined on a Cartesian grid in the bulk and satisfy the DP in the standard $L^2$-norms. We also propose new norm convergence tests that produce more accurate outputs. Promising results are obtained, giving hope for application to fully nonlinear systems, like the Einstein Field Equations, and extracting the resulting gravitational waves free of systematic errors or gauge ambiguities.

gr-qc↗

arXiv2026

Merger remnant and eccentricity dynamics surrogates for eccentric nonspinning black hole binaries

Accurate models of merger remnants are increasingly important for gravitational-wave science, including precision tests of gravity with ringdown, inference of black-hole populations, and modeling hierarchical mergers. For eccentric binaries, remnant mass, spin, and recoil carry nontrivial imprints of eccentricity that are both physically informative and more challenging to model, yet remain less developed than in the quasi-circular case. We present two new models trained on numerical-relativity (NR) simulations of unequal-mass, non-spinning eccentric binary black holes: NRSurE_q4NoSpin_Remnant, which predicts remnant properties, and NRSurE_q4NoSpin_Dynamics, a time-domain surrogate for the evolution of eccentricity and mean anomaly. Both models are trained on NR simulations over a three-dimensional parameter space with mass ratios $q \leq 4$, eccentricity $e < 0.23$, and mean anomaly $\ell \in [0,2\pi)$ radians, where both $e$ and $\ell$ defined at $t=-1000M$ relative to peak amplitude and $M$ is the total mass. We highlight some applications, including the phenomenological impact of eccentricity on remnant properties and the enhancement or suppression of recoil. We also provide error estimates for all modeled quantities, supporting reliable use in current and future gravitational-wave parameter-estimation analyses. Both models will be made available through open-source codes.

gr-qc↗

arXiv2026

Including higher-order modes in a quadrupolar eccentric numerical relativity surrogate using universal eccentric modulation functions

\texttt{gwNRHME} is a framework that converts multi-modal (i.e., containing several spherical harmonic modes) quasi-circular waveforms into their eccentric counterparts, provided the quadrupolar eccentric mode is known, by exploiting universal eccentric modulation functions. Leveraging this framework, we combine the quasi-circular NR surrogate model \texttt{NRHybSur3dq8} with the quadrupolar, non-spinning, eccentric surrogate \texttt{NRSurE\_q4NoSpin\_22} to construct a multi-modal, non-spinning, eccentric model, denoted as \model{}, which includes nine modes: $(2,\{1,2\})$, $(3,\{1,2,3\})$, $(4,\{2,3,4\})$, and $(5,5)$. When compared against 156 eccentric SXS NR waveforms, \model{} achieves median frequency-domain mismatches (computed using the Advanced LIGO design sensitivity) of $\sim 9\times 10^{-5}$, with a standard deviation of $\sim 2 \times 10^{-4}$. To demonstrate the modularity of the framework, we further combine \texttt{NRSurE\_q4NoSpin\_22} with effective-one-body (EOB) models \texttt{SEOBNRv5HM} and \texttt{TEOBResumS-Dali} in their non-spinning limits, yielding eccentric waveforms with median mismatches of $\sim 2\times10^{-4}$ and $\sim 10^{-3}$, respectively, with standard deviation of $\sim 2 \times 10^{-3}$ and $\sim 2 \times 10^{-2}$ respectively. Finally, we provide both a surrogate model, \texttt{gwEccEvolve\_q4NoSpin\_Sur}, and an analytical model, \texttt{gwEccEvNSv2}, for the eccentricity evolution up to $2M$ before merger, based on eccentricity definitions derived from the universal modulation functions. The \texttt{gwNRHME} framework is publicly available through the \texttt{gwModels} package, and the resulting waveform models will be released via the \texttt{gwsurrogate} package.

gr-qc↗

arXiv2026

Rapid inference of gravitational-wave signals in the time domain using a heterodyned likelihood

Parameter estimation of gravitational wave signals is computationally intensive and typically requires millions of likelihood evaluations to construct posterior probability distributions. This computational cost increases significantly in the time domain, which requires non-diagonal covariance matrices to compute the likelihood. Consequently, parameter estimation of long-duration gravitational wave signals, such as binary neutron star mergers, becomes computationally infeasible in time domain. In this work, we detail a framework for the heterodyned likelihood that enables rapid inference in the time domain. Our method is applicable to signals with arbitrary mode content, and leverages the smoothness of the ratio of complex-valued waveform modes, approximating the ratio as a linear function within appropriately chosen time bins. This allows downsampling of the waveform modes and a reformulation of the likelihood, such that it depends only on the bin edges. We demonstrate that this likelihood recovers posteriors that are indistinguishable from those obtained using the standard likelihood in the time domain. We also observe dramatic improvement in speed - for a 128 seconds-long gravitational wave signal, our method is at least $\sim 400$ times faster than the standard time-domain analysis, reducing the wall clock time to just a few hours. We also demonstrate the reliability and unbiasedness of the likelihood using percentile-percentile tests for binary black hole and binary neutron star injections. We use the Gohberg-Semencul representation of the inverse of Toeplitz covariance matrix to accelerate matrix-vector products, which has potential applications even in non heterodyned time-domain inference.

gr-qc↗

arXiv2025

Dynamo and Jet interconnections in GRMHD simulations of black hole accretion disks

We present global 3D GRMHD simulations of black hole (BH) accretion disks designed to investigate how MRI-driven dynamo action regulates jet formation and evolution. Unlike standard SANE/MAD setups that impose a coherent large-scale poloidal loop, our "sub-SANE" initial conditions use multiple same-polarity small-scale magnetic loops. Rapid reconnection erases magnetic memory and enables large-scale dynamo to emerge early from MRI turbulence. We perform two such sub-SANE simulations at different BH spins ($a = 0.5, 0.9375$) and compare them with conventional SANE runs. The sub-SANE disks show regular large-scale dynamo cycles with periods of about ten orbits. Decomposition of the induction equation shows that the turbulent dynamo term is stronger in 3D compared to 2.5D and balances advection in the saturated state, confirming sustained large-scale field generation. These dynamo-generated fields are advected inward with minimal time lag, producing correlated peaks in both poloidal and toroidal field strengths from $r_{\rm max}$ to the horizon. Early in the evolution, these peaks imprint directly onto the jet's electromagnetic energy flux, indicating that the jet mirrors the dynamo wave. Though jets form at early times, the sub-SANE runs eventually undergo jet shutdown. We show that this occurs when the magnetic field at the horizon loses coherence, as quantified by a decline in the signed-to-unsigned flux ratio $\mathcal{C}_{\rm BH}$ below $\approx 0.6$. In contrast, the SANE reference case with similar accretion rate and horizon magnetic flux maintains high magnetic coherence because its initial large-scale field persists, allowing its jet to survive. Our results show that both dynamo-driven field evolution and horizon magnetic-field coherence critically regulate jet longevity, establishing a direct dynamo-jet connection in GRMHD disks.

astro-ph.HE↗

arXiv2025

Biased parameter inference of eccentric, spin-precessing binary black holes

While the majority of gravitational wave (GW) events observed by the LIGO and Virgo detectors are consistent with mergers of binary black holes (BBHs) on quasi-circular orbits, some events are also consistent with non-zero orbital eccentricity, indicating that the binaries could have formed via dynamical interactions. Moreover, there may be GW events which show support for spin-precession, eccentricity, or both. In this work, we study the interplay of spins and eccentricity on the parameter estimation of GW signals from BBH mergers. We inject eccentric signals with no spins, aligned spins, and precessing spins using hybrids, TEOBResumS-DALI, and new Numerical Relativity (NR) simulations, respectively, and study the biases in the posteriors of source parameters when these signals are recovered with a quasi-circular precessing-spin waveform model, as opposed to an aligned-spin eccentric waveform model. We find significant biases in the source parameters, such as chirp mass and spin-precession ($\chi_p$), when signals from highly-eccentric BBHs are recovered with a quasi-circular waveform model. Moreover, we find that for signals with both eccentricity and spin-precession effects, Bayes factor calculations confirm that an eccentric, aligned-spin model is preferred over a quasi-circular precessing-spin model. Our study highlights the complex nature of GW signals from eccentric, precessing-spin binaries and the need for readily usable inspiral-merger-ringdown eccentric, spin-precessing waveform models for unbiased parameter estimation.

gr-qc↗

arXiv2025

Eccentric binary black holes: A new framework for numerical relativity waveform surrogates

Mounting evidence indicates that some of the gravitational wave signals observed by the LIGO/Virgo/KAGRA observatories might arise from eccentric compact object binaries, increasing the urgency for accurate waveform models for such systems. While for non-eccentric binaries, surrogate models are efficient and accurate, the additional features due to eccentricity have posed a challenge. In this letter, we present a novel method for decomposing eccentric numerical relativity waveforms which makes them amenable to surrogate modelling techniques. We parameterize the inspiral in the radial phase domain, factoring out eccentricity-induced dephasing and thus enhancing compressibility and accuracy. This is combined with a second surrogate for the merger-ringdown in the time-domain and a novel technique to take advantage of the approximate periodicity with radial oscillations during the inspiral. We apply this procedure to the $(2,2)$ mode for non-spinning black hole binaries, and demonstrate that the resulting surrogate, NRSurE_q4NoSpin_22, is able to faithfully reproduce the underlying numerical relativity waveforms, with maximum mismatches of $5\times10^{-4}$ and median mismatches of $2\times10^{-5}$. This technique paves the way for high-accuracy parameter estimation with eccentric models, a key ingredient for astrophysical inference and tests of general relativity.

gr-qc↗

arXiv2025

Chase Orbits, not Time: A Scalable Paradigm for Long-Duration Eccentric Gravitational-Wave Surrogates

Orbital eccentricity is a key tracer of the astrophysical origins of compact binaries. Yet it remains absent from routine LIGO-Virgo-KAGRA analyses, in part because of the prohibitive computational cost of generating eccentric template waveforms. The complicated morphology of these waveforms due to the eccentric orbital timescale variations makes it difficult to construct their accurate and efficient surrogate models, especially for waveforms long enough to comprehensively cover the sensitivity bands of current ground-based gravitational-wave detectors. We present a novel and scalable surrogate building technique which makes surrogate modeling of long-duration eccentric binary black hole waveforms both feasible and highly efficient. The technique aims to simplify the harmonic content of intermediate eccentric waveform data-pieces by modeling them in terms of an angular orbital element called the mean anomaly, instead of time. We show that this parameterization yields much more compressed surrogates than the standard time-based parameterizations. We also significantly simplify variations in waveform data-pieces across the parameter space by expressing them in terms of the instantaneous orbital eccentricity and mean anomaly to ease their parametric fitting. Building on these developments, we construct InspiralESIGMASur: a $2.77 \times 10^6M$ (850-1250 orbits) long non-spinning surrogate for the inspiral-only eccentric waveform model InspiralESIGMA [K. Paul et al., Phys. Rev. D 111, 084074 (2025)]. The methods presented in this work make it feasible to build long-duration eccentric surrogates for current as well as future third-generation gravitational-wave detectors.

gr-qc↗

arXiv2025

Gravitational Waves from Strongly Magnetized Eccentric Neutron Star Binaries

We explore the dynamics of neutron star binaries that approach their final inspiral stages with residual eccentricity and strong magnetic fields, features that can arise in systems formed through dynamical capture of relatively young neutron stars. Our analysis focuses on identifying magnetic-field imprints on the gravitational wave signal arising from two mechanisms: magnetic interaction between the neutron stars and electromagnetic radiation from the system's effective dipole. Using a perturbative approach, we obtain the associated gravitational wave energy loss rate and phase evolution, and quantify detectability through cumulative dephasing, horizon distances, and Fisher-matrix analyses. While magnetic effects are intrinsically small, entering at 2 post-Newtonian (PN) order, their cumulative influence over extended inspirals will become distinguishable in future detectors owing to enhanced low-frequency sensitivity. For binaries with comparable magnetic fields, we show that $10^{14}\,\mathrm{G}$ systems will be detectable up to $\sim10$ Mpc with DECIGO and the Einstein Telescope, while $10^{15}\,\mathrm{G}$ fields will be discernible out to several hundred Mpc. For extreme fields of $10^{16}\,\mathrm{G}$, third-generation detectors could probe out to Gpc scales. These findings suggest that magnetic effects in compact binaries can indeed become observable with next-generation detectors, offering a potential probe of magnetar-level fields and binary formation pathways.

astro-ph.HE↗

arXiv2025

Generalized Perturbed Kepler Problem: Gravitational Wave Imprints from Eccentric Compact Binaries

Observations of astrophysical binaries may reveal departures from pure Keplerian orbits due to environmental influences, modifications to the underlying gravitational dynamics, or signatures of new physics. In this work, we develop a unified framework to systematically study such perturbations in the ambit of the perturbed Kepler problem and explore their impact on eccentric orbital dynamics and gravitational wave emission. Unlike traditional parametrized frameworks such as post-Newtonian and post-Einsteinian expansions, our approach offers a more source-specific modeling strategy, making it more natural to trace the physical origins of eccentric binary model parameters. Starting from a general perturbed potential, we derive the modified orbit and compute the associated gravitational fluxes and phase evolution, assessing their observational relevance for both current and future detectors. This framework thus offers a general and physically transparent toolkit for probing such subtle deviations from standard dynamics in gravitational wave data.

gr-qc↗

arXiv2025

Early Warning From Eccentric Compact Binaries: Template Initialization And Sub-dominant Mode Effects

Early warning of gravitational waves (GWs) is essential for multi-messenger observations of binary neutron star and black hole-neutron star merger events. In this study, we investigate early warning prospects from eccentric compact binaries, whose mergers are expected to comprise a significant fraction of detected GW events in the future. Eccentric binaries exhibit oscillatory frequency evolution, causing GW frequencies to recur multiple times through their coalescence. Consequently, generating eccentric waveform templates for early warning requires specification of initial conditions. While the standard approach involves initiating waveform generation when the orbit-averaged frequency enters the detector band, we compare this with an alternative approach that uses the periastron frequency as the starting point. Our analysis shows that initializing at the periastron frequency yields an improved signal-to-noise ratio and sky localization. Additionally, including subdominant modes alongside the dominant $(2,2)$ mode leads to further improvements in sky localization. We explore the parameter space of primary mass $m_1 \in [1.4, 15] \, M_\odot$, spin $\chi_1 \in [0, 0.8]$, and eccentricity $e \leq 0.4$ across three detector configurations: O5, Voyager, and 3G. We find that in the O5 (Voyager) configuration, including eccentricity and subdominant modes, the sky localization area can be reduced by $2-80\% (2-85\%)$ at 1000 sq. deg. with increasing eccentricity from $e_5 = 0.1$ to $e_5 = 0.4$, yielding up to $41$ seconds (1 minute) of extra early warning time. For NSBH systems, subdominant modes contribute up to $70$ $(94)\%$ reduction for O5 (Voyager) scenario. In the 3G detector scenario, the sky area reduction due to eccentricity reaches $80\%$ (from $e_{2.5} = 0.1$ to $e_{2.5} = 0.4$) at 100 sq. deg., and subdominant modes enhance the reduction up to $98\%$ for NSBH systems.

gr-qc↗

arXiv2025

An Accurate Modeling of Nano-hertz Gravitational Wave Signal from Eccentric Supermassive Binary Black Holes: An Essential Step Toward a Robust Discovery

The stochastic gravitational wave background (SGWB) in the nanohertz (nHz) regime, detectable by pulsar timing arrays (PTAs), provides a promising probe of the cosmic population of supermassive black hole binaries (SMBHBs). These binaries are expected to retain significant eccentricity throughout their evolution. We present a new technique to model the nHz SGWB by incorporating eccentricity into a multi-scale adaptive simulation framework. Using the time-domain eccentric waveform model ESIGMAHM, we generate realistic GW signals from astrophysical populations of SMBHBs. Unlike circular binaries, eccentric systems emit across multiple frequencies, introducing spectral correlations between frequency bins. These correlations provide a novel observational signature of the eccentricity distribution of the SMBHB population. In this work, we adopt simplified power-law models for the eccentricity distribution. While this does not capture the full complexity of galactic environments, it effectively highlights the key features of GW emission from eccentric binaries and their imprint on the SGWB. Our approach advances nHz GW signal modeling by incorporating eccentricity at small scales, enabling more realistic predictions and offering a new avenue for probing SMBHB astrophysics with future PTA observations.

astro-ph.CO↗

arXiv2025

Signatures from metastable oppositely-charged black hole binaries in scalar Gauss-Bonnet gravity

We conduct numerical simulations of inspiraling, oppositely-charged black holes in the class of scalar-Gauss-Bonnet theories that exhibit spontaneous black hole scalarization. For quasi-circular, equal-mass binaries near the existence threshold for scalarized solutions, we find a new phenomenon whereby one of the component black holes can suddenly flip the sign of its scalar charge during the inspiral. We confirm this phenomenon with two independent codes and identify two key signatures thereof: a change in the dominant scalar radiation channel (from dipolar to quadrupolar), and, strikingly, the introduction of eccentricity in the orbit. This scenario offers a concrete example of potential nonlinear departures from general relativity in the inspiral of binary black holes in alternative theories of gravity and is of relevance for the development of new tests of gravity.

gr-qc↗

arXiv2025

The SXS Collaboration's third catalog of binary black hole simulations

We present a major update to the Simulating eXtreme Spacetimes (SXS) Collaboration's catalog of binary black hole simulations. Using highly efficient spectral methods implemented in the Spectral Einstein Code (SpEC), we have nearly doubled the total number of binary configurations from 2,018 to 3,756. The catalog now densely covers the parameter space with precessing simulations up to mass ratio $q=8$ and dimensionless spins up to $|\vec{\chi}|\le0.8$ with near-zero eccentricity. The catalog also includes some simulations at higher mass ratios with moderate spin and more than 250 eccentric simulations. We have also deprecated and rerun some simulations from our previous catalog (e.g., simulations run with a much older version of SpEC or that had anomalously high errors in the waveform). The median waveform difference (which is similar to the mismatch) between resolutions over the simulations in the catalog is $4\times10^{-4}$. The simulations have a median of 22 orbits, while the longest simulation has 148 orbits. We have corrected each waveform in the catalog to be in the binary's center-of-mass frame and exhibit gravitational-wave memory. We estimate the total CPU cost of all simulations in the catalog to be 480,000,000 core-hours. We find that using spectral methods for binary black hole simulations is over 1,000 times more efficient than much shorter finite-difference simulations of comparable accuracy. The full catalog is publicly available through the sxs Python package and at https://data.black-holes.org .

gr-qc↗

arXiv2025

Gravitational physics in the context of Indian astronomy: A vision document

Contributions from the Indian gravity community have played a significant role in shaping several branches of astronomy and astrophysics. This document reviews some of the most important contributions and presents a vision for gravity research in the context of astronomy \& astrophysics in India. This is an expanded version of one of the chapters in the recently released Vision Document of the Astronomical Society of India.

astro-ph.IM↗

arXiv2025

Computational Astrophysics, Data Science & AI/ML in Astronomy: A Perspective from Indian Community

In contemporary astronomy and astrophysics (A&A), the integration of high-performance computing (HPC), big data analytics, and artificial intelligence/machine learning (AI/ML) has become essential for advancing research across a wide range of scientific domains. These tools are playing an increasingly pivotal role in accelerating discoveries, simulating complex astrophysical phenomena, and analyzing vast amounts of observational data. For India to maintain and enhance its competitive edge in the global landscape of computational astrophysics and data science, it is crucial for the Indian A&A community to fully embrace these transformative technologies. Despite limited resources, the expanding Indian community has already made significant scientific contributions. However, to remain globally competitive in the coming years, it is vital to establish a robust national framework that provides researchers with reliable access to state-of-the-art computational resources. This system should involve the regular solicitation of computational proposals, which can be assessed by domain experts and HPC specialists, ensuring that high-impact research receives the necessary support. By building such a system, India can cultivate the talent, infrastructure, and collaborative environment necessary to foster world-class research in computational astrophysics and data science.

astro-ph.IM↗

arXiv2024

Simulating binary black hole mergers using discontinuous Galerkin methods

Binary black holes are the most abundant source of gravitational-wave observations. Gravitational-wave observatories in the next decade will require tremendous increases in the accuracy of numerical waveforms modeling binary black holes, compared to today's state of the art. One approach to achieving the required accuracy is using spectral-type methods that scale to many processors. Using the SpECTRE numerical-relativity code, we present the first simulations of a binary black hole inspiral, merger, and ringdown using discontinuous Galerkin methods. The efficiency of discontinuous Galerkin methods allows us to evolve the binary through ~18 orbits at reasonable computational cost. We then use SpECTRE's Cauchy Characteristic Evolution (CCE) code to extract the gravitational waves at future null infinity. The open-source nature of SpECTRE means this is the first time a spectral-type method for simulating binary black hole evolutions is available to the entire numerical-relativity community.

gr-qc↗

arXiv2024

ESIGMAHM: An Eccentric, Spinning inspiral-merger-ringdown waveform model with Higher Modes for the detection and characterization of binary black holes

We present a time-domain inspiral-merger-ringdowm (IMR) waveform model ESIGMAHM constructed within a framework we named ESIGMA for coalescing binaries of spinning black holes on moderately eccentric orbits (Huerta et al. (2018) [Phys. Rev. D 97, 024031]). We now include the effect of black hole spins on the dynamics of eccentric binaries, as well as model sub-dominant waveform harmonics emitted by them. The inspiral evolution is described by a consistent combination of latest results from post-Newtonian theory, self-force, and black hole perturbation theory. We assume that these moderately eccentric binaries radiate away most of their orbital eccentricity before merger, and seamlessly connect the eccentric inspiral with a numerical relativity based surrogate waveform model for mergers of spinning binaries on quasi-circular orbits. We validate ESIGMAHM against eccentric Numerical Relativity simulations, and also against contemporary effective-one-body and phenomenological models in the quasi-circular limit. We find that ESIGMAHM achieves match values greater than $99\%$ for quasi-circular spin-aligned binaries with mass ratios up to $8$, and above $97\%$ for non-spinning and spinning eccentric systems with small or positively aligned spins. Using IMRESIGMA, we quantify the impact of orbital eccentricity on GW signals, showing that next-generation detectors can detect eccentric sources up to $10\%$ louder than quasi-circular ones. We also show that current templated LIGO-Virgo searches will lose more than $10\%$ of optimal SNR for about $20\%$ of all eccentric sources by using only quasi-circular waveform templates. The same will result in a $25\%$ loss in detection rate for eccentric sources with mass ratios $m_1/m_2\geq 4$. Our results highlight the need for including eccentricity and higher-order modes in GW source models and searches for asymmetric eccentric BBH signals.

gr-qc↗