arXiv subjects

Koustav Chandra

Publications and source records attributed to Koustav Chandra.

At least 19 recordsLinked to original sources

arXiv2026

Testing General Relativity with GWTC-4.0 through mixture models

Gravitational-wave observations of compact binary mergers have enabled precision tests of gravity in the strong-field dynamical regime. Current approaches combine single-event results that assume deviations from General Relativity (GR) are uniformly distributed across events, limiting their flexibility and potentially biasing the inferred evidence. We introduce a simple mixture-model framework in which a fraction $\zeta$ of events is consistent with GR, while a fraction $1 - \zeta$ deviates from it, without imposing constraints on the population distribution of the deviation parameters. We apply this method to publicly available results from the LIGO-Virgo-KAGRA (LVK) collaboration on O1-O4a compact binary mergers obtained through FTI, pSEOBNR, and KerrPostMerger tests. We find that the data are consistent with all events satisfying GR. However, we obtain respective Bayes factors $B^{\zeta=1}_{\zeta \neq 1} \simeq 20 $, $10$ and $15$, which are much smaller than those inferred from existing LVK analyses, indicating that the data require greater flexibility in modelling possible deviations than standard approaches permit. In light of our results, we recommend using flexible mixture models to test GR across compact-merger catalogues, unless there are obvious physical motivations to impose more restrictive models, as in the case of graviton-mass estimates.

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arXiv2026

Intrinsic handedness in O1-O4a black-hole mergers: probing orbital precession, remnant retention in dense environments and cosmological mirror asymmetry

Precessing binary black-holes generically produce an imbalance of right- and left- handed gravitational waves, reflecting the breaking of mirror symmetry by the merger dynamics. We study this phenomenon using the observer-independent quantity $V_{\rm GW}$, a gravitational analogue of the optical Stokes parameter that quantifies the intrinsic handedness of the emitted radiation. Using 91 LIGO-Virgo-KAGRA black-hole mergers from the O1-O4a observing runs, we find that $92\%$ of the analyzed events favour non-vanishing $V_{\rm GW}$, indicating a predominance of precessing dynamics across the events. Through a recently established relation between $V_{\rm GW}$ and the remnant black hole recoil, we further constrain the retention of merger remnants in dense stellar environments, finding that at most $8\%$ could remain gravitationally bound to globular or nuclear star clusters and subsequently participate in hierarchical merger channels. We finally investigate the cosmological distribution of black-hole merger handedness. The observed $V_{\rm GW}$ distribution is consistent with symmetry under $V_{\rm GW}\rightarrow -V_{\rm GW}$, and yields an average value $\langle V_{\rm GW}\rangle=-1.9^{+6.1}_{-6.6}\times10^{-3}$ ($90\%$ credibility), consistent with the absence of a preferred handedness and with expectations from large-scale statistical isotropy. In particular, the inclusion of O4a events reduces uncertainties in $\langle V_{\rm GW} \rangle$ by $\sim 40\%$ with respect to O1-O3 events. These results establish black-hole merger handedness as a unified probe of orbital precession, remnant recoil, hierarchical formation, and cosmological mirror symmetry.

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arXiv2026

Eccentricity in Disguise? Insights from GW231123 and Numerically Simulated Binary Black Hole Merger Signals

GW231123 is a gravitational-wave signal originating from the merger of a black hole binary with total mass $\sim 250 M_{\odot}$, the largest ever detected by the LIGO-Virgo-Kagra Collaboration. Remarkably, under standard priors, the system features among the fastest-spinning binary components confidently measured in binary mergers, $ \chi_{1,2} \gtrsim 0.7$ at $90\%$ one-dimensional credibility, according to the most accurate model employed. As typical binary mergers result in remnants with $\chi \sim 0.7$, such spin values are challenging to obtain even from previous (hierarchical) mergers. These inferred properties rely on waveform models lacking eccentric corrections in the merger-ringdown stage. Here, we show that binaries retaining significant eccentricity up to merger can be misinterpreted as near-extremally spinning when non-circular corrections are neglected. Binary-agnostic ringdown analysis instead provides unbiased estimates of the remnant properties, provided that a robust estimate of the signal peak can be obtained. We re-analyse GW231123 using available eccentric numerical-relativity catalogues, finding that although eccentric templates can provide a good fit to the data, quasi-spherical templates are still favoured. Ringdown analyses confirm a secondary likelihood peak correlated with large eccentricity values, but improved eccentric models will be required to assess the reliability of this interpretation. Finally, analysing GW231123 under population-informed parametric priors confirms the exceptional nature of this event within the current black hole binary population.

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arXiv2026

Tarnished by Tools: Cost of Systematics in Golden Dark Siren Cosmology

Golden dark sirens - exceptionally well-localized gravitational-wave (GW) sources without electromagnetic counterparts - offer a powerful route to precision measurements of the Hubble constant, $H_0$, with next-generation (XG) detectors. The statistical promise of this method, however, places stringent demands on waveform accuracy and detector calibration, as even small systematic errors can dominate over statistical uncertainties at high signal-to-noise ratios. We investigate the impact of waveform-modeling systematics on golden dark siren cosmology using a synthetic population of binary black holes consistent with current GW observations and analyzed in the XG-detector era. By comparing state-of-the-art waveform models against numerical-relativity-based reference signals, we quantify modeling inaccuracies from both modeling and data-analysis perspectives and assess how they propagate into biases in luminosity distance, host-galaxy association, and single-event $H_0$ inference. We find that while current waveform models often allow recovery of statistically consistent $H_0$ posteriors, small waveform-induced biases can significantly affect three-dimensional localization and host galaxy ranking, occasionally leading to incorrect redshift assignments. We further derive order-of-magnitude requirements on detector calibration accuracy needed to ensure that calibration systematics remain subdominant for golden dark sirens observed with XG networks. To realize sub-percent $H_0$ measurements with golden dark sirens will require waveform and calibration accuracies that scale as $\mathcal{O}(\rho^{-2})$ with signal-to-noise ratio, motivating sustained advances in waveform modeling, numerical relativity, and detector calibration for the XG era.

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arXiv2025

From Source Properties to Strong-Field Tests: a multipronged analysis of GW250114 with an effective one-body model for generic orbits

We present a detailed analysis of GW250114, the loudest gravitational-wave signal observed to date, using a waveform model capable of describing binary black holes in generic (eccentric and precessing) orbits. Our analysis builds on LIGO-Virgo-KAGRA (LVK)'s results, finding that the source is consistent at a probability of $\geq 96\%$ with the merger of two first-generation, nearly equal-mass, low-spin black holes, forming a remnant within the pair-instability mass gap. The signal's high signal-to-noise ratio ($\gtrsim 75$) enables the detection of the subdominant $(4,\pm4)$ multipoles, whose presence we confirm with higher evidence than previously reported by the LVK. Restricting the analysis even to post-peak data yields $\log_{10}B\gtrsim 1$ in favor of models including the $(4,\pm4)$ mode, demonstrating that this contribution remains detectable well into the post-merger phase. We further perform three independent tests of general relativity, complementary to those performed by the LVK: a modified residual analysis confirms that our semi-analytical model fully describes the signal without detectable discrepancies; a subdominant mode test finds that the amplitude of the $(4,\pm4)$ multipoles agrees with general-relativistic expectations; and a parameterised analysis of the plunge-merger-ringdown regime recovers the GR expectation within the 50\% credible region for the remnant mass and spin, and within the 90\% interval for the $(2,\pm2)$ peak amplitude. Collectively, these results reinforce GW250114 as a landmark event for a precision test of gravity.

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arXiv2025

A parametrized model for gravitational waves from eccentric, precessing binary black holes: theory-agnostic tests of General Relativity with pTEOBResumS

Gravitational waves from binary black hole (BBH) mergers allow us to test general relativity in the strong-field, high-curvature regime. However, existing gravitational wave-based tests have so far assumed non-eccentric signal sources, limiting their applicability to more general astrophysical scenarios. In this work, we present pTEOBResumS, a new parametrized inspiral-merger-ringdown model for null tests of GR that incorporates both orbital eccentricity and spin precession. Building on the effective-one-body model TEOBResumS-Dal\'i, we introduce parametrized deviations from GR both in the inspiral and the merger-ringdown regimes. We validate the model via parameter estimation of synthetic signals, including from numerical relativity simulations of BBHs and a boson star binary. These allow us to establish the model's consistency, demonstrate its capability to identify beyond-GR effects, and gauge the impact of eccentricity in tests of GR. We then analyze a set of BBH events from the first three LIGO-Virgo-KAGRA observing runs, testing whether they are best explained by a GR or non-GR waveform, under either the eccentric, spin-aligned or precessing, quasi-circular hypotheses. We find no significant statistical evidence in favor of deviations from GR. Consistent with previous works, we infer a mild preference for longer remnant quasi-normal mode damping times than expected in GR, though the limited sample and potential systematics reduce its significance. In addition, when weighting by signal strength, joint posteriors combining the individual events are still compatible with GR. We find no strong evidence for imprints of orbital eccentricity in the analyzed events, with the exception of GW200129. For this, our analysis finds a strong preference for an eccentric, GR-consistent description, although as previous works have noted this result could be influenced by data quality issues.

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arXiv2025

$\texttt{GR-Athena++}$ Simulations of Spinning Binary Black Hole Mergers

We present the second release of the $\texttt{GR-Athena++}$ waveform catalog, comprising four new quasi-circular, non-precessing, spinning binary black hole simulations. These simulations are performed at high resolutions and represent a step toward generating high-fidelity gravitational waveforms that can eventually meet the accuracy requirements of upcoming next-generation detectors, including LISA, Cosmic Explorer, and Einstein Telescope. Gravitational waves are extracted at future null infinity ( $\mathscr{I}^{+}$) using both Cauchy characteristic extraction and finite-radius extraction. For each simulation, we provide strain data across multiple resolutions and analyze waveform accuracy via convergence studies and self-mismatch analyses. The absolute phase and relative amplitude differences reach their largest values near the merger, while the smallest errors are of order $\mathscr{O}(10^{-2})$ and $\mathscr{O}(10^{-3})$, respectively. A self-mismatch analysis of the dominant $(2,2)$ mode yields mismatches between $\mathscr{O}(10^{-5})$ and $\mathscr{O}(10^{-7})$ for a total binary mass of $10^{6}$ $M_{\odot}$ over the frequency range $[0.002, 0.1]$ Hz using LISA noise curve. All waveforms are publicly available via $\texttt{ScholarSphere}$.

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arXiv2025

Black-hole ringdown analysis with inspiral-merger informed templates and limitations of classical spectroscopy

The ringdown of a perturbed black hole consists of a superposition of quasi-normal modes (QNMs), with complex frequencies determined by the black hole's mass and spin, while phases and amplitudes depend on binary parameters. Traditional semi-agnostic ringdown analyses ignore the remnant's binary-merger origin, assuming instead generic perturbations. This can lead to an unphysical number of free parameters and usage of priors inconsistent with those imposed by full inspiral-merger-ringdown (IMR) analysis. Here we revisit an alternative approach, first proposed in Calder\'on Bustillo et al. 2021, in which ringdowns are modeled using the post-merger portion of full IMR waveform models. This naturally includes all QNMs without adding extra degrees of freedom and ties priors to binary parameters. We analyze the signal GW150914 with the post-merger portion of the IMR surrogate model NRSur7dq4, finding decisive evidence for the no-hair theorem with a Bayes factor $>650:1$ or $>99.8$% probability, compared to only $3:1$ from semi-agnostic spectroscopy, consistent with previous work. We also find modest evidence ($\simeq 5:1$) for the $(\ell,m,n)=(2,2,1)$ overtone and none for higher modes. Using simulated signals, we validate our formalism by showing accurate recovery of remnant properties. Next, by analysing simulated signals with post-merger signal-to-noise ratios up to 100, we show that classical spectroscopy based on overtone detection alone cannot provide strong no-hair tests. Finally, we demonstrate that the non-orthogonality of QNMs makes the inferred mode content strongly model-dependent, leading to multiple inconsistent interpretations that are equally well-supported by the data. We briefly discuss potential implications for recent events, such as GW231123 and GW250114.

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arXiv2025

Improving the detection significance of gravitational wave transient searches with CNN models

Gravitational wave (GW) transient searches rely on signal-noise discriminators to distinguish astrophysical signals from noise artefacts. These discriminators are typically tuned towards expected signal morphologies, which may limit their effectiveness as detector sensitivity improves and more complex signals, such as from core collapse supernovae or compact binary mergers featuring precession, higher-order harmonics, or eccentricity, become detectable. In this work, we use a Convolutional Neural Network-based approach to classify noise transients from astrophysical transients, aiming to enhance the sensitivity of existing searches. We evaluate our method on two matched filter based searches, PyCBC-IMBH and PyCBC-HM tuned for Intermediate Mass Black Hole (IMBH) binary systems. Our approach improves the sensitive volume-time reach of these searches by approximately 30% at a false alarm rate of once per 100 years. Finally, we apply our method to the first four chunks of the first half of the third observation run and demonstrate a marked improvement in significance. In particular, we significantly improve the first IMBH binary GW event GW190521 with an IFAR exceeding 42000 years.

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arXiv2025

Cosmic Calipers: Precise and Accurate Neutron Star Radius Measurements with Next-Generation Gravitational Wave Detectors

Gravitational waves from merging binary neutron stars carry characteristic information about their astrophysical properties, including masses and tidal deformabilities, that are needed to infer their radii. In this study, we use Bayesian inference to quantify the precision with which radius can inferred with upgrades in the current gravitational wave detectors and next-generation observatories such as the Einstein Telescope and Cosmic Explorer. We assign evidences for a set of plausible equations of state, which are then used as weights to obtain radius posteriors. We find that prior choices and the loudness of observed signals limit the precision and accuracy of inferred radii by current detectors. In contrast, next-generation observatories can resolve the radius precisely and accurately, across most of the mass range to within $\lesssim 5\%$ for both soft and stiff equations of state. We also explore how the choice of the neutron star mass prior can influence the inferred masses and potentially affect radii measurements, finding that choosing an astrophysically motivated prior does not notably impact an individual neutron star's radius measurements.

astro-ph.HE↗

arXiv2024

Binary Black Hole Waveforms from High-Resolution GR-Athena++ Simulations

The detection and subsequent inference of binary black hole signals rely heavily on the accuracy of the waveform model employed. In the highly non-linear, dynamic, and strong-field regime near merger, these waveforms can only be accurately modeled through numerical relativity simulations. Considering the precision requirements of next-generation gravitational wave observatories, we present in this paper high-resolution simulations of four non-spinning quasi-circular binary black hole systems with mass ratios of 1, 2, 3, and 4, conducted using the GR-Athena++ code. We extract waveforms from these simulations using both finite radius and Cauchy characteristic extraction (CCE) methods. Additionally, we provide a comprehensive error analysis to evaluate the accuracy and convergence of the waveforms. Our self-mismatch study shows that the (2, 2) mode of the CCE strains, for the world tube extraction radius of $R=50$, reaches the level of ${\sim} 10^{-12}$ mismatch for mass ratios of 1, 2, 3, and ${\sim} 10^{-11}$ mismatch for the mass ratio of 4. However, when larger extraction radii are considered or when more modes are included the mismatches increase. These results highlight both the promise and limitations of current simulations in achieving the precision required for upcoming detectors such as LISA, Cosmic Explorer, and Einstein Telescope. The waveforms are publicly available on ScholarSphere, and represent the first set of waveforms of the new GR-Athena++ catalog.

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arXiv2024

The Critical Role of LIGO-India in the Era of Next-Generation Observatories

We examine the role of LIGO-India in facilitating multi-messenger astronomy in the era of next generation observatories. A network with two L-shaped Cosmic Explorer (CE) detectors and one triangular Einstein Telescope (ET) would precisely localize nearly the entire annual binary neutron star merger population up to a redshift of 0.5--over 10,000 events would be localized within $10\ \mathrm{deg}^2$, including approximately 150 events within $0.1\ \mathrm{deg}^2$. Luminosity distance would be measured to within 10% for over 9,000 events and within 1% for $\sim 100$ events. Surprisingly, replacing the 20 km CE detector with LIGO-India operating at A$^\sharp$ sensitivity (I$^\sharp$) yields nearly identical performance. The factor-of-five shorter arms are offset by a fourfold increase in baseline relative to a second CE in the U.S., preserving localization accuracy, with over 9,000 events within $10\ \mathrm{deg}^2$ and $\sim 90$ events within $0.1\ \mathrm{deg}^2$. This configuration detects $\sim 6,000$ events with luminosity distance uncertainties under 10%, including $\sim 50$ events with under 1%. Both networks provide $\mathcal{O}(100)$ early-warning detections up to 10 minutes before merger, with localization areas $\leq 10\ \mathrm{deg}^2$. While I$^\sharp$ enables excellent localization and early warnings, its shorter arms and narrower sensitivity band would limit its reach for other science goals, such as detecting population III binary black hole mergers at $z \gtrsim 10$, neutron star mergers at $z \sim 2$, or constraining cosmological parameters.

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arXiv2024

Foreground signals minimally affect inference of high-mass binary black holes in next generation gravitational-wave detectors

Next-generation gravitational-wave observatories are expected to detect over a thousand compact binary coalescence signals daily, with some lasting from minutes to hours. Consequently, multiple signals will overlap in the time-frequency plane, generating a "foreground noise" that predominantly affects the low-frequency range, where binary neutron star inspiral evolution is gradual. This study investigates the impact of such foreground noise on parameter estimation for short-duration binary black hole signals, particularly those with high detector-frame masses and/or located at large redshifts. Our results show a reduction in detection sensitivity by approximately 25\% when the noise power spectrum deviates by up to 50\% from Gaussian noise due to foreground contamination. Despite this, using standard parameter estimation techniques without subtracting overlapping signals, we find that foreground noise has minimal impact, primarily affecting precision. These findings suggest that even in the presence of substantial foreground noise, global-fit techniques, and/or signal subtraction will not be necessary, as accurate recovery of system parameters is achievable with minimal loss in precision.

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arXiv2024

gwforge: A user-friendly package to generate gravitational-wave mock data

The next-generation gravitational-wave detectors, with their improved sensitivity and wider frequency bandwidth, will be capable of observing almost every compact binary signal from epochs before the first stars began to form, increasing the number of detectable binaries to hundreds of thousands annually. This will enable us to observe compact objects through cosmic time, probe extreme matter phenomena, do precision cosmology, study gravity in strong field dynamical regimes and potentially allow observation of fundamental physics beyond the standard model. However, the richer data sets produced by these detectors will pose new computational, physical and astrophysical challenges, necessitating the development of novel algorithms and data analysis strategies. To aid in these efforts, this paper introduces gwforge, a user-friendly, lightweight Python package, to generate mock data for next-generation detectors. We demonstrate the package's capabilities through data simulation examples and highlight a few potential applications: performance loss due to foreground noise, bright-siren cosmology and impact of waveform systematics on binary parameter estimation.

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arXiv2024

Impact of Bayesian Priors on the Inferred Masses of Quasi-Circular Intermediate-Mass Black Hole Binaries

Observation of gravitational waves from inspiralling binary black holes has offered a unique opportunity to study the physical parameters of the component black holes. To infer these parameters, Bayesian methods are employed in conjunction with general relativistic waveform models that describe the source's inspiral, merger, and ringdown. The results depend not only on the accuracy of the waveform models but also on the underlying fiducial prior distribution used for the analysis. In particular, when the pre-merger phase of the signal is barely observable within the detectors' bandwidth, as is currently the case with intermediate-mass black hole binary signals in ground-based gravitational wave detectors, different prior assumptions can lead to different interpretations. In this study, we utilise the gravitational-wave inference library, $\texttt{Parallel Bilby}$, to evaluate the impact of mass prior choices on the parameter estimation of intermediate-mass black hole binary signals. While previous studies focused primarily on analysing event data, we offer a broader, more controlled study by using simulations. Our findings suggest that the posteriors in total mass, mass ratio and luminosity distance are contingent on the assumed mass prior distribution used during the inference process. This is especially true when the signal lacks sufficient pre-merger information and/or has inadequate power in the higher-order radiation multipoles. In conclusion, our study underscores the importance of thoroughly investigating similarly heavy events in current detector sensitivity using a diverse choice of priors. Absent such an approach, adopting a flat prior on the binary's redshifted total mass and mass ratio emerges as a reasonable choice, preventing biases in the detector-frame mass posteriors.

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arXiv2024

Constraining possible $γ$-ray burst emission from GW230529 using Swift-BAT and Fermi-GBM

GW230529 is the first compact binary coalescence detected by the LIGO-Virgo-KAGRA collaboration with at least one component mass confidently in the lower mass-gap, corresponding to the range 3-5$M_{\odot}$. If interpreted as a neutron star-black hole merger, this event has the most symmetric mass ratio detected so far and therefore has a relatively high probability of producing electromagnetic (EM) emission. However, no EM counterpart has been reported. At the merger time $t_0$, Swift-BAT and Fermi-GBM together covered 100$\%$ of the sky. Performing a targeted search in a time window $[t_0-20 \text{s},t_0+20 \text{s}]$, we report no detection by the Swift-BAT and the Fermi-GBM instruments. Combining the position-dependent $γ-$ray flux upper limits and the gravitational-wave posterior distribution of luminosity distance, sky localization and inclination angle of the binary, we derive constraints on the characteristic luminosity and structure of the jet possibly launched during the merger. Assuming a top-hat jet structure, we exclude at 90$\%$ credibility the presence of a jet which has at the same time an on-axis isotropic luminosity $\gtrsim 10^{48}$ erg s$^{-1}$, in the bolometric band 1 keV-10 MeV, and a jet opening angle $\gtrsim 15$ deg. Similar constraints are derived testing other assumptions about the jet structure profile. Excluding GRB 170817A, the luminosity upper limits derived here are below the luminosity of any GRB observed so far.

astro-ph.HE↗

arXiv2024

Everything everywhere all at once: A detailed study of GW230529

This study investigates the origins of GW230529, delving into its formation from massive stars within isolated binary systems. Utilizing population synthesis models, we present compelling evidence that the neutron star component forms second. However, the event's low signal-to-noise ratio introduces complexities in identifying the underlying physical mechanisms driving its formation. Augmenting our analysis with insights from numerical relativity, we estimate the final black hole mass and spin to be approximately $5.3 M_\odot$ and $0.53$, respectively. Furthermore, we employ the obtained posterior samples to calculate the ejecta mass and kilonova light curves resulting from r-process nucleosynthesis. We find the ejecta mass to range within $0-0.06 M_{\odot}$, contingent on the neutron star equation of state. The peak brightness of the kilonovae light curves indicates that targeted follow-up observations with a Rubin-like observatory may have detected this emission.

astro-ph.HE↗

arXiv2024

Unmasking noise transients masquerading as intermediate-mass black hole binaries

In the first three observation runs, ground-based gravitational wave (GW) detectors have observed close to 100 compact binary coalescence (CBC) events. The GW detection rates for CBCs are expected to increase with improvements in the sensitivity of the International Gravitational-Wave Observatory Network (IGWN). However, with improved sensitivity, non-Gaussian instrumental transients or ``glitches'' are expected to adversely affect GW searches and characterisation algorithms. The most detrimental effect is due to short-duration glitches, which mimic the morphology of short-duration GW transients, in particular Intermediate-mass black hole (IMBH) binaries. They can be easily misidentified as astrophysical signals by current searches, and if included in astrophysical analyses, glitches mislabelled as IMBH binaries can affect IMBH population studies. In this work, we introduce a new similarity metric that quantifies the consistency of astrophysical parameters across the detector network and helps to distinguish between IMBH binaries and short-duration, loud glitches which mimic such binaries. We develop this method using a simulated set of IMBH binary signals and a collection of noise transients identified during the third observing run of the Advanced LIGO and Advanced Virgo detectors.

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