arXiv subjects

Apratim Ganguly

Publications and source records attributed to Apratim Ganguly.

At least 19 recordsLinked to original sources

arXiv2026

Impact of the Host Galaxy on the Search for Gravitational-Wave Microlensing by Compact Objects

Gravitational-wave (GW) microlensing occurs when an intervening compact object causes diffraction of the GW, imprinting characteristic frequency-dependent modulations on the observed signal. Current searches for microlensing signatures in the detected GW events, including those used to constrain the fraction of dark matter in the form of compact objects, assume an isolated point mass lens (PML) model. However, astrophysical compact objects are typically embedded within larger structures, such as galaxies, whose potentials introduce additional convergence and tidal shear, thereby perturbing the lensed signal. In this work, we study the impact of the background galaxy in the search for microlensing by compact objects using a PML model. We distribute the compact objects inside a galaxy following the Navarro-Frenk-White density profile. We model the microlensing effects using the potential of a PML embedded in a constant convergence and shear, which are, in turn, calculated from a singular isothermal sphere model of the galaxy lens. We demonstrate that neglecting the galaxy's effects can systematically reduce sensitivity in microlensing searches, particularly for high microlens masses. Thus, future searches may need to incorporate the additional effect of the lens galaxy, particularly those aimed at placing robust constraints on the abundance of compact dark matter.

gr-qc↗

arXiv2026

Biases in Tests of General Relativity from Microlensed Gravitational-Wave Signals

Gravitational-wave (GW) observations of compact binary mergers enable precision tests of general relativity (GR) in the strong-field regime, but their reliability depends on accurate waveform modeling. Unmodeled physical effects can induce systematic biases that mimic deviations from GR. Here we study the impact of GW microlensing on standard LIGO-Virgo-KAGRA tests of GR using GW150914-like simulated signals lensed by isolated point-mass objects with masses in the range $10-10^{5}M_\odot$. We perform Bayesian parameter estimation with unlensed waveform templates and quantify biases in parameterized tests, modified dispersion relation tests, the inspiral-merger-ringdown consistency test (IMRCT) and the meta-IMRCT framework. To compare these tests with a common discriminator, we introduce a unified GR-deviation significance statistic based on the GR quantile, applicable to both one- and multi-dimensional deviation parameters. We find that microlensing-induced waveform distortions can produce significant false deviations from GR, reaching $\sim4σ$ in one-dimensional tests and $\sim4.5σ$ in two-dimensional consistency tests, despite the injected signals being fully GR-compatible. These false deviations arise mainly in the wave-optics regime, where diffraction induces frequency-dependent amplitude and phase modulations, while signals in the long-wavelength and geometric-optics regimes remain largely consistent with GR. We also find that the significance of these apparent deviations does not correlate strongly with microlensing detectability as measured by Bayes factors, showing that GR tests probe waveform projections not captured by global lensing diagnostics. Our results establish wave-optics gravitational lensing as an important astrophysical systematic for present and future precision tests of GR and highlight the need to model propagation effects in next-generation GW analyses.

gr-qc↗

arXiv2026

Higher Lovelock Curvature Terms Favor Local Nakedness in Dust Collapse

We show that higher-curvature Lovelock terms do not restore local cosmic censorship in spherical dust collapse, but instead promote the local visibility of central shell-focusing singularities. On the collapse branch with positive highest-order Lovelock coefficient $c_N$, the highest nonvanishing Lovelock order $N$ controls both the near-singularity collapse and the formation of trapped surfaces. In noncritical dimensions, $D-1-2N>0$, the apparent-horizon curve approaches the singularity curve with trapping exponent $β_N=(D-1)/(D-1-2N)$. Comparing this scale with the first nonvanishing correction $r^\ell$ to the singularity curve gives the local-visibility condition $\ell<β_N$, provided the singularity curve opens outward. Thus increasing $N$ enlarges the class of inhomogeneous initial data producing outgoing radial null rays from the central singularity. In the critical odd-dimensional branch, $D=2N+1$, no apparent horizon forms sufficiently close to the center, so any outward opening of the singularity curve gives local visibility. The locally visible singularities are Królak-strong along the emerging null rays, with Tipler strength reached at threshold. For bound and unbound collapse, the noncritical exponents are unchanged: the energy function modifies the opening of the singularity curve, while in the critical branch it enters the leading terminal collapse velocity.

gr-qc↗

arXiv2026

Neutron stars in Poincaré gauge gravity with quadratic torsion

We study static neutron stars in an algebraic sector of Poincaré gauge gravity with parity-even and parity-odd quadratic torsion invariants. Since torsion is non-propagating, the contorsion equation is algebraic and can be solved in terms of the spin current. For a Weyssenhoff fluid satisfying the Frenkel condition, the metric field equations reduce to ordinary Riemannian Einstein equations sourced by an effective fluid containing spin-squared corrections. We derive the effective energy density, radial pressure, and tangential pressure, allowing both isotropic and anisotropic spin correlations. In contrast with Einstein--Cartan theory, the coefficient of the effective spin-spin interaction is not fixed, but depends on the dimensionless quadratic-torsion couplings. In the Einstein--Cartan limit, using the metric definition of the stress-energy tensor, the unpolarized spin contribution gives $w_{\mathrm{spin}}=-1/3$. We then derive the corresponding modified Tolman--Oppenheimer--Volkoff equations and solve them numerically using the DD2 equation of state. For the positive effective spin-spin coupling branch considered here, the torsion correction makes the stellar configurations more compact, lowers the maximum mass, and reduces the binding energy relative to the general-relativistic sequence. For the smooth weak-polarization profiles considered, spin-correlation anisotropy has only a negligible effect on the mass--radius relation.

gr-qc↗

arXiv2026

Oppenheimer-Snyder Collapse in f(R) Gravity : Stalemate or Resolution?

We study the Oppenheimer--Snyder (OS) collapse problem in metric $f(R)$ gravity by matching a homogeneous dust Friedmann--Lemaître--Robertson--Walker (FLRW) interior to a generalized Vaidya exterior across a timelike hypersurface. In metric $f(R)$ gravity, regular matching requires the continuity not only of the induced metric and extrinsic curvature, but also of the Ricci scalar and its normal derivative. These additional conditions generically exclude the usual Ricci-flat exteriors, such as the Schwarzschild solution. We show that, for an unrestricted generalized Vaidya exterior, the matching conditions fix the boundary data but do not uniquely determine the bulk extension, leaving open the possibility of a physical resolution of the collapse problem. However, once the exterior matter content is restricted to the generalized Vaidya form, the field equations impose a strong constraint, forcing $f_{,R}$ to be linear in the areal radius, $f_{,R}=A(v)\,r+B(v)$. For locally invertible $f_{,R}$ with $f_{,RR}\neq 0$, this sharply reduces the admissible class of exteriors, so that the matching data uniquely determine the exterior solution on each interval where the boundary map is locally invertible. We further show that, for generic viable $f(R)$ models, the branch with $A(v)\neq 0$ does not admit a global extension with finite asymptotic curvature, while the branch $A(v)=0$ places the interior on a constant-curvature sector. This excludes nontrivial dust collapse, although it does not rule out collapse for more general interior matter with constant trace. Thus, generalized Vaidya exteriors reopen the collapse problem at a formal level, but within the restricted matter sector considered here, the OS dust collapse problem remains unresolved and the physically acceptable branch is highly constrained.

gr-qc↗

arXiv2026

Bose-Einstein Condensate Dark Matter in the Core of Neutron Stars: Implications for Gravitational-wave Observations

We investigate neutron stars admixed with dark matter (DM) in the form of a finite-temperature Bos-Einstein condensate (BEC) within a general relativistic two-fluid framework in which the nuclear and dark components interact only gravitationally. Using realistic nuclear matter equations of state (EOS), APR4, MPA1, and SLy, we construct equilibrium configurations and compute mas-radius relations, tidal Love numbers, and dimensionless tidal deformabilities. We quantify how the presence of a BEC dark component modifies the mas-$Λ$ relation relevant for gravitational wave observations, finding that increasing the DM mass fraction generically reduces the maximum mass, radius, and tidal deformability of neutron stars. By comparing theoretical mass-$Λ$ curves with EOS-insensitive posteriors from GW170817, we evaluate, in a conditional sense, the dark matter fractions that would align a given nuclear EOS with the observed tidal constraints; for example, under the assumption that APR4 describes nuclear matter and that the GW170817 components were dark-matter admixed neutron stars, our study favors dark matter fractions of order a few percent, whereas stiffer EOSs require larger fractions to achieve comparable agreement. This interpretation assumes that inspiral waveforms are adequately characterized by tidal deformability and should therefore be regarded as structural rather than a direct detection of dark matter. We also examine finite-temperature effects in the BEC sector and find that, for moderate dark matter fractions, temperature has a negligible impact on the stability and tidal properties of admixed configurations. Our results demonstrate how even modest DM admixtures can influence neutron star structure and tidal observables, highlighting the importance of considering non-standard matter components in multimessenger constraints on dense matter.

gr-qc↗

arXiv2026

Comprehensive analysis of time-domain overlapping gravitational wave transients: A Lensing Study

Next-generation GW detectors will produce a high rate of temporally overlapping signals from unrelated compact binary coalescences. Such overlaps can bias parameter estimation (PE) and mimic signatures of other physical effects, such as gravitational lensing. In this work, we investigate how overlapping signals can be degenerate with gravitational lensing by focusing on two scenarios: Type-II strong lensing and microlensing by an isolated point-mass lens. We simulate quasicircular binary black-hole pairs with chirp-mass ratios $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\in\{0.5,\,1,\,2\}$, SNR ratios $\mathrm{SNR}_{\rm B}/\mathrm{SNR}_{\rm A}\in\{0.5,\,1\}$, and coalescence-time offsets $Δt_{\rm c}\in[-0.1,\,0.1]~\mathrm{s}$. Bayesian PE and fitting-factor studies show that the Type-II lensing hypothesis is favored over the unlensed quasicircular hypothesis ($\log_{10}\mathscr{B}^{\rm L}_{\rm U}>1$) only in a small region of the overlapping parameter space with $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\gtrsim1$ and $|Δt_{\rm c}|\leq0.03~\rm{s}$.. Meanwhile, false evidence for microlensing signatures can arise because, to a reasonable approximation, the model produces two superimposed images whose time delay can closely match $|Δt_{\rm c}|$. Overall, the inferred Bayes factor depends on relative chirp-mass ratios, relative loudness, difference in coalescence times, and also the absolute SNRs of the overlapping signals. Cumulatively, our results indicate that overlapping black-hole binaries with nearly equal chirp masses and comparable loudness are likely to be falsely identified as lensed. Such misidentifications are expected to become more common as detector sensitivities improve. While our study focuses on ground-based detectors using appropriate detectability thresholds, the findings naturally extend to next-generation GW observatories.

gr-qc↗

arXiv2025

Can Eccentric Binary Black Hole Signals Mimic Gravitational-Wave Microlensing?

Gravitational lensing in the wave-optics regime imprints characteristic frequency-dependent amplitude and phase modulations on gravitational-wave (GW) signals, yet to be detected by ground-based interferometers. Similar modulations may also arise from orbital eccentricity, raising the possibility of degeneracies that could lead to false microlensing claims. We investigate the extent to which eccentric binary black hole (BBH) signals can mimic microlensing signatures produced by an isolated point-mass lens. With a simulated population of eccentric signals using numerical relativity simulations and \texttt{TEOBResumS-Dalí} waveform model, we perform a Bayesian model-comparison study, supported by a complementary \textit{mismatch} analysis. We find a strong degeneracy for high eccentricities, low total masses, and high signal-to-noise ratios (SNRs): under these conditions, quasicircular microlensed model can be strongly favored over quasicircular unlensed model, even when the true signal is unlensed. For moderate SNRs ($\sim 30$), binaries with $M_\mathrm{tot}\lesssim 100\,M_\odot$ and eccentricity $e \gtrsim 0.4$ are particularly susceptible to misclassifications. In such cases, inferred microlens parameters exhibit well-constrained posteriors despite being unphysical. Crucially, the degeneracy is completely removed when the recovery uses waveform models that incorporate eccentricity, which overwhelmingly favors the eccentric hypothesis over microlensing. Our results demonstrate that any event exhibiting strong Bayesian evidence for microlensing should also be analyzed with eccentric waveform models and vice-versa to avoid false positives and biased astrophysical inference. This work contributes to developing robust strategies for interpreting signals in the era of precision GW astronomy.

gr-qc↗

arXiv2023

Accelerated parameter estimation in Bilby with relative binning

We describe an implementation of the relative binning technique to speed up parameter estimation of gravitational-wave signals. We first give a pedagogical overview of relative binning, discussing also the expressions for the likelihood marginalized over phase and distance. Then, we describe the details of the code in \texttt{Bilby}, an open-source software package commonly used for parameter estimation of gravitational-wave sources. Our code is able to reproduce the parameters of GW170817 in 14 hours on a single-core CPU, performs well on simulated signals, and passes the percentile-percentile (p-p) tests. We also illustrate that relative binning is an ideal technique to estimate the parameters of signals in next-generation gravitational wave detectors.

gr-qc↗

arXiv2023

Unveiling Microlensing Biases in Testing General Relativity with Gravitational Waves

Gravitational waves (GW) from chirping binary black holes (BBHs) provide unique opportunities to test general relativity (GR) in the strong-field regime. However, testing GR can be challenging when incomplete physical modeling of the expected signal gives rise to systematic biases. In this study, we investigate the potential influence of wave effects in gravitational lensing (which we refer to as microlensing) on tests of GR using GWs for the first time. We utilize an isolated point-lens model for microlensing with the lens mass ranging from $10-10^5~$M$_\odot$ and base our conclusions on an astrophysically motivated population of BBHs in the LIGO-Virgo detector network. Our analysis centers on two theory-agnostic tests of gravity: the inspiral-merger-ringdown consistency test (IMRCT) and the parameterized tests. Our findings reveal two key insights: First, microlensing can significantly bias GR tests, with a confidence level exceeding $5σ$. Notably, substantial deviations from GR $(σ> 3)$ tend to align with a strong preference for microlensing over an unlensed signal, underscoring the need for microlensing analysis before claiming any erroneous GR deviations. Nonetheless, we do encounter scenarios where deviations from GR remain significant ($1 < σ< 3$), yet the Bayes factor lacks the strength to confidently assert microlensing. Second, deviations from GR correlate with pronounced interference effects, which appear when the GW frequency ($f_\mathrm{GW}$) aligns with the inverse time delay between microlens-induced images ($t_\mathrm{d}$). These false deviations peak in the wave-dominated region and fade where $f_\mathrm{GW}\cdot t_\mathrm{d}$ significantly deviates from unity. Our findings apply broadly to any microlensing scenario, extending beyond specific models and parameter spaces, as we relate the observed biases to the fundamental characteristics of lensing.

gr-qc↗

arXiv2023

Follow-up Analyses to the O3 LIGO-Virgo-KAGRA Lensing Searches

Along their path from source to observer, gravitational waves may be gravitationally lensed by massive objects. This results in distortions of the observed signal which can be used to extract new information about fundamental physics, astrophysics, and cosmology. Searches for these distortions amongst the observed signals from the current detector network have already been carried out, though there have as yet been no confident detections. However, predictions of the observation rate of lensing suggest detection in the future is a realistic possibility. Therefore, preparations need to be made to thoroughly investigate the candidate lensed signals. In this work, we present some of the follow-up analyses and strategies that could be applied to assess the significance of such events and ascertain what information may be extracted about the lens-source system from such candidate signals by applying them to a number of O3 candidate events, even if these signals did not yield a high significance for any of the lensing hypotheses. For strongly-lensed candidates, we verify their significance using a background of simulated unlensed events and statistics computed from lensing catalogs. We also look for potential electromagnetic counterparts. In addition, we analyse in detail a candidate for a strongly-lensed sub-threshold counterpart that is identified by a new method. For microlensing candidates, we perform model selection using a number of lens models to investigate our ability to determine the mass density profile of the lens and constrain the lens parameters. We also look for millilensing signatures in one of the lensed candidates. Applying these additional analyses does not lead to any additional evidence for lensing in the candidates that have been examined. However, it does provide important insight into potential avenues to deal with high-significance candidates in future observations.

gr-qc↗

arXiv2023

Detection and parameter estimation challenges of Type-II lensed binary black hole signals

Strong lensing of {gravitational-wave signals} can produce three types of images, denoted as Type-I, Type-II and Type-III, corresponding to the minima, saddle and maxima of the lensing potential of the lensed images. Type-II images, in particular, receive a non-trivial phase shift of $π/2$. This phase shift can introduce additional distortions in the strains produced by the Type-II image of the binary black hole signals depending on the morphology of the signals, e.g., when they have contributions from higher harmonics, precession, eccentricity, etc. {The probability of observing Type-II images is nearly the same as that of strong lensing itself, and thus these signals are likely to be observed in the near future.} In this work, we investigate the potential applicability of these distortions in helping identify Type-II signals from a single detection and the systematic biases that could arise in the inference of parameters if they are recovered with gravitational-wave templates that do not take the distortion into account. We show that the lensing distortions will allow us to confidently identify the Type-II images for highly inclined binaries: at network signal-to-noise ratio (SNR) $ρ=20(50)$, individual Type-II images should be identifiable with ln Bayes factor $\ln \mathcal{B} > 2$ for inclinations $ ι> 5 π/12 (π/3) $. Furthermore, based on the trends we observe in these results, we predict that, at high SNRs ($\gtrsim 100$), individual Type-II images would be identifiable even when the inclination angle is much lower ($\sim π/6$). We then show that neglecting physical effects arising from these identifiable Type-II images can significantly bias the estimates of source parameters. Thus, in the future, using templates that take into account the lensing deformation would be necessary to extract source parameters from Type-II lensed signals.

gr-qc↗

arXiv2017

Dynamic Networks with Multi-scale Temporal Structure

We describe a novel method for modeling non-stationary multivariate time series, with time-varying conditional dependencies represented through dynamic networks. Our proposed approach combines traditional multi-scale modeling and network based neighborhood selection, aiming at capturing temporally local structure in the data while maintaining sparsity of the potential interactions. Our multi-scale framework is based on recursive dyadic partitioning, which recursively partitions the temporal axis into finer intervals and allows us to detect local network structural changes at varying temporal resolutions. The dynamic neighborhood selection is achieved through penalized likelihood estimation, where the penalty seeks to limit the number of neighbors used to model the data. We present theoretical and numerical results describing the performance of our method, which is motivated and illustrated using task-based magnetoencephalography (MEG) data in neuroscience.

stat.ME↗

arXiv2017

Black hole stability under odd-parity perturbations in Horndeski gravity

We study the stability under linear odd-parity perturbations of static spherically symmetric black holes in Horndeski gravity. We derive the master equation for these perturbations and obtain the conditions of no-ghost and Laplacian instability. In order for the black hole solutions to be stable, we study their generalized "Regge-Wheeler potential". It turns out that the problem is reduced to an algebraic problem where three functions characterizing the black hole should be positive outside the horizon to prove the stability. We found that these conditions are similar to the no-ghost and Laplacian instability conditions. We apply our results to various known solutions.

gr-qc↗

arXiv2017

Global structure of static spherically symmetric solutions surrounded by quintessence

We investigate all static spherically symmetric solutions in the context of general relativity surrounded by a minimally-coupled quintessence field, using dynamical system analysis. Applying the 1+1+2 formalism and introducing suitable normalized variables involving the Gaussian curvature, we were able to reformulate the field equations as first order differential equations. In the case of a massless canonical scalar field we recovered all known black hole results, such as the Fisher solution, and we found that apart from the Schwarzschild solution all other solutions are naked singularities. Additionally, we identified the symmetric phase space which corresponds to the white hole part of the solution and in the case of a phantom field, we were able to extract the conditions for the existence of wormholes and define all possible class of solutions such as Cold Black holes, singular spacetimes and wormholes like Ellis wormhole, for example. For an exponential potential, we found that the black hole solution which is asymptotically flat is unique and it is the Schwarzschild spacetime, while all other solutions are naked singularities. Furthermore, we found solutions connecting to a white hole through a maximum radius, and not a minimum radius (throat) such as wormhole solutions, therefore violating the flare-out condition. Finally, we have found a necessary and sufficient condition on the form of the potential to have an asymptotically AdS spacetime along with a necessary condition for the existence of asymptotically flat black holes.

gr-qc↗

arXiv2017

Estimation of Vertex Degrees in a Sampled Network

The need to produce accurate estimates of vertex degree in a large network, based on observation of a subnetwork, arises in a number of practical settings. We study a formalized version of this problem, wherein the goal is, given a randomly sampled subnetwork from a large parent network, to estimate the actual degree of the sampled nodes. Depending on the sampling scheme, trivial method of moments estimators (MMEs) can be used. However, the MME is not expected, in general, to use all relevant network information. In this study, we propose a handful of novel estimators derived from a risk-theoretic perspective, which make more sophisticated use of the information in the sampled network. Theoretical assessment of the new estimators characterizes under what conditions they can offer improvement over the MME, while numerical comparisons show that when such improvement obtains, it can be substantial. Illustration is provided on a human trafficking network.

stat.AP↗

arXiv2015

Dense magnetized plasma associated with a fast radio burst

Fast Radio Bursts are bright, unresolved, non-repeating, broadband, millisecond flashes, found primarily at high Galactic latitudes, with dispersion measures much larger than expected for a Galactic source. The inferred all-sky burst rate is comparable to the core-collapse supernova rate out to redshift 0.5. If the observed dispersion measures are assumed to be dominated by the intergalactic medium, the sources are at cosmological distances with redshifts of 0.2 to 1. These parameters are consistent with a wide range of source models. One fast radio burst showed circular polarization [21(7)%] of the radio emission, but no linear polarization was detected, and hence no Faraday rotation measure could be determined. Here we report the examination of archival data revealing Faraday rotation in a newly detected burst - FRB 110523. It has radio flux at least 0.6 Jy and dispersion measure 623.30(5) pc cm$^{-3}$. Using Galactic contribution 45 pc cm$^{-3}$ and a model of intergalactic electron density, we place the source at a maximum redshift of 0.5. The burst has rotation measure -186.1(1.4) rad m$^{-2}$, much higher than expected for this line of sight through the Milky Way and the intergalactic medium, indicating magnetization in the vicinity of the source itself or within a host galaxy. The pulse was scattered by two distinct plasma screens during propagation, which requires either a dense nebula associated with the source or a location within the central region of its host galaxy. Keeping in mind that there may be more than one type of fast radio burst source, the detection in this instance of source-local magnetization and scattering favours models involving young stellar populations such as magnetars over models involving the mergers of older neutron stars, which are more likely to be located in low density regions of the host galaxy.

astro-ph.HE↗

arXiv2014

Global structure of Black Holes via dynamical system

We recast the system of Einstein field equations for Locally Rotationally Symmetric spacetimes into an autonomous system of covariantly defined geometrical variables. The analysis of this autonomous system gives all the important global features of the maximal extension of these spacetimes. We conclude that the dynamical system analysis can be a powerful mathematical tool for qualitative understanding of the global structure of spacetimes covariantly, without actually solving the field equations.

gr-qc↗