arXiv subjects

Sayak Datta

Publications and source records attributed to Sayak Datta.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical Friction as Environmental Gravitational Self-Force

Dynamical friction (DF) and gravitational-wave radiation reaction are conventionally treated as distinct dissipative mechanisms acting on a compact object inspiraling through a matter environment. We show that DF is not an additional force but a term at order $q^2\epsilon$ in a covariant multiparameter expansion of the MiSaTaQuWa force equation that vanishes identically in the absence of a particle-induced perturbation of the environment's matter fields, where $q$ and $\epsilon$ are respectively the mass ratio and environmental parameter. As a test of this identification, we evaluate it in weak and adiabatic field limit, solving the polar density-perturbation equation sourced by the particle's own order-$q$ vacuum metric perturbation. The reduction reproduces the Chandrasekhar-Ostriker drag force and predicts two features in strong gravity. An $\ell$-dependent splitting of the wake that only recombines into the Ostriker source in the weak-field limit, and a purely relativistic axial contribution with no Newtonian counterpart. This establishes dynamical friction as an intrinsic piece of self-force theory, extending to strong field, generic orbits, and generic environments, with direct consequences for extreme-mass-ratio inspiral waveform modeling for LISA.

gr-qc↗

arXiv2026

Tidal Stripping of Matter Bound to the Secondary in Extreme Mass-Ratio Inspirals

Environmental studies of extreme mass-ratio inspirals (EMRIs) have focused almost entirely on matter surrounding the primary supermassive black hole. We instead consider matter bound to the stellar-mass secondary (e.g., gas or dark matter); which can be progressively tidally stripped during the LISA-band inspiral. This changes the bound mass of the inspiraling object, modifying the gravitational-wave (GW) phase at leading order in the secondary mass. Furthermore, as the signal interpolates from an initially dressed inspiral to a nearly bare one, it can produce a characteristic inflection in the residual phase with constant mass waveform templates. Even for an environmental mass $\sim 10^{-3}\,M_{\odot}$, the cumulative dephasing relative to in band initial bound mass waveform can be larger than unity. In subsolar mass cases, the relative dephasing can reach $O(10^3)\, \rm rad$. Neglecting this effect may bias inferred EMRI parameters at the level of the fractional change in the in-band bound mass. The tidal stripping phenomena carry information about the mass and the compactness of the bound matter, enabling probes of sub-AU, planetary- to subsolar-mass environments surrounding stellar-mass black holes.

gr-qc↗

arXiv2026

Spectral suppression of black hole ringdown tails

The late-time power law tail predicted by Price's law is a generic feature of black hole perturbation theory, yet it is largely absent in numerical relativity waveforms of binary black hole mergers. We show that this suppression arises from the spectral structure of oscillatory sources. For a generic perturbation with carrier frequency $\nu$ and characteristic width $\sigma$, the branch-cut excitation coefficient governing the tail is suppressed by $\alpha=\sigma\nu$. For a Gaussian pulse, the suppression $\sim e^{-\alpha^2/2}$. This suppression is exact and confirmed by the time domain Regge Wheeler evolutions. The same parameter that controls the transition from broadband to frequency selective black hole response is also responsible for the tail suppression. Moreover, we analytically derive the leading- and next-to-leading-order tail coefficients, finding agreement with numerical fits below the $\sim10\%$ level. Our results provide a first principle explanation for the absence of tails in quasi-circular mergers and their enhancement in head-on and eccentric ones.

gr-qc↗

arXiv2026

Shaping black hole resonances I. Black hole ringdown as a spectral filtering process

The ringdown of a perturbed black hole (BH) can be described as a superposition of quasinormal modes (QNMs), whose frequencies are determined by the spacetime geometry while their amplitudes depend also on the perturbing source. However, the physical mechanism governing mode excitation remains unclear and is typically treated on a case by case basis. In this work, we show that QNM excitation is governed by a simple spectral rule: each mode is excited according to the Fourier content of the perturbation evaluated at its characteristic frequency. This result follows from the factorization of the excitation coefficients and establishes a direct, quantitative connection between the spectral properties of the perturbation and the resulting ringdown amplitudes. To make this mechanism explicit and controllable, we construct localized perturbations with independently tunable spectral bandwidth and carrier frequency. We demonstrate analytically and numerically that BHs act as resonant spectral filters. We show analytically that the excitation amplitude of each mode equals the weighted spatial Fourier transform of the initial data evaluated at wavenumber $k\sim\omega_n$ so that the filter selectively excites modes whose frequencies lie within the spectral support of the perturbation while suppressing others. Consequently, the excitation is maximized when the dominant perturbation frequency lies close to the real part of the QNM frequency, and we validate this at the percent level with fits to time-domain numerical evolutions. To robustly perform these fits, we have developed a new fitting algorithm, $\mathtt{QNMToolkit}$, which performs ringdown fits over large ensembles of sliding time-domain windows and quantifies the resulting fitting variance.

gr-qc↗

arXiv2026

Binary black holes in the heat of merger

A black hole binary approaching merger undergoes changes in its inspiral rate as energy and angular momentum are lost from the orbits into the horizons. This effect strengthens as the black holes come closer. We use numerical relativity data to model this so-called tidal heating in the strong gravity regime. We present a frequency-domain approximant for nonspinning black hole binaries that accounts for tidal heating effects up to the merger frequency. The approximant includes horizon parameters that characterize the nature of the compact objects. By applying this model to a binary black hole baseline that incorporates tidal heating, one can construct a more accurate point-particle waveform, one that is devoid of finite-size effects of the component objects. We also discuss its ramifications in modeling binary neutron star systems.

gr-qc↗

arXiv2026

Axial tidal Love numbers of black holes in matter environments

We study the axial (magnetic) tidal Love numbers of a Schwarzschild black hole surrounded by a spherically symmetric matter distribution. While the formalism developed here is general, we specialize to the case of anisotropic fluids as a proxy for dark matter distributions, computing the Love numbers for different density profiles of astrophysical interest. We employ two complementary methods: a small-compactness expansion, yielding closed-form analytic expressions, and direct numerical integration of the perturbation equations. We discuss the connection between different formulations of the fluid perturbations and the resulting Love numbers. We further show that density profiles lacking compact support generically produce logarithmic terms in the asymptotic expansion of the perturbation variable, which obstruct the standard tidal matching procedure and whose origin we trace to the absence of a strictly vacuum exterior. Our findings highlight the importance of controlling the asymptotic structure of the matter distribution when defining tidal observables for black holes dressed by matter, and provide a general framework that can be applied to other spherically symmetric environments.

gr-qc↗

arXiv2026

Geometric properties of slowly rotating black holes embedded in matter environments

Extreme mass-ratio inspirals (EMRIs) provide a precise probe of strong-field gravity, where small deviations from vacuum Kerr geometry can accumulate over many orbital cycles. In realistic astrophysical settings, black holes are embedded in surrounding dark and baryonic matter whose presence and motion can perturb the spacetime. A systematic semi-analytic framework incorporating the rotation of the environment itself into a black-hole geometry, and propagating its effects consistently into conserved quantities and epicyclic observables, has not been explored exhaustively, limiting consistent assessments of environmental effects on precision observables. In this work, we construct a slowly rotating black hole spacetime embedded in an anisotropic matter distribution and explicitly include the angular velocity of the surrounding medium within a controlled slow-rotation expansion. We demonstrate that the environment's velocity field induces corrections to the metric coefficients that propagate into modifications of conserved quantities governing geodesics. We also derive semi-analytic shifts in the innermost stable circular orbit, light-ring location, and radial and vertical epicyclic frequencies, showing that environmental rotation produces systematic and in certain regimes qualitatively distinct behavior relative to static configurations. Consequently, we explicitly show that the environment's nature and motion shift the positions of the epicyclic resonances. The formalism applies to generic anisotropic matter profiles and is not restricted to the specific halo model adopted for numerical illustration. These results establish a direct and quantitatively controlled link between environmental rotation and strong-field orbital observables, enabling consistent incorporation of rotating matter environments into precision EMRI modeling.

gr-qc↗

arXiv2026

A multi-parameter expansion for the evolution of asymmetric binaries in astrophysical environments

Compact binaries with large mass asymmetries - such as Extreme and Intermediate Mass Ratio Inspirals - are unique probes of the astrophysical environments in which they evolve. Their long-lived and intricate dynamics allow for precise inference of source properties, provided waveform models are accurate enough to capture the full complexity of their orbital evolution. In this work, we develop a multi-parameter formalism, inspired by vacuum perturbation theory, to model asymmetric binaries embedded in general matter distributions with both radial and tangential pressures. In the regime of small deviations from the Schwarzschild metric, relevant to most astrophysical scenarios, the system admits a simplified description, where both metric and fluid perturbations can be cast into wave equations closely related to those of the vacuum case. This framework offers a practical approach to modeling the dynamics and the gravitational wave emission from binaries in realistic matter distributions, and can be modularly integrated with existing results for vacuum sources.

gr-qc↗

arXiv2026

Eccentricity evolution of spinning binaries and its dependence on the equation of state of the components

We study the evolution of the eccentricity of an eccentric orbit with spinning components. We develop a prescription to express the evolving eccentricity in terms of reference eccentricity and frequency. For that purpose we considered the spins to be perpendicular to the orbital plane. Using this we found an analytical result for the contribution of spin in eccentricity evolution. As a result, we expressed orbital eccentricity in a series of reference eccentricity and gravitational wave frequency. The prescription developed here can easily be used to find arbitrarily higher-order contributions of reference eccentricity. With this we computed the eccentricity upto $\mathcal{O}(e_0^5)$. This result can be used to construct the waveforms of spinning compact objects in an eccentric orbit. Since, our expression depends on the spin induced quadrupole moments, we also study the impact of component properties on the eccentricity evolution through the quadrupole moment. We find for BNSs the evolution depends on the equation of state very mildly unless the NSs are subsolar mass. For subsolar mass NSs the deviations from BH case is comparatively larger and has equation of state dependence. For binary boson stars the deviations are comparatively larger across the mass values. We argue that it may affect our understanding of formation channels and their corresponding populations. We also argue that this can possibly be used as another tool to constrain exoticness of compact objects in a binary.

gr-qc↗

arXiv2025

Impact of a third body on binary neutron star tidal interactions

For waveform modelling of compact binary coalescence, it is conventionally assumed that the binary is in isolation. In this work, we break that assumption and introduce a third body at a distance. The primary goal is to understand how the distant third body would affect the binary dynamics. However, in the present work, we treat the three-body problem perturbatively and study tidal interaction in the binary due to the third body's presence. We introduce appropriate modifications to the equations governing the orbital motions and the evolution equations of the binary component's quadrupole moment. Further, we obtain the radiated energy and accumulated dephasing for the binary. We show that for b-EMRI, the effect is weak in the tidal sector, while for systems such as b-IMRIs, it would be most relevant to study these effects.

gr-qc↗

arXiv2024

Phenomenological gravitational waveform model of binary black holes incorporating horizon fluxes

Subjected to the tidal field of its companion, each component of a coalescing black hole binary suffers a slow change in its mass (tidal heating) and spin (tidal torquing) during the inspiral and merger. This effect modifies the phase and amplitude of the gravitational waveform. Numerical relativity (NR) waveforms contain these effects inherently, whereas analytical approximants for the early inspiral phase have to include them manually in the energy balance equation. In this work, we construct a frequency-domain gravitational waveform model that incorporates this effect, by recalibrating the inspiral phase of the waveform model IMRPhenomD to incorporate the phase corrections for tidal heating. We also include corrections to the amplitude by adding them directly to the inspiral amplitude model of IMRPhenomD. We demonstrate that the inclusion of the corrections, especially in the phase, confers an overall improvement in the phase agreement between the analytical inspiral model (uncalibrated SEOBNRv2) and NR data. The model presented here is faithful, with less than $1\%$ mismatches against a set of hybrid waveforms (except for one outlier that barely breaches this limit). The recalibrated model shows mismatches of up to $\sim 14\%$ with IMRPhenomD for high mass ratios and spins. Amplitude corrections become less significant for higher mass ratios, whereas the phase corrections leave more impact -- suggesting that the former is practically irrelevant for gravitational wave data analysis in Advanced LIGO (aLIGO), Virgo and KAGRA. Comparing with a set of 219 numerical relativity waveforms, we find that the median of mismatches decreases by $\sim 4\%$ in aLIGO zero-detuned high power noise curve, and by $\sim 1.5\%$ with a flat noise curve. This implies a modest but notable improvement in waveform accuracy.

gr-qc↗

arXiv2024

Enhancing Fenton-like Photo-degradation and Electrocatalytic Oxygen Evolution Reaction (OER) in Fe-doped Copper Oxide (CuO) Catalysts

Although hydrogen generation by water electrolysis is the cheapest of all other available sources, water splitting still occurs with sluggish kinetics. It is a challenging barrier for H2 production on a large scale. Moreover, research is still underway to understand the oxygen evolution reaction (OER) and design the catalysts with improved OER performance. Herein, we report the synthesis, characterization, and OER performance of iron-doped copper oxide (CuO) as low-cost catalysts for water oxidation. The OER occurs at about 1.49 V versus the RHE with a Tafel slope of 69 mV/dec in a 1 M KOH solution. The overpotential of 338 mV at 10 mA/cm2 is among the lowest compared with other copper-based materials. The catalyst can deliver a stable current density of >10 mA/cm2 for more than 10 hours. Additionally, wastewater treatment, particularly synthetic dye wastewater, is vital for preventing water scarcity and adverse effects on human health and ecotoxicology. The as-synthesized catalysts are also utilized for Fenton-like photo-degradation under low-power visible household LED lights toward the most commonly industrially used simulated Methylene blue dye wastewater. Almost complete degradation of the MB dye has been achieved within 50 minutes of visible light irradiation with a first-order rate constant of 0.0973/min. This dual functionality feature can open new pathways as a non-noble, highly efficient, and robust catalyst for OER and wastewater treatments.

physics.app-ph↗

arXiv2024

Effect of pH on photocatalytic degradation of Methylene Blue in water by facile hydrothermally grown TiO2 Nanoparticles under Natural Sunlight

Each year, the production of synthetic dye wastewater reaches a trillion tons, posing a significant challenge to addressing water scarcity on a global level. Hence, the treatment of wastewater to prevent water scarcity is of prime importance, and failing to do so will increase ecotoxicological risks and human health. Textile wastewater contains harmful dye. Photocatalytic degradation of such dye-contaminated wastewater is crucial to purifying the dye-contaminated water. However, this process takes time, uses high-power lamps, and is expensive. Here, we report the effect of the concentration of precursor on the size and surface morphology of TiO2 nanostructures prepared by facile hydrothermal synthesis and its ability to perform as a photocatalyst to degrade the most common industrial textile dye, methylene blue (MB), under natural sunlight. The impact of particle size on the photocatalytic activity and photocarrier migration rate was thoroughly examined. Also, the effect of pH on adsorption and photocatalytic degradation has been evaluated in detail. With several optimized conditions, almost complete dye degradation was achieved within 40 minutes under the direct illumination of natural sunlight. The enhanced photocatalytic performance can be correlated to the synergetic effect of a higher charge transfer mechanism, good catalytic active surface area availability (386 m2/g), and several optimized parameters that affect the reaction efficacy. Additionally, repeated use of NPs without sacrificing performance five times confirmed its stability and Sustainability as a promising candidate for large-scale industrial textile wastewater remedies.

cond-mat.mtrl-sci↗

arXiv2024

Tidal heating as a discriminator for horizons in equatorial eccentric extreme mass ratio inspirals

Tidal heating in a binary black hole system is driven by the absorption of energy and angular momentum by the black hole's horizon. Previous works have shown that this phenomenon becomes particularly significant during the late stages of an extreme mass ratio inspiral (EMRI) into a rapidly spinning massive black hole, a key focus for future low-frequency gravitational-wave observations by (for instance) the LISA mission. Past analyses have largely focused on quasi-circular inspiral geometry, with some of the most detailed studies looking at equatorial cases. Though useful for illustrating the physical principles, this limit is not very realistic astrophysically, since the population of EMRI events is expected to arise from compact objects scattered onto relativistic orbits in galactic centers through many-body events. In this work, we extend those results by studying the importance of tidal heating in equatorial EMRIs with generic eccentricities. Our results suggest that accurate modeling of tidal heating is crucial to prevent significant dephasing and systematic errors in EMRI parameter estimation. We examine a phenomenological model for EMRIs around exotic compact objects by parameterizing deviations from the black hole picture in terms of the fraction of radiation absorbed compared to the BH case. Based on a mismatch calculation we find that reflectivities as small as $|\mathcal{R}|^2 \sim \mathcal{O}(10^{-5})$ are distinguishable from the BH case, irrespective of the value of the eccentricity. We stress, however, that this finding should be corroborated by future parameter estimation studies.

gr-qc↗

arXiv2024

Horizon fluxes of binary black holes in eccentric orbits

We compute the rate of change of mass and angular momentum of a black hole, namely tidal heating, in an eccentric orbit. The change is caused due to the tidal field of the orbiting companion. We compute the result for both the spinning and non-spinning black holes in the leading order of the mean motion, namely $ξ$. We demonstrate that the rates get enhanced significantly for nonzero eccentricity. Since eccentricity in a binary evolves with time we also express the results in terms of an initial eccentricity and azimuthal frequency $ξ_ϕ$. In the process, we developed a prescription that can be used to compute all physical quantities in a series expansion of initial eccentricity, $e_0$. These results are computed taking account of the spin of the binary components. The prescription can be used to compute very high-order corrections of initial eccentricity. We use it to find the contribution to eccentricity up to $\mathcal{O}(e_0^5)$ in the spinning binary. We also provide an approximate expression for $\mathcal{O}(e_0^n)$, where $n$ is any odd number. With this, we compute approximate expression for $\mathcal{O}(e_0^7)$ and $\mathcal{O}(e_0^9)$ for non-spinning binary. Using the computed expression of eccentricity, we derived the rate of change of mass and angular momentum of a black hole, both rotating and non-rotating, in terms of initial eccentricity and azimuthal frequency up to $\mathcal{O}(e_0^6)$. We also compute leading order dephasing in both cases analytically up to $\mathcal{O}(e_0^6)$ and study its impact.

gr-qc↗

arXiv2024

Black holes immersed in dark matter: energy condition and sound speed

In this work, we study the impact of the environment around a black hole in detail. We introduce non-vanishing radial pressure in a manner analogous to compact stars. We examine both isotropic and anisotropic fluid configurations with and without radial pressure respectively. Our focus extends beyond just dark matter density to the vital role of the energy condition and sound speed in the spacetime of a black hole immersed in matter. In cases of anisotropic pressure with vanishing radial pressure, all profiles violate the dominant energy condition near the BH, and the tangential sound speed exceeds light speed for all dark matter profiles. In our second approach, without assuming vanishing radial pressure, we observe similar violations and superluminal sound speeds. To rectify this, we introduce a hard cutoff for the sound speed, ensuring it remains subluminal. As a consequence, the energy condition is also satisfied. However, this results in increased density and pressure near the BH. This raises questions about the sound speed and its impact on the density structure, as well as questions about the validity of the model itself. With the matter distribution, we also compute the metric for different configurations. It reveals sensitivity to the profile structure. The metric components point towards the horizon structure.

gr-qc↗

arXiv2023

Eccentricity-tide coupling: impact on binary neutron stars and extreme mass-ratio inspirals

We study the effect of tidal interaction between two compact bodies in an eccentric orbit. We assume the tidal fields to be static. Therefore, we ignore the dynamic tides and resonant excitations. Using the results, we find the analytical expression for the phase shift of the emitted gravitational wave. In the process, we find that in the leading order, the initial eccentricity $e_0$ and the dimensionless tidal deformability $Λ$ couple as $\sim e_0^nΛ$, where $n$ is a positive number. We only focus on the dominant contribution, i.e., $e_0^2Λ$. We also compute the accumulated dephasing for binary neutron star systems. We find that for optimistic values of eccentricities $e_0 \sim .05$ and $Λ\sim 600$, the accumulated dephasing is $\mathcal{O}(10^{-4})$ radian, requiring a signal-to-noise ratio $\sim 7000$ to be observable. Therefore, these effects can be measured in binary neutron star systems with large eccentricities if the signal-to-noise ratios of the systems are also very large. Hence, in third-generation detectors, it may have an observable impact if the systems have large eccentricities. We also explore the impact of this effect on extreme mass-ratio inspirals (EMRIs). We find that even for supermassive bodies with small values of $Λ\sim 10^{-3}$, this effect has large dephasing in EMRIs $\sim \mathcal{O}(10)$ radian. Therefore, this effect will help in probing the nature of the supermassive bodies in an EMRI.

gr-qc↗

arXiv2023

Probing horizon scale quantum effects with Love

Future gravitational wave detectors have been projected to be able to probe the nature of compact objects in great detail. In this work, we study the potential observability of the small length scale physics near black hole horizon with the tidal deformability of the compact objects in an inspiraling binary. We find that it is possible to probe them with extreme mass ratio inspirals. We discuss how the quantum effects can affect the gravitational wave observables. This as a consequence is bound to shape our understanding of the quantum scale near the horizon.

gr-qc↗