arXiv subjects

Debarati Chatterjee

Publications and source records attributed to Debarati Chatterjee.

At least 19 recordsLinked to original sources

arXiv2026

Transport properties in binary neutron star mergers: Effect of magnetic field

In extreme environments such as binary neutron star mergers, temperatures as high as $50$ MeV and magnetic fields up to $10^{17}$ G, reach a regime where neutrino transport governs the macroscopic thermodynamic and chemical evolution. Existing merger simulations rely on zero magnetic field neutrino emissivity and opacity, potentially missing critical transport physics in highly magnetized neutron star cores. We present an exact framework for computing charged current Urca emissivity and neutrino opacity at finite temperature and magnetic field. We employ the Nucleon Width Approximation framework to account for the collisional broadening effects dominant in the high-density core. Our calculations demonstrate that extreme magnetic fields significantly enhance charged current neutrino opacity, effectively reducing the mean free path for thermal neutrinos.

nucl-th↗

arXiv2026

Magnetized neutron stars: perturbative versus fully-numerical approaches

(1) Background: for the study of highly magnetized neutron stars observed as magnetars, and to quantify the effect of this intense magnetic field onto the star's structure and shape which can be particularly relevant for the study of emission of continuous gravitational waves, both numerical and perturbative approaches have been developed. (2) Methods: we compare these two approaches in General Relativity with the limitation to the case where the magnetic field has a purely poloidal structure. The perturbative one (Konno-99) assumes that the deformation induced by the magnetic field is small and that this field arises only from dipole currents. The full numerical one is based on the library LORENE. (3) Results: we have used both approaches to compute the magnetic field distribution and the deformation of the star, varying the value of the magnetic field at the pole, the compactness of the star and its equation of state. (4) Conclusions: whereas the perturbative approach breaks down for very high polar magnetic field values (typically above a few times $10^{16}$ G), it gives very good results for observed values, even in magnetars. On the contrary, the numerical code exhibits resolution problems for relatively low magnetic field values (typically $10^{10}$ G), which translates into imprecise computation of the star's deformation and mass quadrupole moment.

astro-ph.HE↗

arXiv2026

Realistic Equations of State Informing Neutron Star Post-Merger Gravitational-Wave Frequencies

Binary neutron star mergers are thought to produce hot, rapidly rotating neutron stars with masses that can far exceed their Tolman-Oppenheimer-Volkoff mass. The gravitational-wave emission from such remnants provides a unique opportunity to measure the nuclear equation of state at densities and temperatures not available to terrestrial experiments. Current detector design is informed by gravitational-wave signals from general relativistic hydrodynamics simulations of neutron star mergers, typically with hybrid thermal treatments for the equation of state, where a cold equation of state is modified by adding a thermal component. We use realistic equations of state based on the relativistic mean field model with consistent treatment of thermal effects to compute the distribution of expected peak gravitational-wave frequencies. Marginalising over equation of state and progenitor neutron star masses, we show the peak frequency of emission ranges from $\sim2.5$ to 4 kHz. The width of this distribution suggests the need for broadband observatories with kHz sensitivity, and calls into question some of the so-called post-merger optimised configurations. We show the proposed KAGRA high-frequency design is well-suited to measuring post-merger remnants when compared to the KAGRA broadband design.

astro-ph.HE↗

arXiv2025

Thermal and Magnetic effects on Bulk Viscosity in Binary Neutron Star Mergers

Astrophysical scenarios such as binary neutron star mergers, protoneutron stars, and core-collapse supernovae involve finite temperatures and strong magnetic fields. Previous studies on the effect of magnetic fields on flavor-equilibration processes relied on the Fermi surface approximation, which is not a reliable approximation in the neutrino-transparent regime of matter in supernovae or neutron star mergers. In a recent study, we went beyond the Fermi surface approximation, performing the full phase space integral to obtain direct Urca rates in a background magnetic field. In this work, we extend these calculations to incorporate the collisional broadening (modified Urca) contribution. We use the recently developed nucleon width approximation, which naturally includes the magnetic field dependence of all contributions. We demonstrate the impact of magnetic fields on the flavor-equilibrium condition for two finite-temperature equations of state with different direct Urca thresholds. We also study the impact of magnetic fields on the bulk viscous dissipation of density oscillations relevant in postmerger scenarios.

nucl-th↗

arXiv2025

$f$-mode Oscillations for Hyperons and H-dibaryons in Neutron Stars

The fundamental ($f$-mode) oscillations of neutron stars are studied within the quark meson coupling model, a relativistic Hartree-Fock theory of dense nuclear matter, which takes into account the self-consistent modification of the valence quark structure of the bound baryons in the associated strong Lorentz scalar mean fields. For the first time, hyperons and H-dibaryons are included, along with the effects of potential additional short-range repulsion within this scheme, and their influence on $f$-modes is investigated. Universal relations are studied within the relativistic Cowling approximation and compared against those in the existing literature for potential applications in gravitational wave asteroseismology.

astro-ph.HE↗

arXiv2025

PSR J0614-3329: A NICER case for Strange Quark Stars

Precise measurements of neutron star masses and radii by the NICER mission impose important constraints on the nuclear equation of state. The most recent NICER measurement of PSR J0614-3329 reported an equatorial radius of $R_{eq} = 10.29^{+1.01}_{-0.86}$ km for a mass of $M = 1.44^{+0.06}_{-0.07} M_{\odot}$. Considering all the NICER measurements to date, we find substantial evidence using Bayesian hypothesis ranking for strange quark stars over physically motivated models of neutron stars compatible with this low radius. This provides a strong case for quark matter in neutron stars and also for the possible existence of strange quark stars, a consequence of the Bodmer-Witten hypothesis, suggesting that they could be considered among the population of compact stars during analyses of astrophysical data. Using a wide sample of equations of state, we report the nucleonic equations of state that best fit current observations and rule out one model of strange quark matter.

astro-ph.HE↗

arXiv2025

$g$-mode oscillations of dark matter admixed neutron stars

We investigate $g$-mode oscillations in dark matter admixed neutron stars employing a relativistic mean field model to describe hadronic matter and a model for self-interacting fermionic dark matter motivated by the neutron decay anomaly. Following the construction of such admixed configurations, we derive the equilibrium and adiabatic speeds of sound therein, leading to a computation of the star's $g$-mode spectrum in the Cowling approximation. In particular, we explore the effect of dark matter self-interaction, the nucleon effective mass and dark matter fraction on the principal $g$-mode frequency, and its first overtone. We show that the effect on $g$-mode frequency depends predominantly on the dark matter fraction, and demonstrate an equation of state-independent constraint for the latter. Prospects of identifying the presence of dark matter in neutron stars using $g$-mode are discussed.

gr-qc↗

arXiv2025

$f$-mode oscillations in hot Neutron Stars: Effect of hyperons and neutrino trapping

In this work, we present an equation of state formalism for hot Neutron Stars (NSs) which consistently includes the effects of finite temperature, hyperons as well as neutrino trapping, relevant for the study of proto-neutron stars, binary neutron star mergers and supernova explosions. Within a non-linear relativistic mean field description, the framework allows for a systematic variation of nuclear parameters within the range allowed by uncertainties in nuclear experimental data, ensuring compatibility with nuclear theory, astrophysical and heavy-ion data. We then investigate the role of nuclear and hypernuclear parameters as well as thermal effects on NS macroscopic properties and $f$-mode oscillations in hot neutron stars within Cowling approximation. Our results reveal that in hyperonic neutron stars with trapped neutrinos, the saturation nuclear density shows moderate to strong correlation with NS astrophysical observables. We also investigate whether thermal effects break universal relations and provide fit relations for hot NS configurations in the neutrino-trapped regime.

astro-ph.HE↗

arXiv2025

A White Paper on The Multi-Messenger Science Landscape in India

The multi-messenger science using different observational windows to the Universe such as Gravitational Waves (GWs), Electromagnetic Waves (EMs), Cosmic Rays (CRs), and Neutrinos offer an opportunity to study from the scale of a neutron star to cosmological scales over a large cosmic time. At the smallest scales, we can explore the structure of the neutron star and the different energetics involved in the transition of a pre-merger neutron star to a post-merger neutron star. This will open up a window to study the properties of matter in extreme conditions and a guaranteed discovery space. On the other hand, at the largest cosmological scales, multi-messenger observations allow us to study the long-standing problems in physical cosmology related to the Hubble constant, dark matter, and dark energy by mapping the expansion history of the Universe using GW sources. Moreover, the multi-messenger studies of astrophysical systems such as white dwarfs, neutron stars, and black holes of different masses, all the way up to a high redshift Universe, will bring insightful understanding into the physical processes associated with them that are inaccessible otherwise. This white paper discusses the key cases in the domain of multi-messenger astronomy and the role of observatories in India which can explore uncharted territories and open discovery spaces in different branches of physics ranging from nuclear physics to astrophysics.

astro-ph.HE↗

arXiv2025

Astrophysics with Compact Objects: An Indian Perspective, Present Status and Future Vision

Astrophysical compact objects, viz., white dwarfs, neutron stars, and black holes, are the remnants of stellar deaths at the end of their life cycles. They are ideal testbeds for various fundamental physical processes under extreme conditions that are unique in nature. Observational radio astronomy with uGMRT and OORT facilities has led to several important breakthroughs in studies of different kinds of pulsars and their emission mechanisms. On the other hand, accretion processes around compact objects are at the core of Indian astronomy research. In this context, AstroSat mission revolutionized spectro-temporal observations and measurements of accretion phenomena, quasi-periodic oscillations, and jet behaviour in binary systems hosting compact objects. Moreover, recently launched XPoSat mission is set to provide an impetus to these high-energy phenomena around compact objects by enabling us to conduct polarization measurements in the X-ray band. Further, during the past decade, numerous gravitational wave signals have been observed from coalescing black holes and neutron stars in binary systems. Recent simultaneous observation of the GW170817 event in both gravitational waves and electromagnetic channels has ushered in the era of multi-messenger astronomy. In the future, synergistic efforts among several world-class observational facilities, e.g., LIGO-India, SKA, TMT, etc., within the Indian astrophysics community will provide a significant boost to achieve several key science goals that have been delineated here. In general, this article plans to highlight scientific projects being pursued across Indian institutions in this field, the scientific challenges that this community would be focusing on, and the opportunities in the coming decade. Finally, we have also mentioned the required resources, both in the form of infrastructural and human resources.

astro-ph.HE↗

arXiv2025

Tidal heating in binary inspiral of strange quark stars

We investigate tidal heating associated with the binary inspiral of strange quark stars and its impact on the resulting gravitational wave signal. Tidal heating during the merger of neutron stars composed of nuclear matter may be considered negligible, but it has been demonstrated recently that the presence of hyperons at high densities could significantly enhance the dissipation during inspiral. In this work, we evaluate the bulk viscosity arising from non-leptonic weak processes involving quarks and show that it can be several orders of magnitude higher than the viscosity of nuclear matter at temperatures relevant to the inspiral phase of the merger of strange stars. We model strange quark matter in the normal phase using a non-ideal bag model including electrons and ensure compatibility with astrophysical constraints. By analysing equal-mass binary systems with component masses ranging from 1.4 to 1.8 $\, M_{\odot}$, we find that temperatures close to 0.1 MeV are reached by the end of the inspiral phase. We also estimate the effect on the gravitational waveform and conclude that the additional phase shift could range from $0.1$ to $0.5$ radians for strange quark masses of 200 MeV, making it potentially detectable by next-generation gravitational wave detectors. Given that tidal heating from hyperons is dominant only for very massive neutron stars having masses 1.8 to 2.0 $\, M_{\odot}$, a successful detection of this phase shift during the inspiral of binary systems with relatively low masses of 1.4 to 1.6 $\, M_{\odot}$ could be a smoking gun signature for the existence of strange quark stars.

gr-qc↗

arXiv2025

Tidal dissipation in binary neutron star inspirals from hyperon bulk viscosity: Phase modeling and parameter estimation bias

During the inspiral of a binary neutron star, viscous processes in the neutron star matter can damp out the tidal energy induced by its companion and convert it to thermal energy. This tidal dissipation/heating process introduces a net phase shift in the gravitational wave signal. In our recent work, we showed based on a Newtonian estimate that tidal dissipation from bulk viscosity originating from the non-leptonic weak interactions involving hyperons could have a detectable phase shift in the gravitational-wave (GW) signal in the next-generation GW detectors. Using simulated signals, we demonstrate that not accounting for this physical effect in waveform models can result in systematic biases in tidal deformability measurements of high-mass neutron star ($\geq 1.8M_{\odot}$) binary observations in next-generation GW detectors. By employing Newtonian orbital dynamics, we model this tidal dissipation induced dephasing as a phenomenological function of the characteristic velocity. We incorporate its effect in gravitational waveforms of equal-mass binary neutron stars. Those waveforms are used to perform a full Bayesian parameter estimation, which confirms that our model can alleviate possible biases in tidal deformability estimation. We also illustrate that the model can accurately measure the additional phase due to tidal dissipation in a $2M_{\odot}$ neutron star in observations with next-generation GW detectors and discuss its significance in extreme matter studies.

gr-qc↗

arXiv2025

Fundamental oscillations as a tool to distinguish boson stars from neutron stars and black holes

Massive boson stars are self-gravitating configurations of self-interacting scalar fields and can be modeled by a massive scalar field with a quartic self-interaction potential. It has been shown that the equation of state and static structure properties, such as mass and radius, follow scaling relations independent of microscopic dark matter properties. In this work, we demonstrate for the first time that non-radial fundamental ($f$-)mode characteristics also follow a scaling in the strong interaction limit, opening up the outstanding prospect of evaluating the mode properties for boson stars for arbitrary masses spanning the scalar dark matter parameter space allowed by current observations. We provide the scaling relations within full general relativity and obtain the mode characteristics corresponding to the maximum boson star mass configuration. We apply these to determine the $f$-mode properties for boson stars solely as a function of their mass and compactness, which allows distinguishing them from those of neutron stars and black hole quasinormal modes in comparable mass range. In particular, we show that the frequencies are always lower than those of corresponding black holes of the same mass by a factor of 4.5. This provides a smoking gun for the distinguishability of boson stars from other compact objects using gravitational wave observations.

gr-qc↗

arXiv2025

Detectability of Massive Boson Stars using Gravitational Waves from Fundamental Oscillations

Boson Stars are macroscopic self-gravitating configurations made of complex scalar fields. These exotic compact objects would manifest as dark Boson stars and, in the absence of electromagnetic signatures, could mimic properties of compact stars in the gravitational wave spectrum. In a recent study, using the simplest potential for massive Boson stars, we demonstrated that fundamental non-radial oscillations ($f$-modes) obey scaling relations that allow them to be distinguished from neutron stars and black holes. In this work, we provide analytical fits for these scaling relations, valid for the dark matter parameter space compatible with current astrophysical and cosmological data, that can be directly incorporated into future studies of massive Boson stars in the strong coupling regime, avoiding the need for numerical calculations. We also provide analytical fits for empirical and universal relations for gravitational wave asteroseismology, which can be used to infer microscopic dark matter properties following a successful detection. Further, we investigate the possibility of detection of $f$-modes and the dark matter parameter space that can be probed with current and future gravitational wave detectors across multiple frequency bands. Assuming a burst gravitational wave model and demanding a signal-to-noise ratio of 5, we show that the current and future detectors can, in principle, probe Boson star $f$-modes up to cosmological distances: 1 Mpc with aLIGO, 30 Mpc with Cosmic Explorer and Einstein Telescope, and in the best case scenario, about 300 Mpc with LISA.

gr-qc↗

arXiv2024

Investigating the role of nuclear parameters in Neutron Star oscillations: a model comparison

Recent studies based on the relativistic mean field (RMF) model found certain nuclear empirical parameters, in particular the nucleon effective mass, to be strongly correlated with observable properties of Neutron Stars (NSs), such as the frequencies of $f-$mode oscillations. This shows the potential to constrain the values of effective mass from future observations of $f-$modes. One of our primary goals of this work is to investigate whether such correlations are physical or an artifact of the underlying nuclear model. To test this, we perform a comparative study of the correlations between NS astrophysical observables and nuclear physics parameters using two different equation of state models based on RMF theory and non-relativistic Meta-Modelling (MM) scheme. The nuclear meta-model does not assume any underlying nuclear model and therefore allows us to test the model dependence of the results. The calculations of the $f-$mode characteristics are performed within the relativistic Cowling approximation. We use state-of-the-art nuclear microscopic calculations at low density and multi-messenger astrophysical data at high-density within a Bayesian-inspired scheme to constrain the parameter space of the nuclear models. From the posterior distribution, we probe the underlying correlations among nuclear parameters and with NS observables. We find that the correlation between the symmetry energy and its slope is physical, while that of the nucleon effective mass with NS observables is model-dependent. The study shows that the effective mass governs the high density behaviour in RMF models, while in the MM it is controlled by the higher order saturation parameters, and hence probes the possibility of constraining them from future $f$-mode observations. The findings of this investigation are interesting both for astrophysics as well as nuclear physics communities.

astro-ph.HE↗

arXiv2024

Effect of Magnetic Fields on Urca Rates in Neutron Star Mergers

Isospin-equilibrating weak processes, called ``Urca" processes, are of fundamental importance in astrophysical environments like (proto-)neutron stars, neutron star mergers, and supernovae. In these environments, matter can reach high temperatures of tens of MeVs and be subject to large magnetic fields. We thus investigate Urca rates at different temperatures and field strengths by performing the full temperature and magnetic-field dependent rate integrals for different equations of state. We find that the magnetic fields play an important role at temperatures of a few MeV, especially close to or below the direct Urca threshold, which is softened by the magnetic field. At higher temperatures, the effect of the magnetic fields can be overshadowed by the thermal effects. We observe that the magnetic field more strongly influences the neutron decay rates than the electron capture rates, leading to a shift in the flavor equilibrium.

nucl-th↗

arXiv2024

Investigating the role of nuclear parameters on oscillation modes in hot Neutron Stars

Recent studies have revealed that certain nuclear parameters are more dominant than others in governing global neutron star properties, such as its structure or oscillation mode characteristics. Although neutron stars can in general assumed to be cold, in astrophysical scenarios such as newly born neutron stars or remnants of binary neutron star mergers, finite temperature effects play a non-negligible role. In this work, we perform a consistent and systematic investigation of the role of nuclear parameters and thermal effects on neutron star properties and fluid oscillation modes within a full general relativistic scheme. We impose constraints on the parameter space of the relativistic mean field model using state-of-the-art information from terrestrial experiments and multi-messenger astrophysical data. We find effective nucleon mass to be the most important nuclear parameter controlling astrophysical observables of hot neutron stars, similar to the cold beta equilibrated matter. However, we conclude that the interplay among saturation properties and astrophysical observables depends not only on the thermal configurations considered but also on the constraints imposed. We also investigated the role of nuclear saturation parameters on some universal relations for hot NSs which are important in gravitational wave asteroseismology. Our investigation confirmed that these relations are mostly insensitive to nuclear saturation properties and mainly affected by variation of charge fraction in the star.

astro-ph.HE↗

arXiv2024

Communicating the gravitational-wave discoveries of the LIGO-Virgo-KAGRA Collaboration

The LIGO-Virgo-KAGRA (LVK) Collaboration has made breakthrough discoveries in gravitational-wave astronomy, a new field that provides a different means of observing our Universe. Gravitational-wave discoveries are possible thanks to the work of thousands of people from across the globe working together. In this article, we discuss the range of engagement activities used to communicate LVK gravitational-wave discoveries and the stories of the people behind the science, using the activities surrounding the release of the third Gravitational-Wave Transient Catalog as a case study.

astro-ph.IM↗