arXiv subjects

Monika Sinha

Publications and source records attributed to Monika Sinha.

At least 19 recordsLinked to original sources

arXiv2026

Influence of $Δ$ resonances in hyperonic neutron stars on gravitational-wave inspiral signatures

The composition of dense matter in neutron star interiors remains uncertain, with the possible appearance of the non-strange $Δ$ resonances providing an important source of theoretical uncertainty. In this work, we investigate the gravitational wave signatures of hyperonic neutron stars containing $Δ$ baryons within a density-dependent relativistic mean-field framework based on the DDME2 parametrization. Within this framework, the baryons interact among themselves via mesons. For $Δ$ baryons, the interaction is uncertain to date. We investigate how variations of the scalar and vector $Δ-$meson couplings, subject to $x_{σΔ}-x_{ωΔ}=0.2$, modify the tidal properties and inspiral gravitational-wave signals. We find that the allowed variation in the $Δ-σ$ coupling leads to substantial changes in the tidal properties of the neutron stars, which in turn leave distinct imprints on the gravitational-wave waveforms, particularly in their frequency evolution, accumulated phase, and strain amplitude. These waveform differences arise through changes in tidal deformability and are not unique signatures of $Δ$ resonances. Future gravitational-wave observations may therefore constrain the $Δ-$meson couplings within the adopted dense-matter framework, subject to degeneracies with alternative EOS models.

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arXiv2026

Neutron star cooling with $Δ$ dUrca processes

The cooling of neutron stars (NSs) provides insight into their composition and equation of state. However, a complete understanding of this phenomenon is still an open question. The early stage of NS cooling is dominated by neutrino emission from the core, which rapidly cools the star. Urca processes are one such mechanism where neutrinos escape from the core, carrying away most of the heat. Nucleonic and hyperonic direct Urca processes are known rapid cooling mechanisms. The possible appearance of $Δ$ baryons in dense matter introduces additional weak-interaction channels, including $Δ$ direct Urca processes. Although such processes have been discussed in the context of $Δ$-admixed NS matter, their neutrino emissivities and impact on NS cooling have not been systematically investigated. In this work, for the first time we incorporate the neutrino emissivities of the relevant $Δ$ direct Urca processes into NS cooling simulations. We investigate the thermal evolution of $Δ$-admixed NSs and assess the impact of these additional neutrino emission channels on their cooling behavior. We find that $Δ$ direct Urca processes can provide an efficient neutrino-cooling mechanism and significantly accelerate the thermal evolution of NSs. Our results demonstrate that $Δ$ baryons and their associated weak-interaction processes can play an important role in explaining NSs with unusually low surface temperatures and provide a new connection between the composition of dense matter and NS cooling observations.

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arXiv2026

Neutrino Emissivity in $Δ$-direct-Urca Processes Inside Neutron Stars

Observed massive neutron stars may contain heavier baryons such as $Δ$ isobars and hyperons in their interiors. In the presence of $Δ$ isobars, the neutrino emitting nucleonic Urca processes can be accompanied by $Δ$-direct-Urca processes involving nucleons and hyperons. Over the past three decades, significant theoretical effort has been dedicated to understanding the thermal evolution of neutron stars with the nucleonic and hyperonic Urca processes emerging as dominant cooling mechanisms. While recent studies have proposed the presence of $Δ$ isobars in the dense core of neutron stars, the specific contribution of the $Δ$-direct-Urca process to neutrino emissivity has not been explicitly quantified. In this work, we address this gap by deriving the neutrino emissivity associated with $Δ$-direct-Urca processes in dense matter inside neutron stars. We show that once $Δ$-direct-Urca processes become kinematically allowed, their emissivities are comparable to those of nucleonic-direct-Urca processes.

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arXiv2026

Neutron star matter with hyperons: Bayesian comparison of nucleonic and SU(6)/SU(3) hyperonic models

We investigate neutron star matter with hyperons within a density-dependent relativistic mean-field framework using Bayesian inference, considering three composition scenarios: purely nucleonic matter, hyperonic matter under SU(6) flavor symmetry, and hyperonic matter under SU(3) symmetry with free vector-sector parameters. The analysis incorporates constraints from empirical nuclear matter properties, theoretical inputs at low densities, and multimessenger observations of neutron stars. We find that the SU(6) scheme, grounded in the quark model and isospin counting rule, leads to a significantly softer equation of state. In contrast, the additional flexibility of the SU(3) framework enhances vector repulsion and yields a comparatively stiffer equation of state consistent with observational bounds across the explored parameter space; in particular, the posterior distributions favor values of the vector coupling ratio $α_v$ lower than the SU(6) limit $α_v = 1$. These differences are reflected in neutron star observables, including mass--radius relations, tidal deformabilities, direct Urca thresholds, and oscillation properties, all of which remain compatible with current constraints within the SU(3) scenario. We further examine structural signatures through the curvature of the mass--radius relation and find that, although hyperon-rich configurations can induce noticeable variations, such features depend sensitively on the stiffness of the equation of state and are therefore not universally robust indicators. Bayesian model comparison further shows that present constraints do not meaningfully discriminate between the purely nucleonic and SU(3) hyperonic scenarios, while providing positive, but not decisive, evidence against the more restrictive SU(6) framework.

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arXiv2026

Thermal conductivity of dense $npΛ$ matter in neutron star cores

The possible presence of hyperons in the cores of massive neutron stars has important implications for their microscopic transport properties and thermal evolution. Despite recent progress in modelling core transport, a dedicated analysis regarding the thermal conductivity of a specific $npΛ$ mixture remains absent from the existing literature. In this work, we investigate the thermal conductivity of dense, $β$-equilibrated $npΛ$ matter within the framework of the variational linearized Boltzmann kinetic approach, employing the density-dependent DDME2 equation of state across a baryon density range of $(0.5\text{--}4.5)\,n_0$. We find that neutrons still dominate thermal transport, while the onset of $Λ$ hyperons induces only a remarkably small reduction in neutron conductivity compared to pure nucleonic matter. These results suggests that the core thermal relaxation timescale remains practically unaltered in the presence of $Λ$ hyperons.

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arXiv2026

Modeling large glitches with core superfluidity in a Hybrid star

Many pulsars exhibit a peculiar behaviour in their pulse profile of a sudden increase in their rotational period, which is popularly known as a pulsar glitch. Some of them show giant glitches with relative amplitude $ΔΩ/Ω\sim 10^{-6}-10^{-5}$. With the model of pinned neutron vortices inside the neutron star (NS) crust, this large glitch cannot be explained so far. However, the increasing evidence of massive pulsars indicates the appearance of exotic degrees of freedom in the inner core of the pulsars. Given this, we consider the pulsar as a hybrid star (HS). This model opens up the possibility of vortex-pinning inside the core. Under the Gibbs equilibrium conditions, it is possible for hadrons and the quark phase to coexist. Due to the global charge neutrality condition, quark pasta structures are formed in the background of hadronic matter. We consider these pasta structures as pinning sites of superfluid vortices. We show that considering the core contribution, our calculations come to be of the order of $ΔΩ/Ω\sim 10^{-6}$, which is close to the observations shown by the Vela-like pulsars.

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arXiv2026

Dense Matter and Compact Stars in Strong Magnetic Fields

Compact stars serve as natural systems where matter exists at densities far beyond those achievable in laboratory experiments. Among them, magnetars are expected to possess interior magnetic fields that may reach values of the order of $10^{17}-10^{18}$ G. These extreme conditions are expected to alter the microscopic and macroscopic properties of dense matter. In this review, we examine how strong magnetic fields affect fermionic matter through mechanisms such as Landau quantization and anomalous magnetic moment interactions. We further discuss the behaviour of magnetized hadronic matter within relativistic mean-field approaches and consider the possible emergence of additional degrees of freedom, including hyperons, $Δ$ resonances, meson condensates and quark matter. The consequences of these effects for neutron-star structure and observational constraints are also briefly outlined.

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arXiv2026

$r$-mode stabilization in rotating hyperon-rich neutron stars and its implications for GW190814

The GW190814 event, involving a black hole of mass $22.2$--$24.3 M_{\odot}$ and a compact object of mass $2.50$--$2.67 M_{\odot}$, challenges our understanding of the mass gap between the heaviest neutron stars and the lightest black holes. If the secondary is a neutron star exceeding $2.5 M_{\odot}$, hyperons are likely to appear in its core, softening the equation of state. Rapid rotation can offset some of this softening, enabling higher maximum masses, but it may simultaneously excite the Chandrasekhar--Friedman--Schutz $r$-mode instability. Bulk viscosity arising from nonleptonic weak interactions in hyperonic matter provides an efficient damping mechanism that can stabilize such configurations. In this work, we investigate the combined effects of rotation, thermal evolution, and hyperon-induced bulk viscosity on the stability of massive neutron stars. We demonstrate a direct connection between the suppression of $r$-mode instabilities and the long-term dynamical stability of hyperon-rich stars, offering a plausible interpretation of the GW190814 secondary as a rapidly rotating, hyperon-rich neutron star rather than a low-mass black hole. Our unified framework extends beyond previous studies restricted to static equations of state or extreme viscous damping assumptions, providing new insights into the stability of massive, exotic neutron star configurations.

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arXiv2025

Kaon-meson coupling from SU(3) flavour symmetry and application to antikaon condensed dense matter in neutron star

Observations of massive pulsars suggest that the central density of neutron stars can exceed several times the nuclear saturation density, creating a favourable environment for the appearance of exotic states, such as strange and non-strange baryons, meson condensates, and deconfined quark matter. The antikaon condensate is the most studied and plausible candidate among meson condensates. However, little is known about the exact interaction mechanisms between antikaons and mediator mesons. In this work, we investigate these interactions by, for the first time, employing SU(3) flavor symmetry to study antikaon condensation in dense matter. We determine hadron couplings in the mesonic sector using SU(3) flavour symmetry. Among the three key parameters we calculate $θ_v$, the mixing angle between the octet meson $ω_8$ and the singlet meson $ϕ_1$; the ratio of the octet to singlet couplings $z$; and leave the weight factor that balances the symmetric and antisymmetric couplings $α_v$ as a free parameter to explore its impact on the system. Using this approach, we derive the couplings for antikaon interactions with both singlet and octet mesons in the nonet vector meson family and examine the corresponding implications for dense matter featuring antikaon condensation. Our findings reveal that the equation of state for dense matter becomes progressively stiffer with increasing values of $α_v$, which delays the onset of antikaon condensation and increases the maximum achievable mass of neutron stars.

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arXiv2025

Imprints of Non-Symmetric Dark Matter Halos on Magnetars: A Two-Fluid Perspective

In this study, we investigate the impact of dark matter on the structure and deformation of magnetars. We assume a perturbative approach for the magnetic field deformation and that the dark matter only interacts gravitationally with hadronic matter. Assuming that dark matter is significantly softer than hadronic matter, we find that the magnetic field can affect dark matter through the deformation of space-time. The number of stars having a dark matter halo outside the visible surface of the star increases with an increase in dark matter fraction and the stiffness of the dark matter equation of state. As the magnetic field deforms the stars from sphericity, we can have a situation where we have a non-symmetric dark matter halo outside the star. The deformation of the dark matter halo gives rise to the discrepancies in the observed period $P$ (and period derivative $\dot{P}$) and gravitational wave signatures. The observed visible surfaces predict a lower period and gravitational wave strain than that with a dark matter halo. This can have interesting observational gravitational signatures unique to magnetars having a dark matter halo.

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arXiv2025

Investigating Universal Relations in Compact Stars featuring $Δ-$Admixed Exotic Dense Matter

The dense material in a compact star from a supernova remnant is beyond terrestrial experimentation, so phenomenological modeling is used to match astrophysical observations. This is crucial due to the complex sensitivity of compact star features to dense matter properties. Despite modeling flexibility, certain universal relationships among compact star features hold true, regardless of the matter model. Our study examines these universal relationships, focusing on the moment of inertia, tidal Love number, and quadrupole moment, as well as correlations between non-radial oscillation frequencies and star compactness. We consider baryonic stars with cores of heavier baryons. Our findings show that baryonic stars with cores of heavier baryons follow the universal relations, and the f-mode oscillation frequency's universality relative to tidal deformability is notable, with an error margin under 1$\%$.

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arXiv2025

Exploring $Δ$-resonance in neutron stars: implications from astrophysical and nuclear observations

This study presents the first comprehensive Bayesian inference of neutron star matter, incorporating $Δ$-resonances alongside hyperons and nucleons within a density-dependent relativistic hadron (DDRH) framework. Using constraints from nuclear saturation properties, chiral effective field theory ($χ$EFT), NICER radius measurements, and tidal deformability data from GW170817, we systematically explore the impact of $Δ$-resonances on the equation of state (EoS) of dense matter and neutron star observables. Our results demonstrate that the inclusion of $Δ$-baryons softens the EoS at low densities while maintaining sufficient stiffness at high densities to support $2M_{\odot}$ neutron stars. This naturally reconciles neutron star radius constraints with the recent observation of the low-mass compact object in HESS J1731-347 while simultaneously exhibiting excellent agreement with GW170817 tidal deformability constraints, reinforcing the astrophysical viability of $Δ$-admixed neutron stars. Additionally, $Δ$-resonances are found to populate the outer layers of the neutron star core, which may have implications for neutron star mergers and their cooling. Furthermore, we show that the presence of $Δ$-baryons might significantly influence neutron star cooling via the direct Urca process. We also investigate quasi-normal $f$-mode oscillations within a fully general relativistic framework, revealing strong correlations between the $f$-mode frequency, neutron star compactness, and tidal deformability. With the inclusion of $Δ$-resonances and adherence to astrophysical constraints, we obtain $f_{1.4} = 1.97^{+0.17}_{-0.22}$ kHz and the damping time $τ_{f_{1.4}} = 0.19^{+0.05}_{-0.03}$ s at the $1σ$ confidence level.

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arXiv2025

Exploring non-radial oscillation modes in dark matter admixed neutron stars

Because of their extreme densities and consequently, gravitational potential, compact objects such as neutron stars can prove to be excellent captors of dark matter particles. Considering purely gravitational interactions between dark and hadronic matter, we construct dark matter admixed stars composed of two-fluid matter subject to current astrophysical constraints of maximum mass and tidal deformability. We choose a wide range of parameters to construct the dark matter equation of state, and the DDME2 parameterization for the hadronic equation of state. We then examine the effect of dark matter on the stellar structure, tidal deformability and non-radial modes considering the relativistic Cowling approximation. We find the effect on p-modes is substantial, with frequencies decreasing up to the typical $f-$mode frequency range for most stars with a dark matter halo. The effects on the f-mode frequency are less extreme. Finally, we find the most probable and $1σ$ values of the dark matter parameters used in this study.

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arXiv2024

Bayesian Inference of dense matter equation of state of neutron star with antikaon condensation

In this paper, we employ the Density Dependent Relativistic Hadron (DDRH) field theoretical Model in a Bayesian analysis to investigate the equation of state (EOS) of dense matter featuring antikaon condensation for $K^-$ and $\bar{K}^0$ inside neutron stars. The vector coupling parameters within the kaonic sector are determined through the iso-spin counting rule and quark model. Our study integrates various constraints, including $χ$EFT calculations, nuclear saturation properties, and astrophysical observations from pulsars PSR J0030+0451 and PSR J0740+66 and from the GW170817 event. We present posterior distributions of model parameters derived from these constraints, enabling us to explore the distributions of nuclear matter properties and neutron star (NS) characteristics such as radii, tidal deformabilities, central energy densities, and speed of sound. The antikaon potential at the 68(90)\% confidence intervals is determined to be $-129.36^{+12.53(+32.617)}_{-3.837(-5.696)}$ MeV. This aligns with several studies providing estimates within the range of $-120$ to $-150$ MeV. We find that the maximum neutron star mass is constrained to around 2M$_\odot$ due to the significant softening of the EOS caused by antikaon condensation. This softening results in a considerable decrease in the speed of sound. Although antikaon condensation for $K^-$ is not feasible inside the canonical neutron stars, it becomes feasible for higher NS masses. The condensation of both $K^-$ and $\bar{K}^0$ is probably present in the interior of neutron star with mass greater than 2M$_\odot$. We also discuss the interconnections among input variables, isoscalar and isovector aspects of the EOS, and specific NS properties in the context of antikaon condensation.

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arXiv2024

Universal relations for compact stars with exotic degrees of freedom

The nature of the highly dense matter inside the supernova remnant compact star is not constrained by terrestrial experiments and hence modeled phenomenologically to accommodate the astrophysical observations from compact stars. The observable properties of the compact stars are highly sensitive to the microscopic model of highly dense matter. However, some universal relations exist between some macroscopic properties of compact stars independent of the matter model. We study the universal relation including the stars containing exotic degrees of freedom such as heavier strange and non-strange baryons, strange quark matter in normal and superconducting phases, etc. We examine the universal relations for quantities moment of inertia - tidal love number - quadrupole moment. We also study the correlation of non-radial f-mode and p-mode frequencies with stellar properties. We find the f-mode frequency observes the universal relation with dimensionless tidal deformability but the p-mode frequency does not show a good correlation with stellar properties. The p-mode frequency is sensitive to the composition of the matter. We find that universal relation is also applicable for stars with exotic matter in the core of the star with several models of exotic matter.

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arXiv2024

Non-radial oscillations in newly born compact star considering effects of phase transition

The massive stars end their lives by supernova explosions leaving central compact objects that may evolve into neutron stars. Initially, after birth, the star remains hot and gradually cools down. We explore the matter and star properties during this initial stage of the compact stars considering the possibility of the appearance of deconfined quark matter in the core of the star. At the initial stage after the supernova explosion, the occurrence of non-radial oscillation in the newly born compact object is highly possible. Non-radial oscillations are an important source of GWs. There is a high chance for GWs from these oscillations, especially the nodeless fundamental (f-) mode to be detected by next-generation GW detectors. We study the evolution in frequencies of non-radial oscillation after birth considering phase transition and predicting the possible signature for different possibilities of theoretical compact star models.

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arXiv2023

Ambipolar diffusion velocity and magnetic field evolution in magnetar core: Generalised theoretical approach

The magnetic field associated with neutron stars is generally believed to be threaded inside the star. In the presence of a magnetic field, the plasma present in the interior of the star goes through several processes that lead to magnetic field evolution. It is thought that magnetar activities are mainly due to field decay. The most important process of field decay inside the core of the star is the ambipolar diffusion of the charged particles present in the interior plasma. The decay rate due to ambipolar diffusion is directly connected to the ambipolar velocity of the charged particles under the influence of the present magnetic field. The ambipolar velocity of the charged particles depends on the internal dynamics of the particles. We outline a general method to solve the particle dynamics in the presence of a magnetic field using a magnetohydrodynamic equation for ambipolar velocity. The equation is general and applies to all possible surrounding conditions \eg temperature, and matter states like normal or superfluid.

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arXiv2023

Fast neutron star cooling in light of the PREX-2 experiment

The nuclear symmetry energy and its behaviour with density has been recently evaluated with enhanced value by PREX-2 experiment. This new values enables direct Urca neutrino emission process to be functioning in the dense matter inside neutron stars. With this new outlook we study the cooling rate of canonical mass neutron stars and compare with available observational cooling data. We find most of the isolated neutron star thermal profile is compatible with the cooling of canonical mass star including superfluidity suppression.

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