arXiv subjects

Tuhin Malik

Publications and source records attributed to Tuhin Malik.

At least 19 recordsLinked to original sources

arXiv2026

Observable Signatures of a Quarkyonic Phase in Neutron Stars

Quarkyonic matter in $β$-equilibrium is a potential description of cold dense matter in neutron stars (NSs), that introduces non-interacting quarks alongside nucleons and leptons in NS cores. In this paper, we impose observational and theoretical constraints on the model to perform Bayesian inference on it, and find that it is possible to have quarkyonic matter equations of state that satisfy all current astrophysical observations, thereby reinforcing the argument for its use alongside traditional ones. To differentiate between NSs where a quarkyonic phase does and does not appear in the core, we identify some novel signatures based on the mass-radius relation. Focusing on canonical ($1.4\ \mathrm{M_\odot}$) NSs, we find the populations of NSs with and without quarkyonic cores show separability on the basis of the slope and curvatures of the mass-radius curve, the central sound speed of the star, and the radius difference between two NSs of $2\ \mathrm{M_\odot}$ and $1.4\ \mathrm{M_\odot}$. Our results indicate that observing a neutron star with these signatures matching the values for quarkyonic core NSs would provide a strong evidence for the existence of a quarkyonic phase or a similar crossover transition in its core.

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arXiv2026

Imprints of Higgs-portal fermionic dark matter on neutron-star tidal deformability and the mass-radius slope

We investigate the structure of neutron stars (NSs) admixed with fermionic dark matter (DM) using three density-dependent relativistic mean-field (DDRMF) functionals (DDME, DDB, and GDFM) for $β$-equilibrated nucleonic matter. Modeling DM as the lightest neutralino interacting via Higgs exchange, we treat the DM Fermi momentum $k_F^{\rm DM}$ as a control parameter in the range $0.02$-$0.06$ GeV. We solve the coupled mean-field and Tolman-Oppenheimer-Volkoff equations to obtain the mass-radius relation, maximum mass $M_{\rm max}$, radial sound speed profile $c_s^2$, and tidal deformability $Λ$. In all models, DM softens the equation of state, systematically reducing $M_{\rm max}$, the radius $R_{1.4}$ (at $1.4M_{\odot}$), and the tidal deformability $Λ_{1.4}$ (at $1.4M_{\odot}$) as $k_F^{\rm DM}$ increases. Consequently, the $2 M_\odot$ pulsar limit and NICER data place a model-dependent upper limit on the DM content, while the GW170817 tidal bound requires a minimal DM content for the stiffest functional. Using a recent Bayesian inference of the DDRMF equation of state as the nucleonic reference band, we evaluate if this DM imprint can be distinguished from nucleonic uncertainties using only measureable quantities. Analyzing the tidal deformability $Λ$ and mass-radius slope $dR/dM$ at fixed mass, we find that $Λ$ is a sharp discriminator: at $1.4 M_\odot$ and $k_F^{\rm DM}=0.06$ GeV, the DM-induced reduction of $Λ$ reaches $\simeq 8$ times the nucleonic $1σ$ width (model-independently $7.5$-$7.8σ$). The DM track leaves the nucleonic $1σ$ band for $k_F^{\rm DM}\gtrsim0.03$-$0.04$ GeV, whereas $dR/dM$ becomes diagnostic only for the heavier ($1.8$-$2.0 M_\odot$) branch.

hep-ph↗

arXiv2026

Neutron stars with an agnostic Dark sector: Core and Halo configurations from a two-fluid approach

The study of dark matter admixed neutron stars has the potential to advance our understanding of dark matter particle candidates. However, the large parameter space of dark matter particle masses restricts a systematic, model-independent study. In this analysis, we employ agnostic hadronic and dark matter equations of state to construct dark-matter-admixed neutron stars within a two-fluid formalism. Dark matter is characterised solely by its low-density equation of state and mass, and is modelled as a Fermi gas, while hadronic matter is anchored at low and high densities by chiral effective field theory and perturbative quantum chromodynamics calculations. A speed-of-sound parametrisation covers the intermediate density region for hadronic matter and the high-density region for dark matter, so the dark matter equation of state is constrained only by thermodynamic consistency, free from bias toward a softer or stiffer equation of state. Within this agnostic framework, we find that dark matter does not generically compactify the star: light dark matter forms extended halos that raise the tidal deformability, while heavy dark matter forms compact cores that lower it. Consequently, the dominant observational constraint shifts from gravitational-wave tidal deformability for light, halo-dominated models to NICER mass--radius data for heavy, core-dominated models. Using current data at $1σ$, we constrain the dark matter fraction to $f_{\mathrm{DM}} \lesssim 0.11$ for light dark matter. Being almost independent of any assumed dark-sector microphysics, our framework yields conservative, broadly applicable bounds on the dark-matter content of neutron stars. Neutron stars with similar masses but very different tidal deformabilities could be a smoking-gun signature of dark matter in Neutron stars.

astro-ph.HE↗

arXiv2026

Amortized Simulation-Based Inference of Relativistic Mean-Field Couplings for Neutron-Star Equations of State

We present a simulation-based inference framework for constraining microscopic relativistic mean-field parameters of neutron-star equations of state. Neural posterior estimation is applied to two representative RMF families, a density-dependent DDB model and a nonlinear RMF-NL model, using nuclear saturation properties, chiral effective-field-theory pure-neutron-matter pressures, and the maximum-mass constraint as conditioning observables. The inferred posteriors are validated against the conventional nested sampler (PyMultiNest) calculations and tested with the TARP coverage diagnostic. For both RMF parametrizations, the neural posterior reproduces the nested-sampling constraints on model couplings, nuclear-matter properties, and neutron-star observables with no significant bias. The amortized estimator generates $3\times 10^{4}$ posterior samples in about $2.5\,\mathrm{s}$ on a CPU, enabling a rapid inference workflow without the need for retraining for updated data. This constitutes a proof of concept that NPE-emulated RMF models, once validated, can be safely used for superfast exploratory inference. As an additional mock-observation test, imposing $R_{1.4}=12\,{\rm km}$ and $M_{\rm max}>1.97\,M_\odot$ leads to consistent predictions for the maximum-mass configuration, with DDB giving $M_{\rm max}=2.10^{+0.09}_{-0.07}\,M_\odot$, $R_{\rm max}=10.71^{+0.14}_{-0.21}\,{\rm km}$ and RMF-NL giving $M_{\rm max}=2.05^{+0.10}_{-0.06}\,M_\odot$, $R_{\rm max}=10.69^{+0.18}_{-0.19}\,{\rm km}$; although fixing $R_{1.4}$ confines both families to a narrow EOS region, RMF-NL remains marginally softer than DDB at high density, consistent with its slightly lower maximum mass.

astro-ph.HE↗

arXiv2026

Causality alone bounds the maximum radius difference between different-mass neutron stars

We investigate how the assumption of a common causal equation of state (EoS) correlates the radii of neutron stars at different masses and thereby reduces the uncertainties inferred from independent observations. We show that causality, anchored only to the chiral effective field theory ($χ$EFT) EoS near saturation density, places a closed-form upper bound on the radius difference, $R(2.0\,M_\odot)\le 1.16\,R(1.4\,M_\odot)-1.1\,$km. The bound is saturated exactly by a one-parameter family of EoSs that we construct analytically. Imposing this prior-independent causal ceiling on the independent NICER posteriors of PSR J0437-4715, PSR J0614-3329, and PSR J0740+6620 retains only 7.5% of their joint product distribution and removes the large-radius tail of the PSR J0740+6620 posterior. Unlike full EoS-informed inferences, our construction cleanly isolates the consequences of the generic physical assumptions of a common causal EoS from those associated with a particular choice of EoS prior, providing a transparent benchmark for interpreting neutron-star observations.

astro-ph.HE↗

arXiv2026

Hyperonic equation of state for neutron stars: A systematic Bayesian comparison of density-dependent and non-linear relativistic mean-field models

A systematic Bayesian inference study of the equation of state (EOS) of dense matter with strangeness is presented, extending five relativistic mean-field (RMF) models with both constant and density-dependent couplings to include the full baryon octet. The hyperon-nucleon couplings in the scalar channel are varied within ranges informed by hypernuclear data, while vector isoscalar couplings are fixed by the SU(6) symmetry quark model. Observational constraints from NICER (PSR J0030, J0437, J0740) and GW170817, theoretical constraints from chiral effective field theory ($χ$EFT) and perturbative QCD (pQCD), and experimental constraints from nuclear saturation properties are imposed simultaneously. We find that the inclusion of hyperons systematically reduces the maximum neutron star mass by $0.05$-$0.10,M_\odot$ across all models while increasing the radius at $1.4,M_\odot$ by $0.5$-$0.8$ km. The speed of sound exhibits a characteristic softening at densities $2$-$3,ρ_{\rm sat}$ coinciding with hyperon onset. All hyperonic models remain consistent with the $2,M_\odot$ constraint. Models with a more flexible isovector channel span a larger proton fraction when only nucleons are included. However, the extra flexibility is visibly suppressed by hyperons, meaning that the average proton distribution is independent of model flexibility when hyperons are included. Less flexible models show comparable or slightly increased proton fractions due to EOS stiffening when hyperons are included. Only a residual number of hyperonic equations of state give rise to a mass-radius curve with a negative slope at low masses. A $1.8,M_\odot$ neutron star with a radius larger than or similar to the radius of a $1.2,M_\odot$ star would provide strong evidence that the star contains baryonic degrees of freedom beyond nucleons.

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arXiv2026

Hybrid Star Properties with NJL and MFTQCD Model: A Bayesian Approach

The composition of the core of neutron stars (NS) is still under debate. One possibility is that because of the high densities reached in their cores, matter could be deconfined into quark matter. We investigate the existence of hybrid stars, using microscopic models to describe different phases of matter. Within the adopted microscopic models we calculate properties of NS and properties of matter. We want to probe the pQCD calculations influence and analyze properties that identify a transition to deconfined matter. Bayesian approach is applied to generate 8 sets of equations of state (EOS). A Maxwell construction is adopted to describe the deconfinement transition. For the hadron phase, we consider a stiff and a soft EOS obtained from the Relativistic Mean Field model with nonlinear meson terms. For the quark phase, we use 2 different models: the Nambu-Jona-Lasinio model with multiquark interactions and the Mean Field Theory of QCD, a model similar to the vector MIT bag model. Bayesian inference was applied to determine the model parameters that satisfy the X-ray observations from NICER and have phase transition at densities between 0.15 - 0.40 fm$^{-3}$. We also applied restrictions from the pQCD calculations to half of the sets. Hybrid stars are compatible with current observational data. The pQCD restrictions reduce the value of the $M_{max}$. However, even applying this restriction, the models were able to reach values of $2.1 - 2.3 M_\odot$. The conformal limit was still not attained at the center of the most massive stars. The vector interactions are essential to describe hybrid stars with a mass above $2 M_\odot$. The multiquark interactions introduced may affect the limits of some quantities considered as indicators of the presence of a deconfined phase. It is possible to find a set of EOS, that predict that inside NS the renormalized matter trace anomaly is always positive.

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arXiv2026

A Bayesian Inference of Hybrid Stars with Large Quark Cores

Neutron stars (NSs) are interesting objects capable of reaching densities unattainable on Earth. The properties of matter under these conditions remain a mystery. Exotic matter, including quark matter, may be present in the NS core. In this work, we explore the possible compositions of NS cores, in particular, the possible existence of large quark cores. We use the Relativistic Mean Field (RMF) model with nonlinear terms for the hadron phase and the Nambu-Jona-Lasinio (NJL) model and Mean Field Theory of Quantum Chromodynamics (MFTQCD) for the quark phase. Through Bayesian inference, we obtain different sets of equations: four sets with hybrid equations and one set with only the hadron phase. We impose constraints regarding the properties of nuclear matter, X-ray observational data from NICER, gravitational wave data from the binary neutron star merger GW170817, perturbative QCD (pQCD) calculations, and causality. The MFTQCD allows for a phase transition to quark matter at low densities, just above saturation density, while for the NJL sets, the phase transition occurs above twice the saturation density. As a result, the MFTQCD model predicts the presence of quark matter in the inner core of 1.4 M$_\odot$ NSs, while NJL models suggest a low probability of quark matter in the interior of a 1.4 M$_\odot$ NS. Both models predict the existence of quark matter in 2 M$_\odot$ NSs. The slope of the mass-radius curve has been shown to carry information about the presence of quark matter. In particular, a positive slope at 1.8 M$_\odot$ indicates the presence of non-nucleonic matter. A hybrid star with a stiff quark equation of state could explain a larger radius in more massive stars, such as two solar mass stars, compared to canonical NSs.

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arXiv2026

Medium effects on light clusters from heavy-ion collisions within a relativistic mean-field description

Central $^{136,124}$Xe$+^{124,112}$Sn collisions from INDRA data are analysed using a Bayesian inference on light nuclei multiplicities to estimate the thermodynamical parameters and in-medium modification of the cluster self-energies within a relativistic mean-field model. An excellent description of experimentally measured abundances of H and He isotopes is obtained. We examine two possible modelling of in-medium effects as an increased in-medium effective mass, or an increased vector repulsion. We show that these physical pictures cannot be discriminated by the data. In both cases, the temperature dependence of the meson couplings leads to a faster weakening of the light cluster abundances with temperature than previous studies predicted. Possible systematic errors due to out-of-equilibrium effects affecting the experimental abundances, are considered by repeating the Bayesian inference with reduced information. The abundance prediction of the species excluded from the constraint is well compatible with the experimental data, suggesting that there is no a priori need of accounting for non-equilibrium effects or finite state interactions that potentially affect the deuteron yield.

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arXiv2026

Are NICER and GW170817 constraints suggesting a compactified scenario for Neutron stars?

Astrophysical observations from NICER and gravitational wave data constrain the properties of matter at the cores of neutron stars, enabling us to probe high-density matter with greater accuracy. To understand its implications for neutron stars, three distinct class-agnostic equation-of-state ensembles are constructed using the speed-of-sound parametrisation, which can describe matter in neutron-star cores. Bayesian analysis is employed to constrain the parameters, namely, the squared speed of sound and chemical potential, using the observational data. The Bayesian inference shows that the observations effectively constrain the low-density region of the equation of state. The astrophysical bound favours a softer, low-density equation of state in which the phase transition occurs at intermediate densities, thereby reducing the upper mass bounds for neutron stars. For the equation of state with density discontinuity, the discontinuities are preferably small. The equation of state with maximum mass configuration shows considerable stiffening from very low density, providing pressure support to generate maximum mass. In contrast, the equation of state with the maximum compact stellar configuration has a softer low-density equation of state, followed by pronounced stiffening, yielding the maximum compact configuration. The observationally favoured EoS shares the same qualitative structure as the maximum-compactness EoS: relative softness at intermediate densities transitioning to stiffness at high densities, a configuration gravity naturally favours.

astro-ph.HE↗

arXiv2026

Investigating Twin Star Equation of States in Light of Recent Astrophysical Observations

Twin stars are predicted to exist in nature if the hadron-to-quark phase transition is strong enough to form a new branch of hybrid stars, separated from the branch of neutron stars. We adopt an agnostic approach, using transition energy density, transition pressure, the discontinuity strength, and a constant speed of sound for quark matter as our parameter space to construct a large possibility of hybrid equations of state, and thereby encapsulating a comprehensive picture of the twin star scenario. First, we report the complete conditions on our parameter space imposed by the general relativistic hydrostatic equilibrium solutions. For a fixed transition energy density and speed of sound for quark matter, we define distinct ranges of transition pressures based on the allowed strengths of discontinuity. Below a maximum transition pressure, a range of discontinuity exists that increases as the transition pressure decreases. Thereby, we identify the loci of the limits on discontinuities as the `witch-hat' curves. Based on the causality limit, the witch-hat curves can be punctured or incomplete. Strong constraints on this picture are drawn from the inferences from GW170817 and the NICER measurements. We computed the maximum mass for twin stars to be $2.05~M_\odot$, the allowed strongest discontinuity in rest-mass density to be $7.76ρ_\mathrm{sat}$, and the upper bound on transition rest-mass density to be $4.03ρ_\mathrm{sat}$. Subsequently, we compute the implications of the stiffness of the quark matter equation of state on this picture. Different confidence levels for observational inferences are considered to assess the extent of inclusion (and rejection) of hybrid equations of state and, consequently, their effects on the limits of the maximum mass of twin stars and phase transition properties.

astro-ph.HE↗

arXiv2026

Learning the relations between neutron star and nuclear matter properties with symbolic regression

The equation of state (EOS) of dense matter in neutron stars (NSs) remains uncertain, particularly at supra-nuclear densities where complex nuclear interactions and the potential presence of exotic matter, like hyperons, come into play. The complex relationships existing between nuclear matter and neutron star properties are investigated. The focus is on their nonlinearities and interdependencies. In our analysis, we apply a machine learning algorithm known as symbolic regression, paired with principal component analysis, to datasets generated from Bayesian inference over relativistic mean-field models. A systematic Principal Component Analysis has allowed to break down the percentage contribution of each element or feature in the relationships obtained. This study examines two main models (datasets): the NL model, which includes nucleonic degrees of freedom; and the NL-hyp model, which includes hyperons in addition to nucleons. Our analysis confirms a robust correlation between the tidal deformability of a 1.4 $M_\odot$ neutron star and $β$-equilibrium pressure at twice the nuclear saturation density. This correlation remains once hyperons are included. The contribution of the different nuclear matter properties at saturation to the radius and tidal deformability was calculated. It was shown that the isovector properties have the largest impact, with a contribution of about 90\%. We also studied the relationship between the proton fraction at different densities and various symmetry energy parameters defined at saturation density. For the hyperon data set, we took into account the effects of the negatively charged hyperon $Ξ$ in order to recover the relationships. Our study reveals the individual impact of various symmetry energy parameters on proton fractions at different densities.

nucl-th↗

arXiv2025

Covariant Energy Density Functionals for Neutron Star Matter Equation of State Modeling: Cross-Comparison Analysis Using \texttt{CompactObject}

This study analyzes and contrasts different phenomenological methods used to model the nuclear equation of state (EOS) for neutron star matter based on covariant energy density functionals (CEDF). Using two complementary methodologies, we seek to capture a comprehensive picture of the potential behaviors of ultra-dense nucleonic matter and identify the most plausible models based on current observational and experimental constraints. Observational data from radio pulsar timing, gravitational wave detection of GW170817, and X-ray timing provide critical benchmarks for testing the models. We have derived the EOS posteriors for various CEDF models within the \texttt{CompactObject} package, utilizing recent observational data on neutron stars, state-of-the-art theoretical constraints from chiral effective field theory ($χ$EFT) calculations for pure neutron matter at low densities, and pQCD-derived constraints. Our analysis has demonstrated that while all considered CEDF models broadly reproduce current astrophysical and theoretical constraints, subtle yet important differences persist among them, with each framework exhibiting distinct characteristics at supra-nuclear density. This is in particular true for the proton fraction inside neutron stars, but also supported by the models' behavior with respect to the pure neutron matter EOS and the density dependence of the speed of sound. Our study highlights the sensitivity of dense matter predictions to the underlying EOS parameterizations and the priors considered.

nucl-th↗

arXiv2025

Constraints on maximum neutron star mass from proto-neutron star evolution

A proto-neutron star (PNS) gets formed after a successful supernova when the stellar remnant decouples from the ejecta. In this study, we explore a relativistic framework for the finite-temperature $β$-equilibrium limit of equation of state (EOS), constrained via a Bayesian inference methodology. The EOS is constrained by minimal approximations on a few nuclear saturation properties, low-density pure neutron matter constraints from chiral effective field theory, and a neutron star (NS) maximum mass greater than 2.0 $M_{\odot}$. Two sets of EOS derived from the relativistic mean field model for nucleonic and hyperonic matter constrained by a Bayesian inference calculation at the zero temperature limit are used. The thermal adiabatic index ($Γ_{\rm Th}$) is calculated as a function of the baryonic density across several temperatures for both the sets. Our results suggest that the maximum NS mass is of the order of 2.15 $M_\odot$ if hyperons are present. In addition, the present study suggests that an observation of NS with mass larger than $2.2\ M_{\odot}$ can indirectly indicates the absence of hyperons in its core. The deleptonization of hyperonic PNS reduces the stellar maximum mass rendering the PNS exceeding the zero temperature maximum stellar (baryonic) mass limit becomes metastable which is prone to collapse into a black hole while PNS below such a mass threshold evolves to a stable NS.

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arXiv2025

Inferring the Equation of State from Neutron Star Observables via Machine Learning

We have conducted an extensive study using a diverse set of equations of state (EoSs) to uncover strong relationships between neutron star (NS) observables and the underlying EoS parameters using symbolic regression method. These EoS models, derived from a mix of agnostic and physics-based approaches, considered neutron stars composed of nucleons, hyperons, and other exotic degrees of freedom in beta equilibrium. The maximum mass of a NS is found to be strongly correlated with the pressure and baryon density at an energy density of approximately 800 MeV.fm$^{-3}$. We have also demonstrated that the EoS can be expressed as a function of radius and tidal deformability within the NS mass range 1-2$M_\odot$. These insights offer a promising and efficient framework to decode the dense matter EoS directly from the accurate knowledge of NS observables.

nucl-th↗

arXiv2025

Probing the Internal Structure of Neutron Stars: A Comparative Analysis of Three Different Classes of Equations of State

Sound speed can be an important tool in unraveling the nature of matter that exists at the cores of neutron stars. In this study, we investigate three major classes of equations of state; monotonous, non-monotonous and discontinuous depending on the nature of the sound speed in neutron stars. The monotonous EoS refers to hadronic models, the non-monotonous refers to the quarkyonic or smooth crossover models and discontinuous refers to discontinuous first-order phase transition models. We generate a large ensemble of EoS for three classes with the model agnostic speed of sound interpolation approach. Our main aim is to check which class of EoS is most favoured by present astrophysical bounds. It is seen that although non-monotonous and discontinuous is favoured thermodynamically, the usual neutron star observations like mass-radius, and f-mode oscillation fail to provide a satisfactory result. The universal relations are also seen to be futile as they show considerable spread and significant overlaps among the different classes. The Bayesian analysis shows slight bias towards the non-monotonous model but fails to provide a decisive answer.

astro-ph.HE↗

arXiv2025

The footprint of nuclear saturation properties on the neutron star $f$ mode oscillation frequencies: a machine learning approach

We investigate the intricate relationships between the non-radial $f$ mode oscillation frequencies of neutron stars (NS)s and the corresponding nuclear matter equation of state (EOS) using a machine learning (ML) approach within the ambit of the relativistic mean field (RMF) framework for nuclear matter. With two distinct parameterizations of the Walecka model, namely, (1) with non-linear self interactions of the scalar field (NL) and, (2) a density dependent Bayesian model (DDB), we perform a thorough examination of the $f$ mode frequency in relation to various nuclear saturation properties. The correlations between the $f$ mode frequencies and nuclear saturation properties reveal, through various analytical and ML methods, the complex nature of NSs and their potential as the cosmic laboratory for studying extreme states of matter. A principal component analysis (PCA) has been performed using mixed datasets from DDB and NL models to discriminate the relative importance of the different components of the EOS on the $f$ mode frequencies. Additionally, a {\it Random forest feature importance} analysis also elucidates the distinct roles of these properties in determining the $f$ mode frequency across a spectrum of NS masses. Our findings are further supported by symbolic regression searches, yielding high-accuracy relations with strong Pearson coefficients and minimal errors. These relations suggest new methodologies for probing NS core characteristics, such as energy density, pressure, and speed of sound from observations of non-radial $f$ mode oscillations of NSs.

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arXiv2025

Calibrating the medium effects of light clusters in heavy-ion collisions

We propose a Bayesian inference estimation of in-medium modification of the cluster self-energies from light nuclei multiplicities measured in selected samples of central $^{136,124}$Xe$+^{124,112}$Sn collisions with the INDRA apparatus. The data are interpreted with a relativistic quasi-particle cluster approach in the mean-field approximation without any prior assumption on the thermal parameters of the model. An excellent reproduction is obtained for H and He isotope multiplicities, and compatible posterior distributions are found for the unknown thermal parameters. We conclude that the cluster-$σ$-meson coupling is temperature dependent, becoming weaker when the temperature increases, in agreement with microscopic quantum statistical calculations. This implies a faster decrease of the light cluster abundances with temperature than previously estimated.

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