arXiv subjects

Shamim Haque

Publications and source records attributed to Shamim Haque.

5 recordsLinked to original sources

arXiv2026

Dynamical response of twin stars to perturbations

If a strong first-order phase transition takes place at sufficiently high rest-mass densities in the equation of state (EOS) modelling compact stars, a new branch will appear in the mass-radius sequence of stable equilibria. This branch will be populated by stars comprising a quark-matter core and a hadronic-matter envelope, i.e., hybrid stars, which represent ``twin-star'' solutions to equilibria having the same mass but a fully hadronic EOS. While both branches are stable to linear perturbations, it is unclear which of the twin solutions is the ``favoured'' one, that is, which of the two configurations is expected to be found in nature. We assess this point by performing a large campaign of general-relativistic simulations aimed at assessing the response of compact stars on the two branches to perturbations of various strength. In this way, we find that, independently of whether the stars populate the hadronic or the twin branch, their response is characterised by a critical-perturbation strength such that the star will oscillate on the original branch for subcritical perturbations and migrate to the neighbouring branch for supercritical perturbations while conserving rest-mass. Because the critical values are different for stars with the same rest-mass but sitting on either branch, it is possible to define as favoured the part of the branch that has the largest critical perturbation, thus correcting the common wisdom that stellar models on the twin branch are the favoured ones. Interestingly, we show that the binding energies on the two branches can be used to deduce without simulations which of the stellar configurations is more likely to be found in nature.

gr-qc↗

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 $7.76\rho_\mathrm{sat}$, and the upper bound on transition rest-mass density to be $4.03\rho_\mathrm{sat}$. 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↗

arXiv2025

Distinct Signatures of the Nature of Phase Transition in Binary Neutron Star Mergers

Binary neutron-star mergers offer crucial insights into the matter properties of neutron stars. We present the possible imprints in the gravitational wave signal from the nature of phase transition from such events. Our study employs a one-parameter family of equation of states built using a polytropic approach with a control parameter $\Delta p$ surveying the features of hadron-quark phase transition, from Maxwell construction to the Gibbs construction. It allows us to explore the extent of mixed phases and analyse their direct impact on merger dynamics. Post-merger gravitational wave emissions reveal the expression of specific signatures in the spectrogram and power spectral density, serving as a distinct signature of equations of state with mixed phases. We found additional peaks in power spectral density that are exclusively generated from the post-merger remnant experiencing a phase transition. Additionally, the nature of phase transition leaves specific imprints on the spectrogram, leading to a two-folded signature from gravitational wave analysis. Furthermore, we establish the first correlation between $\Delta p$ and the threshold mass for prompt collapse. Our analysis shows that $\Delta p \lesssim 0.04$ is required if GW170817 formed a long-lived remnant or has experienced a delayed collapse into a black hole.

astro-ph.HE↗

arXiv2023

Quarkyonic Model for Neutron Star Matter: A Relativistic Mean-Field Approach

The concept of quarkyonic matter presents a promising alternative to the conventional models used to describe high-density matter and provides a more nuanced and detailed understanding of the properties of matter under extreme conditions that exist in astrophysical bodies. The aim of this study is to showcase the effectiveness of utilizing the quarkyonic model, in combination with the relativistic mean-field formalism, to parameterize the equation of state at high densities. Through this approach, we intend to investigate and gain insights into various fundamental properties of a static neutron star, such as its compositional ingredients, speed of sound, mass-radius profile, and tidal deformability. The obtained results revealed that the quarkyonic matter equation of state (EOS) is capable of producing a heavy neutron star with the mass range of $\sim$ $2.8 M_\odot$. The results of our inquiry have demonstrated that the EOS for quarkyonic matter not only yields a neutron star with a significantly high mass but also showcases a remarkable degree of coherence with the conformal limit of the speed of sound originating from deconfined QCD matter. Furthermore, we have observed that the tidal deformability of the neutron star, corresponding to the EOSs of quarkyonic matter, is in excellent agreement with the observational constraints derived from the GW170817 and GW190425 events. This finding implies that the quarkyonic model is capable of forecasting the behavior of neutron stars associated with binary merger systems. This aspect has been meticulously scrutinized in terms of merger time, gravitational wave signatures, and collapse times using numerical relativity simulations.

nucl-th↗

arXiv2022

Effects of Onset of Phase Transition on Binary Neutron Star Mergers

Quantum Chromodynamics predicts phase transition from hadronic matter to quark matter at high density, which is highly probable in astrophysical systems like binary neutron star mergers. To explore the critical density where such phase transition can occur, we performed numerical relativity simulations of binary neutron star mergers with various masses (equal and unequal binaries). We aim to understand the effect of the onset of phase transition on the merger dynamics and gravitational wave spectra. We generated a set of equations of states by agnostically changing the onset of phase transition, having the hadronic matter part and quark matter part fixed. This particular arrangement of the equation of states explores the scenario of mergers where mixed phases of matter are achieved before or during the merger. Under these circumstances, if the matter properties with hadronic and quark degrees differ significantly, it is reflected in the stability of the final merger product for the intermediate mass binary. We performed a case study on mixed species merger, where one of the binary companions is hybrid star. If quark matter appears at low densities, we observe significant change in post-merger gravitational wave analysis in terms of higher peak frequencies and post-merger frequencies in power spectral density. We report indications expressed as spikes in phase difference plots at merger time for mixed mergers. We found that the expression of phase transition in post-merger gravitational wave signals is more significant for unequal mass binary than for equal mass binary having the same total baryonic mass.

astro-ph.HE↗