arXiv subjects

Ishfaq Ahmad Rather

Publications and source records attributed to Ishfaq Ahmad Rather.

15 recordsLinked to original sources

arXiv2026

Ultra-compact twin stars with hybrid equations of state from bosonic dark matter

The properties of compact stars with a strong first-order phase transition to quark matter and with an additional fluid of self-interacting bosonic dark matter (DM) are studied. We find that the inclusion of DM changes considerably the stability of mass-radius configurations relative to the naive one-fluid criterion. For compact star configurations with similar masses and different radii, so-called twin stars, the presence of DM removes the unstable segment between the hadronic and the hybrid branch, so that the stable mass-radius sequence becomes continuous after the onset of the phase transition to quark matter. We furthermore find stable ultra-compact objects (UCOs), defined by a total compactness $C = M_\text{tot}/R_\text{grav} \ge 1/3$. We observe two distinct classes of UCOs: a DM-halo class with $f_\text{DM} \gtrsim 0.9$, and a DM-core class at $f_\text{DM} \lesssim 0.02$. The two classes can be separated by the surface redshift of the normal matter, which reaches $z=0.73$--$0.77$ for the DM-core class and stays below $0.45$ for the DM-halo class. Finally, we find hybrid star solutions of 'ultimate twins' with similar mass and visible radius, but different dark matter content, leading to different tidal deformabilities and surface redshifts. Future X-ray and gravitational measurements of ultra-compact neutron stars with radii and masses outside the allowed neutron star range can thereby probe the presence and the properties of DM in addition to a first-order phase transition to quark matter.

astro-ph.HE↗

arXiv2026

Reaction-constrained composition $g$-modes in neutron stars with antikaon condensates, hyperons, and $Δ(1232)$ resonances

We study core composition $g_1$ modes of cold, nonrotating neutron stars containing antikaon condensates, hyperons, and $Δ(1232)$ baryons and present, to our knowledge, the first calculation in full general relativity of the continuous-composition $g_1$-mode frequency and gravitational-wave damping time for stars with a $K^-$ condensate. Using the BigApple relativistic mean-field equation of state, we compute frequencies, damping times, and frozen-composition tidal overlaps, and identify the buoyancy channels with a species-resolved Ledoux decomposition validated by mode-frequency sensitivities. We compare fully frozen matter with a fast-$K$ limit for $n\leftrightarrow p+K^-$ and a strong-equilibrium limit for the $Δ$ quartet. Fast-$K$ equilibration retains $36\%$--$44\%$ of the peak local kaon buoyancy and $65.7\%$--$73.4\%$ of the frozen terminal-configuration frequencies, while increasing the damping times by factors of $14.4$--$31.8$; the mode remains above the nucleonic band. Strong $Δ$ equilibration removes most of the direct $Δ$-induced enhancement, returning the $NΔ$ mode toward the nucleonic band, whereas the high-frequency $NYΔ$ branch survives through the frozen $Λ$ gradient. Eigenfunction tracking confirms a continuous $g_1$ branch, and representative DD-ME2 calculations reproduce this hierarchy. The direct full-GR frozen-composition phase shifts satisfy $|ΔΦ_{g_1}|\leq1.410\times10^{-3}$ rad, a factor of 21 below the $0.03$-rad favorable-event scale for the Einstein Telescope. An exotic species therefore produces a distinct composition mode only if its composition gradient, or a coupled slowly equilibrating gradient, survives over the oscillation period.

astro-ph.HE↗

arXiv2026

The petit four of color-superconducting phases in proto-neutron star evolution

At high densities and moderate temperatures, hadronic matter is expected to undergo a first-order phase transition into a color-superconducting (CSC) state. A proto-neutron star describes the earliest evolutionary stages during the first seconds to minutes after core-collapse supernovae and therefore has the potential to assess the appearance of CSC phases at such high densities and moderate temperatures. To address this, we incorporate proto-neutron star conditions, considering neutrino-trapped and neutrino-transparent ones, into the equation of state including color-superconducting phases in a recently developed RG-consistent NJL model. Since the total baryon number of a proto-neutron star is conserved during its later evolution, tracking stellar configurations from an initial mass of the hot proto-neutron star to the final cold neutron star along isolines of baryon number allows us to investigate whether color-superconducting phases can form at any point along this trajectory. By mapping this multidimensional transition in the hot furnace of a core-collapse supernovae cooling from a neutrino-trapped birth state to a cold, neutrino-transparent final state, we reveal four distinct core evolution scenarios-our "petit four" of proto-neutron star evolution: a delayed collapse from the CSC phase to a black hole, a persistent CSC phase, a vanishing CSC phase, and a fleeting CSC phase. For our specific parameterization of the hadronic and the CSC equation of state, we find that a stable color-superconducting phase can only be sustained in the final cold neutron star for a narrow, high-mass region.

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arXiv2026

Constraining Axion-Like Particle mediated Dark Matter with Observational Constraints: A Statistical and Machine Learning Approach

We present a comprehensive study of axion-like particle (ALP) mediated dark matter (DM) effects on neutron star (NS) structure within a relativistic mean-field framework with non-linear mesonic interactions constrained by nuclear and astrophysical data. We explore DM masses $m_χ\in [0,1000]\,\mathrm{GeV}$ and Fermi momenta $q_f \in [0,0.06]\,\mathrm{GeV}$, generating over 30{,}000 equations of state using two representative hadronic models, a stiff EoS (EoS1) and a soft EoS (EoS18), including a consistent crust description. A multi-level statistical filtering scheme based on voting, likelihood, and kernel density estimation is applied using constraints from radio and X-ray pulsars, GW170817, and the low-mass compact object HESS~J1731$-$347. We find that models satisfying the PSR~J0614$-$3329 radius constraint automatically comply with the HESS bound, allowing ALP-mediated DM to explain low-mass compact objects while remaining consistent with $2\,M_\odot$ NSs. For the stiff EoS, we obtain a lower bound $m_χ\gtrsim 43\,\mathrm{GeV}$, with preferred values $q_f = 0.034^{+0.020}_{-0.012}$ and $m_χ\in [101,949]\,\mathrm{GeV}$, while the soft EoS yields no strict lower bound, though large $m_χ$ and $q_f$ are disfavored. We also develop a supervised interpolation model using \texttt{AutoGluon} to infer DM parameters from NS mass--radius curves, achieving $R^2>0.998$, and show that $m_χ$ is mainly constrained by global radius ratios, whereas $q_f$ is driven by the tidal deformability $Λ_{1.4}$.

astro-ph.HE↗

arXiv2025

Radial and Non-Radial Oscillations of Protoneutron Stars with Hyperonic Composition

This paper explores radial and non-radial oscillations of protoneutron stars (PNSs) as they evolve from hot, neutrino-rich configurations through deleptonization to cold, catalyzed states. The equation of state (EoS) is modeled using a density-dependent relativistic mean-field framework, with stellar evolution characterized by changes in entropy and lepton fraction. Both nucleonic and hyperonic compositions are considered. Non-radial $f$- and $p_1$-mode oscillations are computed using both the Cowling approximation and the full General Relativistic framework. Trapped neutrinos initially increase the error in the Cowling approximation for $f$-modes, which decreases during deleptonization and rises again in the cold phase. In contrast, $p_1$-mode errors peak during intermediate stages due to evolving pressure and density gradients. The emergence of hyperons modestly raises oscillation frequencies in both modes. Existing universal relations for $f$-mode frequency and damping time lack model independence for PNSs, motivating a more robust relation. In particular, our proposed universal relation involving the moment of inertia and $\tildeη$ shows strong agreement across all evolutionary phases, offering a temperature-sensitive, model-independent scaling for asteroseismology. Radial oscillations of a $1.4\,M_\odot$ PNS are also studied for different EoSs. Our results show that displacement ($ξ$) and pressure perturbation ($η$) profiles are highly sensitive to thermal state, composition, and compactness. Hyperonic stars show higher frequencies, altered node structures, and stronger pressure perturbations due to EoS softening. Differences in frequency separation $Δν_n$ and fundamental frequency $ν_0$ between nucleonic and hyperonic models provide clear observational diagnostics for probing the interiors of PNSs and constraining the EoS of dense matter.

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arXiv2025

Impact of hyperons on structural properties of neutron stars and hybrid stars within the regularized four-dimensional Einstein-Gauss-Bonnet gravity

We investigate the impact of hyperons and phase transition to quark matter on the structural properties of neutron stars within the regularized four-dimensional Einstein-Gauss-Bonnet gravity (4DEGB). We employ the density-dependent relativistic mean-field model (DDME2) for the hadronic phase and the density-dependent quark mass (DDQM) model for the quark phase to construct hadronic and hybrid equations-of-state (EoSs) that are consistent with the astrophysical constraints. The presence of hyperons softens the EoS and with a phase transition, the EoS further softens, and the speed of sound squared drops to around 0.2 for the maximum mass configuration, which lies in the pure quark phase. Adjusting the Gauss-Bonnet coupling constant, $α$, within its allowed range results in a decrease in the mass-radius relationship for negative $α$, and an increase for positive $α$. In addition, functions are fitted to the maximum mass and its associated radius as a function of the constant $α$ to observe its impact on these properties. We find that positive values of $α$ support massive stars consistent with the 2\,$M_{\odot}$ constraint and NICER measurements, while negative values, although compatible with low-mass radius observations, fail to reach the observed maximum mass, particularly for EoSs involving phase transitions. Therefore, astrophysical observations may be used to effectively constrain the allowed range of $α$.

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arXiv2025

Effect of Dark matter and $σ$-cut potential on radial and non-radial oscillation modes in neutron stars

We study the mesonic nonlinear (NL) interaction equation of state (EoS) employing the relativistic mean-field model and investigate the effect of $σ$-cut potential (NL-$σ$ cut) and dark matter (NL DM) on the non-radial and radial oscillation modes of neutron stars. For NL-$σ$ cut, we include the $σ$-cut potential $U_{cut} (σ)$ to study its effect. For the dark matter, we use the neutron decay anomaly model. For each model, we investigate two extreme EoSs, stiff and soft, that cover the entire allowed parameter range from the given model, consistent with the current astrophysical constraints. The EoS and the stellar properties, such as mass and radius, are calculated, and the effect of $σ$-cut and DM is discussed. Both non-radial and radial oscillation modes are computed in the general relativistic framework. We study the non-radial $f$ and $p_1$ mode frequency, damping time, and some qusi-universal relations connecting the frequencies of the $f$-mode to the average density and compactness. The analysis showed that the $f$ and $p_1$ mode frequencies at both 1.4~$M_{\odot}$ and the maximum mass configuration are higher in the NL DM model compared to the NL and NL-$σ$ models. The consistent alignment between our prior parameterizations and current calculations strongly supports the existence of quasi-universal relations that hold true irrespective of the particular matter components involved. For the radial oscillations, we compute 10 lowest-order modes ($f$, $p$), study the radial perturbations as well as the large frequency separation with NL-$σ$ cut and NL DM EoS, showing that the microphysics involved in the NS EoS is imprinted on the frequency separation between different nodes.

astro-ph.HE↗

arXiv2025

Non-Radial Oscillation Modes in Hybrid Stars with Hyperons and Delta Baryons

We study the effects of hyperons, delta baryons, and quark matter phase transitions on $f$-mode oscillations in neutron stars. Using the density-dependent relativistic mean-field model (DDME2) for the hadronic phase and the density-dependent quark mass (DDQM) model for the quark phase, we construct hadronic and hybrid equations of state (EoSs) consistent with astrophysical constraints. Including hyperons and delta baryons soften the EoS, reducing maximum mass, while phase transition to the quark matter further softens the EoS, decreasing the speed of sound and hence the maximum mass. We confirm the well-known overestimation of $f$-mode frequencies by the Cowling approximation (by about 10-30\%) compared to full General Relativity calculation, and show that this discrepancy persists across models including hyperons, $Δ$ baryons, and a phase transition to quark matter. While the discrepancy generally decreases with stellar mass, it increases near the maximum mass in the presence of a phase transition compared to EoSs without this phenomenology. We derive universal relations connecting the frequencies of the $f$-mode to the average density, compactness, and tidal deformability, finding significant deviations due to hyperons and delta baryons. These deviations could provide distinct observational signatures in gravitational wave data, offering new insights into dense matter physics and advancing gravitational wave asteroseismology of neutron star interiors. Empirical relations for mass-scaled and radius-scaled frequencies are also provided, highlighting the importance of GR calculations for accurate modeling.

astro-ph.HE↗

arXiv2025

Comprehensive Analysis of Constructing Hybrid Stars with an RG-consistent NJL Model

In this work, we investigate the properties of hadronic and quark matter that would allow for a first order phase transition between them within neutron stars. To this end, we use a parameterizable Relativistic Mean-Field (RMF) description for the hadronic phase and a Renormalization Group-consistent Nambu-Jona-Lasino (RG-NJL) model for the quark phase. This also enables us to consider sequential phase transitions involving a two-flavor color-superconducting (2SC) and a color-flavor-locked (CFL) phase. We find large ranges for all parameters that facilitate a phase transition, even when constrained by current astrophysical data. We further attempt to filter out stars with a high chance of detectability by mass-radius measurement, i.e., stars with identical mass but different radii, so-called twin stars. However, we find that such configurations are outside the constrained parameter spaces. Instead, most of the mass-radius relations that feature a phase transition appear to be indistinguishable from a purely hadronic description.

astro-ph.HE↗

arXiv2024

Radial Oscillations in Hybrid Stars with Slow Quark Phase Transition

This study investigates the radial oscillations of hybrid neutron stars, characterized by a composition of hadronic external layers and a quark matter core. Utilizing a density-dependent relativistic mean-field model that incorporates hyperons and baryons for describing hadronic matter, and a density-dependent quark model for quark matter, we analyze the ten lowest eigenfrequencies and their corresponding oscillation functions. Our focus lies on neutron stars with equations-of-state involving N, N + $Δ$, N + H, and N + H + $Δ$, featuring a phase transition to quark matter. Emphasizing the effects of a slow phase transition at the hadron-quark interface, we observe that the maximum mass is attained before the fundamental mode's frequency decreases for slow phase transitions. This observation implies the stability of stellar configurations with higher central densities than the maximum mass, called Slow Stable Hybrid Stars (SSHSs), even under small radial perturbations. The length of these SSHS branch depends upon the energy density jump between two phases and the stiffness of the quark EoS.

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arXiv2024

Astrophysical constraints on color-superconducting phases in compact stars within the RG-consistent NJL model

We determine parameters of the renormalization group-consistent three-flavor color-superconducting Nambu-Jona-Lasinio (NJL) model that are suited to investigate possible compact-star configurations. Our goal is to provide quark-matter equations of state (EoS) that can be used for hadron-quark hybrid-star constructions. To that end, we mainly focus on the parameters of the quark-matter model. By varying the vector and diquark coupling constants, we analyze their impact on the EoS, the speed of sound, the maximum diquark gap, and the mass-radius relation. In almost all configurations, a stable color-flavor-locked (CFL) phase appears in the core of the maximum-mass configurations, typically spanning several kilometers in radius. In other cases, the star's two-flavor color-superconducting (2SC) branch of the EoS becomes unstable before reaching the CFL transition density. At neutron-star densities, the speed of sound squared reaches up to $c_s^2 \sim 0.6$ and the CFL gap up to $Δ\sim250\,$MeV. We argue that adding a hadronic EoS at lower densities by performing a Maxwell construction does not increase the maximum mass substantially. Thus we use the $2.0 M_{\odot}$ constraint to constrain the NJL model parameters that are suited for the construction of hybrid-star EoS. We construct three examples of the hybrid-star model, demonstrating that there is room for different color-superconducting compositions. The hybrid EoSs obtained in this way can have no 2SC matter or different ratios of 2SC and CFL quark matter in the core. We show that early hadron-quark transitions are possible that can modify the tidal deformability at 1.4 $M_\odot$. We find that these EoSs are consistent with the imposed constraints from astrophysics and perturbative QCD. They allow for different hybrid-star scenarios with a hadronic EoS that is soft at low to intermediate densities ($\sim 1-3\, n_{\text{sat}}$).

hep-ph↗

arXiv2024

Radial Oscillations of Hybrid Stars and Neutron Stars including Delta baryons: The Effect of a Slow Quark Phase Transition

We study radial oscillations of hybrid neutron stars composed of hadronic external layers followed by a quark matter core. We employ a density-dependent relativistic mean-field model including hyperons and $Δ$ baryons to describe hadronic matter, and a density-dependent quark model for quark matter. We obtain the ten lowest eigenfrequencies and the corresponding oscillation functions of N, N+$Δ$, N+H, and N+H+$Δ$ equations-of-state with a phase transition to the quark matter at 1.4 and 1.8 ${M_{\odot}}$, focusing on the effects of a slow phase transition at the hadron-quark interface. We observe that the maximum mass is reached before the fundamental mode's frequency vanishes for slow phase transitions, suggesting that some stellar configurations with higher central densities than the maximum mass remain stable even when they undergo small radial perturbations. Future gravitational wave detectors and multi-messenger astronomy, complemented by robust microscopic models enabling exploration of various neutron star compositions, including hyperon content, are anticipated to impose precise limitations on the equation of state of baryonic matter under high-density conditions.

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arXiv2023

Quark Models and Radial Oscillations: Decoding the HESS J1731-347 Compact Object's Equation of State

We investigate the peculiar nature of strange stars through an analysis of different quark models, i.e. vBag model and CFL model equation of states at different parameter sets, and focus on understanding the equation of state governing the intriguing central compact object (CCO) within the supernova remnant HESS J1731-347, with a mass and radius of $M = 0.77^{+0.20}_{-0.17} M_{\odot}$ and $R = 10.4^{+0.86}_{-0.78}$ km, respectively. Additionally, we compare the radial oscillations of two models to determine the frequency of the HESS J1731-347 compact object at its maximum mass. The frequencies of radial oscillations are computed for each of the four EoSs considered. In total, the 10 lowest radial frequencies for each of those EoSs have been computed. By delving into these aspects, we aim at deepening our understanding of strange stars and their connection to the observed HESS J1731-347 mass-radius relationship.

astro-ph.HE↗

arXiv2022

Relativistic Mean Field Study of Neutron Stars and Hyperon Stars

This thesis focuses on a variety of active research topics, such as nuclear matter, neutron stars, and phase transition within the framework of the RMF model. We use the previously successful effective field theory-driven Relativistic Mean Field (RMF) and density-dependent RMF (DD-RMF)formalisms for analyzing hadron matter to examine the infinite nuclear matter and neutron stars. The presence of exotic phases such as quarks has been investigated using the MIT Bag model and its variants, such as the vBag model, at various bag constants. The other exotic phases, such as hyperons, have also been studied under the influence of a strong magnetic field.

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arXiv2021

Effect of Inner Crust EoS on Neutron star properties

The neutron star maximum mass and the radius are investigated within the framework of the relativistic mean-field (RMF) model. The variation in the radius at the canonical mass, $R_{1.4}$, using different inner crust equation of state (EoS) with different symmetry energy slope parameter is studied. It is found that although the NS maximum mass and the corresponding radius do not vary much with different inner crust EoSs, the radius and the tidal deformability at 1.4$M_{\odot}$ vary with the different choice of crust EoS and variation of about 1-2 km is seen in the radius at the canonical mass. For non-unified EoSs, the crust with a low symmetry energy slope parameter produces a low NS radius at the canonical mass. The properties of maximally rotating neutron stars are also studied. The variation in the radius of rotating star at the canonical mass 1.4$M_{\odot}$ is also seen with the slope parameter. Similar to the static neutron star, the radius at 1.4$M_{\odot}$ of rotating neutron star is affected by slope parameter of the inner crust. Other important quantities like moment of inertia, frequency, rotational kinetic energy to gravitational energy ratio are also calculated. The variation in these quantities with the crust slope parameter is found to be more proportional to the mass and the radius of NS.

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