arXiv subjects

Rana Nandi

Publications and source records attributed to Rana Nandi.

At least 19 recordsLinked to original sources

arXiv2025

Implication of multimessenger observations on the relativistic mean-field equation of state of dense nuclear matter and skin thickness of nuclei

The composition and properties of infinite nuclear matter under extreme conditions of temperature and pressure remain incompletely understood. In this work, we constrain the equation of state (EoS) of nuclear matter - constructed within the framework of the Relativistic Mean Field (RMF) model - by combining results from chiral effective field theory and multimessenger observations of neutron stars. Using the saturation properties of nuclear matter, we generate a wide ensemble of EoS, which are subsequently constrained within a Bayesian framework. The resulting posterior distributions provide tight bounds on both the saturation parameters and the coupling constants of the RMF model. Our results indicate that the GW170817 event and the latest NICER observation favor a relatively soft EoS, leading to lower crust-core transition densities and thinner neutron star crusts. The radius of a $1.4\,M_\odot$ neutron star is tightly constrained to $12.508_{-0.241}^{+0.257}$ km, while the maximum mass reaches $2.174_{-0.123}^{+0.174}\,M_\odot$. Furthermore, our analysis reveals that the $\omega$-$\rho$ coupling, which governs the density dependence of the symmetry energy, becomes increasingly significant under successive astrophysical constraints. Finally, the predicted neutron skin thickness of $^{48}$Ca agrees well with the CREX measurement, whereas that of $^{208}$Pb remains in tension with PREX-II. In contrast to earlier studies, we do not observe a clear correlation between the neutron skin thickness of $^{208}$Pb and the symmetry energy slope parameter $L$.

nucl-th↗

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↗

arXiv2024

Structural response of neutron stars to rapid rotation and its impact on the braking index

Pulsars are rotating neutron stars that are observed to be slowing down, implying a loss of their rotational energy. There can be several different physical mechanisms involved in their spin-down process. The properties of fast-rotating pulsars depend on the nature of the neutron star matter, which can also affect the spin-down mechanisms. In this work, we examine three different physical phenomena contributing to the spin-down: magnetic dipole radiation, gravitational mass quadrupole radiation due to the ``mountain" formation, gravitational mass current quadrupole radiation or the r-modes, and calculate the expressions for the braking indices due to all of them. We have also considered the implications of the uncertainties of the equation of the state of neutron star matter and rapid rotation on the braking indices corresponding to the aforementioned processes and their combinations. In all cases, the rapid rotation results in a departure of the braking index from the standard values when the rotational effects are ignored. If generated with a saturation amplitude within the range of $10^{-4} - 10^{-1}$, the r-mode oscillations dominate the spin-down of millisecond pulsars. Moreover, we explore the braking index in the context of millisecond magnetars. We also study the effects of different choices of baryon mass on the braking indices.

astro-ph.HE↗

arXiv2024

Inferring neutron star properties through gravitational waves from r-modes and their relativistic counterparts

We present two frameworks to infer some of the properties of neutron stars from their electromagnetic radiation and the emission of continuous gravitational waves due to r-modes and their relativistic counterparts, termed axial-led hybrid modes. In the first framework, assuming a distance measurement via electromagnetic observations, we infer three neutron star properties: the moment of inertia, a parameter related to the mode's saturation amplitude, and the component of magnetic dipole moment perpendicular to the rotation axis. Unlike signals from mountains, axial-led hybrid oscillations provide additional information through a parameter ($\kappa$) that satisfies a universal relation with the star's compactness. In the second framework, we utilize this and the relation between the moment of inertia and compactness, in addition to assuming an equation of state and utilizing pulsar frequency measurements, to directly measure the neutron star's distance, along with the parameters above. We employ a Fisher information matrix-based approach for quantitative error estimation in both frameworks. We find that the error in the distance measurement dominates the errors in the first framework for any reasonable observation time. In contrast, the second framework enables accurate parameter inference because it does not depend on electromagnetic distance measurements. Although its applicability is limited to a restricted parameter space and relies on assumptions about the equation of state, the simulated errors in this framework are found to be independent of the equation of state. Finally, we discuss the potential utility and critical limitations of our analyses, and propose possible solutions and directions for future research.

astro-ph.HE↗

arXiv2024

MURCA driven Bulk viscosity in neutrino trapped baryonic matter

We examine bulk viscosity, taking into account trapped neutrinos in baryonic matter, in the context of binary neutron star mergers. Following the merging event, the binary star can yield a remnant compact object with densities up to $5$ nuclear saturation density and temperature upto $50$ MeV resulting in the retention of neutrinos. We employ two relativistic mean field models, NL3 and DDME2, to describe the neutrino-trapped baryonic matter. The dissipation coefficient is determined by evaluating the Modified URCA interaction rate in the dense baryonic medium, and accounting for perturbations caused by density oscillations. We observe the resonant behavior of bulk viscosity as it varies with the temperature of the medium. The bulk viscosity peak remains within the temperature range of $\sim 13-50$ MeV, depending upon the underlying equation of states and lepton fractions. This temperature range corresponds to the relevant domain of binary neutron star mergers. We also note that in presence of neutrinos in the medium the bulk viscosity peak shifts towards higher temperature and the peak value of bulk viscosity also changes. The time scale of viscous dissipation is dictated by the beta-off-equilibrium susceptibilities derived from the nuclear equation of state. The resulting viscous decay time scale ranges from $32-100$ milliseconds, which aligns with the order of magnitude of the post-merger object's survival time in some specific scenarios.

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arXiv2022

Low Density Neutron Star Matter with Quantum Molecular Dynamics: The Role of Vector Interactions

The effect of isospin-dependent nuclear forces on the inner crust of neutron stars is modeled within the framework of Quantum Molecular Dynamics (QMD). To successfully control the density dependence of the symmetry energy of neutron-star matter below nuclear saturation density, a coupling potential between the $ω$ and $ρ$ meson fields is introduced. This approach is inspired by the baryon density and isospin density-dependent repulsive Skyrme force of asymmetric nuclear matter. In isospin-asymmetric nuclear matter, the system shows nucleation, as nucleons are arranged into shapes resembling nuclear pasta. The dependence of clusterization in the system on the isospin properties is also explored by calculating two-point correlation functions. We show that, as compared to previous results that did not involve the $ω$-$ρ$ potential, the energy symmetry slope $L$ is successfully controlled by varying the $ω$-$ρ$ coupling strength. Nevertheless, the effect of changing the slope of the nuclear symmetry energy $L$ on the crust-core transition density does not seem significant. To the knowledge of the authors, This is the first implementation of a $ω$-$ρ$ coupling in a QMD model for the inner crust of neutron stars.

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arXiv2021

Semi Universal relation to understand matter properties at neutron star interiors

The occurrence of quark matter at the center of neutron stars is still in debate. This study defines some semi-empirical parameters that quantify the occurrence and the amount of quark matter at star interiors. These parameters show semi-universal relations across all the EoS. One parameter depends on the shifting of the keplerian mass-radius curve from the static one and shows it is a constant across all EoS. The Z-parameter shows how tidal deformability depends on the quark content of the star and the stiffness of the EoS. The quark content of the star also affects the compactness of the star, and its dependence is almost universal. The empirical parameter gives a bound on the quark content of the star and shows that if the amount of the quark content increases, the stars are likely to collapse into a black hole. It is seen that the change in the mass and radius after PT is linearly proportional to the mass of the parent NS. Given a hadronic EoS, bag constant, and quark coupling constant, one can have a critical mass of the neutron star and the maximum mass of the hybrid star for phase transition without any baryonic mass loss.

astro-ph.HE↗

arXiv2021

GW190814: On the properties of the secondary component of the binary

We show that the odds of the mass-gap (secondary) object in GW190814 being a neutron star (NS) improve if one allows for a stiff high-density equation of state (EoS) or a large spin. Since its mass is $\in (2.50,2.67) M_{\odot}$, establishing its true nature will make it either the heaviest NS or the lightest black hole (BH), and can have far-reaching implications on NS EoS and compact object formation channels. When limiting oneself to the NS hypothesis, we deduce the secondary's properties by using a Bayesian framework with a hybrid EoS formulation that employs a parabolic expansion-based nuclear empirical parameterization around the nuclear saturation density augmented by a generic 3-segment piecewise polytrope (PP) model at higher densities and combining a variety of astrophysical observations. For the slow-rotation scenario, GW190814 implies a very stiff EoS and a stringent constraint on the EoS specially in the high-density region. On the other hand assuming the secondary object is a rapidly rotating NS, we constrain its rotational frequency to be $f=1170^{+389}_{-495}$ Hz, within a $90\%$ confidence interval. In this scenario, the secondary object in GW190814 would qualify as the fastest rotating NS ever observed. However, for this scenario to be viable, rotational instabilities would have to be suppressed both during formation and the subsequent evolution until merger, otherwise the secondary of GW190814 is more likely to be a BH.

astro-ph.HE↗

arXiv2021

Towards mitigation of apparent tension between nuclear physics and astrophysical observations by improved modeling of neutron star matter

Observations of neutron stars (NSs) by the LIGO-Virgo and NICER collaborations have provided reasonably precise measurements of their various macroscopic properties. In this paper, we employ a Bayesian framework to combine them and place improved joint constraints on the properties of NS equation of state (EoS). We use a hybrid EoS formulation that employs a parabolic expansion-based nuclear empirical parameterization around the nuclear saturation density augmented by a generic 3-segment piecewise polytrope model at higher densities. Within the $90 \%$ credible level this parameterization predicts $R_{1.4} = 12.57_{-0.92}^{+0.73}$ km and $Λ_{1.4} = 550_{-225}^{+223}$ for the radius and dimensionless tidal deformability, respectively, of a $1.4 M_{\odot}$ NS. Finally, we show how the construction of the full NS EoS based solely on the nuclear empirical parameters at saturation density leads to certain tension with the astrophysical data, and how the hybrid approach provides a resolution to it.

astro-ph.HE↗

arXiv2020

Maximum mass of hybrid star formed via shock induced phase transition in cold neutron stars

This article studies the maximum mass limit of the quark star formed after the shock-induced phase transition of a cold neutron star. By employing hadronic and quark equation of state that satisfies the current mass bound, we use combustion adiabat conditions to find such a limit. The combustion adiabat condition results in a local or a global maximum pressure at an intermediate density range. The maximum pressure corresponds to a local or global maximum mass for the phase transformed hybrid star. The phase transition is usually exothermic if we have a local maximum mass. The criteria for exothermic or endothermic phase transition depend on whether the quark pressure/energy ratios to nuclear pressure/energy are smaller or greater than 1. We find that exothermic phase transition in a cold neutron star usually results in hybrid stars whose mass is smaller than a parent neutron star. The phase transition is endothermic for a global maximum pressure; thereby, one gets a global maximum mass. Hybrid stars much massive than phase transformed local maximum mass can be formed, provided there is some external energy source during the phase transition process. However, for some cases, even massive hybrid stars can form with exothermic phase transition for EoSs having global maximum pressure.

astro-ph.HE↗

arXiv2020

Effect of quantizing magnetic field on the inner crusts of hot Neutron Stars

In the present work we study the effects of strongly quantizing magnetic fields and finite temperature on the properties of inner crusts of hot neutron stars. The inner crust of a neutron star contains neutron-rich nuclei arranged in a lattice and embedded in gases of free neutrons and electrons. We describe the system within the Wigner-Seitz (WS) cell approximation. The nuclear energy is calculated using Skyrme model with SkM* interaction. To isolate the properties of nuclei we follow the subtraction procedure presented by Bonche, Levit and Vautherin, within the Thomas-Fermi approximation. We obtain the equilibrium properties of inner crust for various density, temperatures and magnetic fields by minimizing the free energy of the WS cell satisfying the charge neutrality and $β-$equilibrium conditions. We infer that at a fixed baryon density and temperature, strong quantizing magnetic field reduces the cell radii, neutron and proton numbers in the cell compared with the field free case. However, the nucleon number in the nucleus increases in presence of magnetic field. The free energy per nucleon also decreases in magnetized inner crust. On the other hand, we find that finite temperature tends to smear out the effects of magnetic field. Our results can be important in the context of $r-$process nucleosynthesis in the binary neutron star mergers.

nucl-th↗

arXiv2020

Finding quark content of neutron stars in light of GW170817

The detection of gravitational waves from GW170817 has provided a new opportunity to constrain the equation of state (EOS) of neutron stars. In this article, we investigate the possible existence of quarks inside the neutron star core in the context of GW170817. The nucleon phase is treated within the relativistic nuclear mean-field approach where we have employed a fully comprehensive set of available models, and the quark phase is described in the Bag model. We show that the nucleonic EOSs which are inconsistent with the tidal deformability bound become consistent when phase transition to quark matter via Gibbs construction is allowed. We find that several nucleonic EOSs support the presence of pure quark matter core with a small mass not more than $0.17M_\odot$ confined within a radius of 0.9 km. We also find that the strong correlation between tidal deformability and neutron star radii observed for pure nucleonic stars does persist even with a nucleon-quark phase transition and provides an upper limit on the radius of $R_{1.4} \lesssim 12.9$ km for a $1.4M_\odot$ neutron star.

astro-ph.HE↗

arXiv2019

Observed glitches in 8 young pulsars

The abrupt change in the pulse period of a pulsar is called a pulsar glitch. In this paper, we present eleven pulsar glitches detected using the Ooty Radio Telescope (ORT) and the upgraded Giant Metrewave Radio Telescope (uGMRT) in high cadence timing observations of 8 pulsars. The measured relative amplitude of glitches ($Δν/ν$) from our data ranges from $10^{-6}$ to $10^{-9}$. Among these glitches, three are new discoveries, being reported for the first time. We also reanalyze the largest pulsar glitch in the Crab pulsar (PSR J0534+2200) by fitting the ORT data to a new phenomenological model including the slow rise in the post-glitch evolution. We measure an exponential recovery of 30 days after the Vela glitch detected on MJD 57734 with a healing factor $Q=5.8\times 10^{-3}$. Further, we report the largest glitch ($Δν/ν= 3147.9 \times 10^{-9}$) so far in PSR J1731$-$4744.

astro-ph.HE↗

arXiv2019

Constraining the relativistic mean-field model equations of state with gravitational wave observations

The first detection of gravitational waves from the binary neutron star merger event GW170817 has started to provide important new constraints on the nuclear equation of state at high density. The tidal deformability bound of GW170817 combined with the observed two solar mass neutron star poses a serious challenge to theoretical formulations of realistic equations of state. We analyze a fully comprehensive set of relativistic nuclear mean-field theories by confronting them with the observational bounds and the measured neutron-skin thickness. We find that only a few models can withstand these bounds which predict a stiff overall equation of state but with a soft neutron-proton symmetry energy. Two possible indications are proposed: Circumstantial evidence of hadron-quark phase transition inside the star and new parametrizations that are consistent with ground state properties of finite nuclei and observational bounds. Based on extensive analysis of these sets, an upper limit on the radius of a $1.4M_\odot$ neutron star of $R_{1.4}\lesssim 12.9$ km is deduced.

astro-ph.HE↗

arXiv2019

Role of crustal physics in the tidal deformation of a neutron star

In the late inspiral phase, gravitational waves from binary neutron star mergers carry the imprint of the equation of state due to the tidally deformed structure of the components. If the stars contain solid crusts, then their shear modulus can affect the deformability of the star and, thereby, modify the emitted signal. Here, we investigate the effect of realistic equations of state (EOSs) of the crustal matter, with a realistic model for the shear modulus of the stellar crust in a fully general relativistic framework. This allows us to systematically study the deviations that are expected from fluid models. In particular, we use unified EOSs, both relativistic and non-relativistic, in our calculations. We find that realistic EOSs of crusts cause a small correction, of $\sim 1\%$, in the second Love number. This correction will likely be subdominant to the statistical error expected in LIGO-Virgo observations at their respective advanced design sensitivities, but rival that error in third generation detectors. For completeness, we also study the effect of crustal shear on the magnetic-type Love number and find it to be much smaller.

gr-qc↗

arXiv2018

Glitch Behavior of Pulsars and Contribution from Neutron Star Crust

Pulsars are highly magnetized rotating neutron stars with a very stable rotation speed. Irrespective of their stable rotation rate, many pulsars have been observed with the sudden jump in the rotation rate, which is known as pulsar glitch. The glitch phenomena are considered to be an exhibit of superfluidity of neutron matter inside the neutron star's crustal region. The magnitude of such rapid change in rotation rate relative to their stable rotation frequency can quantify the moment of inertia of the crustal region to the total moment of inertia of the star called as the fractional moment of inertia (FMI). In this paper, we have calculated FMI for different masses of the star using six different representative unified equations of state (EoS) constructed under Relativistic Mean Field (RMF) framework. We have performed an event-wise comparison of FMI obtained from data with that of theoretically calculated values with and without considering the entrainment effect. It is found that larger glitches can't be explained by crustal FMI alone, even without the entrainment.

astro-ph.HE↗

arXiv2018

Hybrid stars in the light of GW170817

We have studied the effect of tidal deformability constraint given by the binary neutron star merger event GW170817 on the equations of state (EOS) of hybrid stars. The EOS are constructed by matching the hadronic EOS described by relativistic mean field (RMF) model and parameter sets NL3, TM1 and NL3$ωρ$ with the quark matter EOS described by modified MIT bag model, via Gibbs' construction. It is found that the tidal deformability constraints along with the lower bound on maximum mass ($M_{\rm max}=2.01\pm0.04M_\odot$) significantly limit the bag model parameter space ($B_{\rm eff}^{1/4}$, $a_4$). We also obtain upper limits on the radius of $1.4M_\odot$ and $1.6M_\odot$ stars as $R_{1.4}\leq13.2-13.5$ km and $R_{1.6}\leq13.2-13.4$ km, respectively for different hadronic EOS considered here.

astro-ph.HE↗

arXiv2018

Transport properties of nuclear pasta phase with quantum molecular dynamics

We study the transport properties of nuclear pasta for a wide range of density, temperature and proton fractions, relevant for different astrophysical scenarios adopting a quantum molecular dynamics model. In particular, we estimate the values of shear viscosity as well as electrical and thermal conductivities by calculating the static structure factor $S(q)$ using simulation data. In the density and temperature range where the pasta phase appears, the static structure factor shows irregular behavior. The presence of a slab phase greatly enhances the peak in $S(q)$. However, the effect of irregularities in $S(q)$ on the transport coefficients is not very dramatic. The values of all three transport coefficients are found to have the same orders of magnitude as found in theoretical calculations for the inner crust matter of neutron stars without the pasta phase and therefore, is in contrast to earlier speculations that a pasta layer might be highly resistive, both thermally and electrically.

astro-ph.HE↗