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In this work, we investigate the impact of the possibility of a small, subsolar mass compact star, such as the recently reported central compact object of HESS J1731-347, on the equation of state (EOS) of neutron stars. We have used a hybrid approach to the nuclear EOS developed recently where the matter around nuclear saturation density is described by a parametric expansion in terms of nuclear empirical parameters and represented in an agnostic way at higher density using piecewise polytropes. We have incorporated the inputs provided by the latest neutron skin measurement experiments from PREX-II and CREX, simultaneous mass-radius measurements of pulsars PSR J0030+0451 and PSR J0740+6620, and the gravitational wave events GW170817 and GW190425. The main results of the study show the effect of HESS J1731-347 on the nuclear parameters and neutron star observables. Our analysis yields the slope of symmetry energy $L=45.71^{+38.18}_{-22.11}$ MeV, the radius of a $1.4 M_\odot$ star, $R_{1.4}=12.18^{+0.71}_{-0.88}$ km, and the maximum mass of a static star, $M_{\rm max}= 2.14^{+0.26}_{-0.17} M_\odot$ within $90\%$ confidence interval, respectively.

On 29 May 2023, the LIGO Livingston observatory detected the gravitational-wave signal GW230529_181500 from the merger of a neutron star with a lower mass-gap compact object. Its long inspiral signal provides a unique opportunity to test General Relativity (GR) in a parameter space previously unexplored by strong-field tests. In this work, we performed parameterized inspiral tests of GR with GW230529_181500. Specifically, we search for deviations in the frequency-domain GW phase by allowing for agnostic corrections to the post-Newtonian coefficients. We performed tests with the Flexible Theory Independent (FTI) and Test Infrastructure for General Relativity (TIGER) frameworks using several quasi-circular waveform models that capture different physical effects (higher modes, spins, tides). We find that the signal is consistent with GR for all deviation parameters. Assuming the primary object is a black hole, we obtain particularly tight constraints on the dipole radiation at $-1$PN order of $|\delta\hat{\varphi}_{-2}| \lesssim 8 \times 10^{-5}$, which is a factor $\sim17$ times more stringent than previous bounds from the neutron star--black hole merger GW200115_042309, as well as on the 0.5PN and 1PN deviation parameters. We discuss some challenges that arise when analyzing this signal, namely biases due to correlations with tidal effects and the degeneracy between the 0PN deviation parameter and the chirp mass. To illustrate the importance of GW230529_181500 for tests of GR, we mapped the agnostic $-1$PN results to a class of Einstein-scalar-Gauss-Bonnet (ESGB) theories of gravity. We also conducted an analysis probing the specific phase deviation expected in ESGB theory and obtain an upper bound on the Gauss-Bonnet coupling of $\ell_{\rm GB} \lesssim 0.51~\rm{M}_\odot$ ($\sqrt{\alpha_{\rm GB}} \lesssim 0.28$ km), which is better than any previously reported constraint., Comment: 19 pages, 9 figures

In this article, we study the neutron stars with antikaon condensates and their radial oscillations. We incorporate the antikaons $(K^-,\bar{K}^0)$ within a relativistic mean field theory with density dependent couplings. A transition to the kaon condensed phase is treated as a second order phase transition for different possible antikaon optical potentials. Then, we solve the structure equations of stars and calculate the fundamental and higher order frequencies of their radial pulsations. We study the eigenfunctions of different radial modes for a $2M_\odot$ star with equations of state having different potential depths of antikaons. We also study the large frequency separations for the $2M_\odot$ star. We find distinct features of the appearances of antikaons on the eigenfunctions. The shape of the eigenfunctions corresponding to the radial perturbations is affected due to the appearance of the antikaons, but for the pressure perturbation, the shape remains unchanged.

In this work, we propose a meta-modelling technique to nuclear matter on the basis of a relativistic density functional with density-dependent couplings. Identical density dependence for the couplings both in the isoscalar and isovector sectors is employed. We vary the coupling parameters of the model to capture the uncertainties of the empirical nuclear matter parameters at saturation. Then, we construct a large ensemble of unified equations of state in a consistent manner both for clusterized and uniform matter in $\beta$-equilibrium at zero temperature. Finally, we calculate neutron star properties to check the consistency with astrophysical observations within a Bayesian framework. Out of the different sets of astrophysical data employed, constraint on tidal deformability from the GW170817 event was found to be the most stringent in the posteriors of different neutron star properties explored in the present study. We demonstrate in detail the impact of the isovector incompressibility ($K_{sym}$) on high-density matter that leads to a considerable variation in the composition of neutron star matter. A couple of selected models with extreme values of $K_{sym}$, which satisfy various modern nuclear physics and neutron star astrophysics constraints, are uploaded in the \textsc{CompOSE} \cite{Typel:2013rza} database for use by the community.

More than 20 years ago, Glendenning, Kettner and Weber proposed the existence of stable white dwarfs with a core of strange quark matter. More recently, by studying radial modes, Alford, Harris and Sachdeva concluded instead that those objects are unstable. We aim to clarify this issue. We investigate the stability of these objects by looking at their radial oscillations while incorporating boundary conditions at the quark-hadron interface, corresponding to either a rapid or a slow conversion of hadrons into quarks. Our analysis shows that objects of this type are stable if the star is not strongly perturbed, and ordinary matter cannot transform into strange quark matter because of the Coulomb barrier separating the two components. On the other hand, ordinary matter can be transformed into strange quark matter if the star undergoes a violent process, as in the preliminary stages of a type Ia supernova, and this causes the system to become unstable and to collapse into a strange quark star. In this way, accretion-induced collapse of strange dwarfs can be facilitated, and km-sized objects with subsolar masses can be produced., Comment: 5 pages, 6 figures. Version accepted for publication on A&A Letters

The existence of massive compact stars $(M\gtrsim 2.1 M_{\odot})$ implies that the conformal limit of the speed of sound $c_s^2=1/3$ is violated if those stars have a crust of ordinary nuclear matter. Here we show that, if the most massive objects are strange quark stars, i.e. stars entirely composed of quarks, the conformal limit can be respected while observational limits on those objects are also satisfied. By using astrophysical data associated with those massive stars, derived from electromagnetic and gravitational wave signals, we show, within a Bayesian analysis framework and by adopting a constant speed of sound equation of state, that the posterior distribution of $c_s^2$ is peaked around 0.3, and the maximum mass of the most probable equation of state is $\sim 2.13 M_{\odot}$. We discuss which new data would require a violation of the conformal limit even when considering strange quark stars, in particular we analyze the possibility that the maximum mass of compact stars is larger than $2.5M_{\odot}$, as it would be if the secondary component of GW190814 is a compact star and not a black hole. Finally, we discuss how the new data for PSR J0740+6620 obtained by the NICER collaboration compare with our analysis (not based on them) and with other possible interpretations.

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., Comment: Matches with the MNRAS published version

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 $\Lambda_{1.4} = 550_{-225}^{+223}$ 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.

In this work, we compare two powerful parameter estimation methods namely Bayesian inference and Neural Network based learning to study the quark matter equation of state with constant speed of sound parametrization and the structure of the quark stars within the two-family scenario. We use the mass and radius estimations from several X-ray sources and also the mass and tidal deformability measurements from gravitational wave events to constrain the parameters of our model. The results found from the two methods are consistent. The predicted speed of sound is compatible with the conformal limit.

We present a Bayesian analysis to constrain the equation of state of dense nucleonic matter by exploiting the available data from symmetric nuclear matter at saturation and from observations of compact X-ray sources and from the gravitational wave event GW170817. For the first time, such analysis is performed by using a class of models, the relativistic mean field models, which allow to consistently construct an equation of state in a wide range of densities, isospin asymmetries and temperatures. The selected class of models contains five nuclear physics empirical parameters at saturation for which we construct the joint posterior distributions. By exploring different types of priors, we find that the equations of state with the largest evidence are the ones featuring a strong reduction of the effective mass of the nucleons in dense matter which can be interpreted as an indication of a phase transition to a chiral symmetry restored phase. Those equations of state in turn predict $R_{1.4} \sim 12$ km. Finally, we present a preliminary investigation on the effect of including $\Lambda$ hyperons showing that they appear in stars more massive than about $1.6 M_{\odot}$ and lead to radii larger than about $R_{1.4} \sim 14$ km. Within the model here explored, the formation of such particles provide a poor agreement with the constraints from GW170817.

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 ($\Delta \nu/\nu$) 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 ($\Delta \nu/\nu = 3147.9 \times 10^{-9}$) so far in PSR J1731$-$4744., Comment: 11 pages, 12 figures, 3 tables; Accepted for publication in MNRAS

We study the effect of superfluidity on the tidal response of a neutron star in a general relativistic framework. In this work, we take a dual-layer approach where the superfluid matter is confined in the core of the star. Then, the superfluid core is encapsulated with an envelope of ordinary matter fluid which acts effectively as the low-density crustal region of the star. In the core, the matter content is described by a two-fluid model where only the neutrons are taken as superfluid and the other fluid consists of protons and electrons making it charge neutral. We calculate the values of various tidal love numbers of a neutron star and discuss how they are affected due to the presence of entrainment between the two fluids in the core. We also emphasize that more than one tidal parameter is necessary to probe superfluidity with the gravitational wave from the binary inspiral.

We shortly summarize the two-families scenario in which both hadronic stars and strange quark stars can exist and we describe the main predictions one can obtain from it. We then concentrate on the observables that most likely will be measured in the near future, i.e. masses, radii, tidal deformabilities and moments of inertia and we present a list of objects that are candidate strange quark stars in this scheme. We show that the estimates of the radii derived up to now from observations are all compatible with the two-families scenario and in particular all the objects having large radii can easily be interpreted as strange quark stars., Comment: Submitted to the AIP Conference Proceedings of the Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era of Gravitational Wave Astronomy (January 3 - 7, 2019, Xiamen, China)

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., Comment: 26 pages, 10 figures

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., Comment: Matches with the published version

We investigate the tidal deformability of a superfluid neutron star. We calculate the equilibrium structure in the general relativistic two-fluid formalism with entrainment effect where we take neutron superfluid as one fluid and the other fluid is comprised of protons and electrons, making it a charge neutral fluid. We use a relativistic mean field model for the equation of state of matter where the interaction between baryons is mediated by the exchange $\sigma$, $\omega$ and $\rho$ mesons. Then, we study the linear, static $l=2$ perturbation on the star to compute the electric-type Love number following Hinderer's prescription., Comment: Accepted for publication in Physical Review D

Particles with strangeness content are predicted to populate dense matter, modifying the equation of state of matter inside neutron stars as well as their structure and evolution. In this work, we show how the modeling of strangeness content in dense matter affects the properties of isolated neutrons stars and the tidal deformation in binary systems. For describing nucleonic and hyperonic stars we use the many-body forces model (MBF) at zero temperature, including the $\phi$ mesons for the description of repulsive hyperon-hyperon interactions. Hybrid stars are modeled using the MIT Bag Model with vector interaction (vMIT) in both Gibbs and Maxwell constructions, for different values of bag constant and vector interaction couplings. A parametrization with a Maxwell construction, which gives rise to third family of compact stars (twin stars), is also investigated. We calculate the tidal contribution that adds to the post-Newtonian point-particle corrections, the associated love number for sequences of stars of different composition (nucleonic, hyperonic, hybrid and twin stars), and determine signatures of the phase transition on the gravitational waves in the accumulated phase correction during the inspirals among different scenarios for binary systems. On the light of the recent results from GW170817 and the implications for the radius of $\sim1.4\,\mathrm{M_{\odot}}$ stars, our results show that hybrid stars can only exist if a phase transition takes place at low densities close to saturation.

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., Comment: 9 pages, 3 figures, 1 table, revised, accepted for publication in ApJ

In this work, we study the properties and structure of a massive and rapidly rotating protoneutron star (PNS) with hyperon content. We follow several stages of quasi-stationary evolution in an approximate way at four discrete steps. We use a density-dependent (DD) relativistic mean field theory (RMF) model and calculate different quantities such as mass, equatorial radius, moment of inertia, and quadrupole moment to get different rotating configurations upto the mass-shedding limit. We study the effect of the appearance of $\Lambda$, the lightest of all hyperons, on each of the evolutionary stages of the PNS. We also check its sensitivity to the inclusion of $\phi$ vector meson as a mediator of $\Lambda-\Lambda$ interaction in detail. Finally, we investigate the universal relations between moment of inertia and compactness in the context of a hot and young compact object.

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$\omega\rho$ 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., Comment: version accepted by ApJ

Strong magnetic fields have important effects on the crustal properties of magnetars. Here we study the magneto-elastic oscillations of magnetars taking into consideration the effect of strong magnetic fields on the crustal composition (magnetised crust). We calculate global magneto-elastic (GME) modes as well as modes confined to the crust (CME) only. The composition of the crust directly affects its shear modulus which we calculate using three different nucleon-nucleon interactions: SLy4, SkM and Sk272. The shear modulus of the crust is found to be enhanced in strong magnetic fields $\geq 10^{17}$ G for all those Skyrme interactions. It is noted that the shear modulus of the crust for the SLy4 interaction is much higher than those of the SkM and Sk272 interactions in presence of magnetic fields or not. Though we do not find any appreciable change in frequencies of fundamental GME and CME modes with and without magnetised crusts, frequencies of first overtones of CME modes are significantly affected in strong magnetic fields $\geq 10^{17}$ G. However, this feature is not observed in frequencies of first overtones of GME modes. As in earlier studies, it is also noted that the effects of crusts on frequencies of both types of maneto-elastic modes disappear when the magnetic field reaches the critical field ($B > 4 \times 10^{15}$ G). Frequencies of GME and CME modes calculated with magnetised crusts based on all three nucleon-nucleon interactions, stellar models and magnetic fields, are compared with frequencies of observed quasi-periodic oscillations (QPOS) in SGR 1806-20 and SGR1900+14. As in earlier studies, this comparison indicates that GME modes are essential to explain all the frequencies as CME modes can explain only the higher frequencies., Comment: 28 pages, 8 figures, accepted for publication in PRC. arXiv admin note: substantial text overlap with arXiv:1207.3247

A comparative study of the $\Lambda$ hyperon equations of state of Banik, Hempel and Banyopadhyay (BHB) \citep{bhb} and \citet{shen11} (denoted as HShen $\Lambda$) for core collapse supernova (CCSN) simulations is carried out in this work. The dynamical evolution of a protoneutron star (PNS) into a black hole is investigated in core collapse supernova simulations in the general relativistic one dimensional code using the BHB$\Lambda \phi$ and HShen $\Lambda$ equation of state (EoS) tables and different progenitor models from Woosley and Heger \citep{Woos}. Radial profiles of the mass fractions of baryons, the density as well as the temperature in the PNS at different moments in time, are compared for both EoS tables. The behaviour of the central density of the PNS with time is demonstrated for those two $\Lambda$ hyperon EoS tables and compared with their corresponding nuclear EoS tables. It is observed that the black hole formation time is higher in the BHB$\Lambda \phi$ case than in the HShen $\Lambda$ EoS for the entire set of progenitor models adopted here, because the repulsive $\Lambda$-$\Lambda$ interaction makes the BHB$\Lambda \phi$ EoS stiffer. Neutrino emission with the $\Lambda$ hyperon EoS ceases earlier than that of its nuclear counterpart. The long duration evolution of the shock radius and gravitational mass of the PNS after a successful supernova explosion with enhanced neutrino heating are studied with the BHB$\Lambda \phi$ EoS and $s$20WH07 progenitor model. The PNS is found to remain stable for 4 s and might evolve into a cold neutron star., Comment: LaTeX; 25 pages including two tables and 10 figures; to be published in the Astrophysical Journal

The measurement of $1.97 \pm 0.04 M_{solar}$ for PSR J1614-2230 and $2.01 \pm 0.04M_{solar}$ for PSR J0348+0432 puts a strong constraint on the neutron star equation of state and its exotic composition at higher densities. In this paper, we investigate the possibility of exotic equation of state within the observational mass constraint of $2M_{solar}$ in the framework of relativistic mean field model with density-dependent couplings. We particularly study the effect of antikaon condensates in the presence of hyperons on the mass-radius relationship of the neutron star., Comment: 6 figures, submitted to Phy. Rev. C