Neutron Star Models Research Articles

Neutron Stars constraints on a late G transition

It has been suggested recently that the Hubble tension could be eliminated by a sharp, ∼10% increase of the effective gravitational constant at z∼0.01. This would decrease the luminosities of type 1a supernovae in just the needed amount to explain the larger value of the Hubble parameter. In the present paper we call attention to a dramatic effect of such transition on neutron stars. A neutron star that existed at z=0.01 would contract, conserving the baryon mass but undergoing a mass reduction. We computed neutron star models, with a realistic equation of state, and obtained that this reduction is typically 0.04M⊙. This amounts to an energy of 7×1052 erg. The transition will affect all neutron stars that formed along the history of each galaxy prior to the transition. Given the large number of neutron stars per galaxy, the liberated energy is huge. An estimate of the expected fluxes of neutrinos and x-rays yields values exceeding observational upper limits, thus rendering the late G transition scenario non-viable.

Differential rotation in neutron stars at finite temperatures

IntroductionThis paper investigates the impact of differential rotation on the bulk properties and onset of rotational instabilities in neutron stars at finite temperatures up to 50 MeV.MethodsUtilizing the relativistic Brueckner-Hartree-Fock (RBHF) formalism in full Dirac space, the study constructs equation of state (EOS) models for hot neutron star matter, including conditions relevant for high temperatures. These finite-temperature EOS models are applied to compute the bulk properties of differentially rotating neutron stars with varying structural deformations.ResultsThe findings demonstrate that the stability of these stars against bar-mode deformation, a key rotational instability, is only weakly dependent on temperature. Differential rotation significantly affects the maximum mass and radius of neutron stars, and the threshold for the onset of bar-mode instability shows minimal sensitivity to temperature changes within the examined range.DiscussionThese findings are crucial for interpreting observational data from neutron star mergers and other high-energy astrophysical events. The research underscores the necessity of incorporating differential rotation and finite temperature effects in neutron star models to predict their properties and stability accurately.

Exploring the physical properties of anisotropic compact objects and constraining their mass–radius relations beyond standard limit in [formula omitted]-gravity

The progress in theoretical modeling of compact high-mass stars in the context of alternative gravity has become important in minimizing challenges associated with neutron star (NS) radius measurements. On this basis, we have presented a rigorous study of compact objects beyond the standard limit in gravity F(Q) in particular F(Q)=b1Q+b2 where b1 and b2 are constants. After formulating the basic equations and finding their relevant solutions by assuming a well-behaved ansatz for the metric potential as well as for anisotropy monitored by the parameters μ1,μ2,b1, along with imposing the boundary conditions on the system under treatment, we have developed a stellar model for anisotropic stars. Specifically, we explored the physical properties of these anisotropic stars and provided novel mass–radius (M−R) relations with models falling in the mass gap of the events GW190814 and GW200210, as well as the effects of slow rotation and moment of inertia on these findings. Interestingly, the behavior of the M−R curves represents a polytropic-type equation of state (EOS) for a negative Λ, while a positive Λ corresponds to a quark matter EOS. However, the analysis reveals that the predicted radii of the compact object observed in the GW190814 event, with a mass of 2.5–2.67 M⊙, is approximately 11.4 km for the de Sitter (dS) case and 11.8 km for the anti-de Sitter (AdS) case, assuming a specific value of the parameter b1=1.1. Furthermore, for b1>1.1, the model predicts the existence of more compact stars with a maximum mass of approximately 3M⊙, which is in good agreement with the R2 model of NSs maintaining the Sly EOS. From the I−M curves, it can be observed that higher values of b1 and μ1 in F(Q) gravity can sustain high moments of inertia (I) of stars with high masses. Interestingly, we obtained the range of I for the dS space as [1.92,3.92]×1045gcm2 and for the AdS space as [2.01,4.03]×1045gcm2 for 1.0≤b1≤1.2.

A NEW SPHERICALLY SYMMETRIC NEUTRON STAR MODEL EMBEDDING CLASS I IN DURGAPAL METRIC

In this paper, we develop a novel spherically symmetric neutron star model by combining the two useful mathematical aspects of astrophysics, Karmakar’s condition and Durgapal metric for anisotropic stellar objects. We use Karmakar’s condition to embed a class I reducibility criteria in the model. The solutions are simplified by considering 𝒆𝝂=𝑩(𝟏+𝒙)𝟔, where 𝒙=𝑪𝒓𝟐, beyond the fifth model given by Durgapal. The model does not have the central singularity. The model well behaves for the realistic physical properties of the neutron stars. The energy density, the pressure in the radial direction, and the pressure in the tangential direction are positively finite and decrease from the center to the surface of the neutron stars. The redshift and the compression moduli are also positive finite. In the offered model, we take the values of the constants as 1.5760 ≤ A ≤ 2.295, 0.0602 ≤ B ≤ 0.5577, and 7.492 × 10-10 ≤ C ≤ 3.397 × 10-9 for the neutron stars EXO 1785-248, SMC X-1, Her X-1, 4U 1538-52, LMC X-4, and Cen X-3. The Buchdahl condition is well satisfied by the model.

Compiled properties of nucleonic matter and nuclear and neutron star models from nonrelativistic and relativistic interactions

Compiled properties of nucleonic matter and nuclear and neutron star models from nonrelativistic and relativistic interactions

A Geometric Neutron Star Model of Repeating and Nonrepeating Fast Radio Bursts

Fast radio bursts (FRBs) are millisecond-duration extragalactic radio transients. They fall into the categories of repeaters and apparent nonrepeaters. However, such a classification causes a lack of motivation to investigate the physical picture. Here, we propose a unified geometric model to distinguish between repeaters and apparent nonrepeaters, in which the quasi-tangential (QT) propagation effect within the magnetosphere of a neutron star is considered. In this model, apparent nonrepeaters arise from sources whose emitting region has a smaller impact angle with respect to the magnetic axis, while repeaters come from sources whose emitting region has a larger impact angle. The observational discriminant polarization properties between repeaters and apparent nonrepeaters are an important clue for verifying this unified geometric model since the polarization is sensitive to the QT propagation effect. Moreover, our model effectively explains all of the other discriminant properties, including bandwidth, duration, peak luminosity, energy, brightness temperature, time–frequency downward drifting, and repetition rate, providing compelling evidence for the magnetospheric origin of FRBs.

Streaming instability in neutron star magnetospheres: No indication of soliton-like waves

Context. Coherent radiation of pulsars, magnetars, and fast radio bursts could, in theory, be interpreted as radiation from solitons and soliton-like waves. Solitons are meant to contain a large number of electric charges confined on long timescales and can radiate strongly via coherent curvature emission. However, solitons are also known to undergo a wave collapse, which casts doubts on the correctness of the soliton radio emission models of neutron stars. Aims. We investigated the evolution of the caviton type of solitons self-consistently formed by the relativistic streaming instability and compared their apparent stability in 1D calculations with more generic 2D cases, in which the solitons are seen to collapse. Three representative cases of beam Lorentz factors and plasma temperatures were studied to obtain soliton dispersion properties. Methods. We utilized 1D electrostatic and 2D electromagnetic relativistic particle-in-cell simulations at kinetic microscales. Results. We find that no solitons are generated by the streaming instability in the 2D simulations. Only superluminal L-mode (relativistic Langmuir) waves are produced during the saturation of the instability, but these waves have smaller amplitudes than the waves in the 1D simulations. The amplitudes tend to decrease after the instability has saturated, and only waves close to the light line, ω = ck, remain. Solitons in the 1D approach are stable for γb ≳ 60, but they disappear for low beam Lorentz factors, γb < 6. Conclusions. Our examples show that the superluminal soliton branch that is formed in 1D simulations will not be generated by the relativistic streaming instability when more dimensional degrees of freedom are present. The soliton model cannot, therefore, be used to explain the coherent radiation of pulsars, magnetars, and fast radio bursts – unless one can show that there are alternative plasma mechanisms for the soliton generation.

A Statistical Approach to Neutron Stars' Crust-Core Transition Density and Pressure.

In this paper, a regression model between neutron star crust-core pressure and the symmetry energy characteristics was estimated using the Akaike information criterion and the adjusted coefficient of determination Radj2. The most probable value of the transition density, which should characterize the crust-core environment of the sought physical neutron star model, was determined based on the obtained regression function. An anti-correlation was found between this transition density and the main characteristic of the symmetry energy, i.e., its slope L.

Differential rotation in compact objects with hyperons and delta isobars

AbstractNeutron stars may experience differential rotation on short, dynamical timescales following extreme astrophysical events like binary neutron star mergers. In this work, the masses and radii of differentially rotating neutron star models are computed. We employ a set of equations of states for dense hypernuclear and ‐admixed‐hypernuclear matter obtained within the framework of CDF theory in the relativistic Hartree‐Fock (RHF) approximation. Results are shown for varying meson‐ couplings, or equivalently the ‐potential in nuclear matter. A comparison of our results with those obtained for nonrotating stars shows that the maximum mass difference between differentially rotating and static stars is independent of the underlying particle composition of the star. We further find that the decrease in the radii and increase in the maximum masses of stellar models when ‐isobars are added to hyperonuclear matter (as initially observed for static and uniformly rotating stars) persist also in the case of differentially rotating neutron stars.

Neutron stars with polytropic equation of state

In this work, the neutron star model is considered in the framework of general relativity with the polytropic equation of state p=ργ, where γ=1+1k and k is known as a polytropic index. The homotopy perturbation method is used to solve Tolman–Oppenheimer–Volkoff (TOV) equation which gives the mass of the stellar structure. The solution of Einstein’s field equations is obtained and stability, causality, and physical viability of the considered model are examined. Using the observational results of mass and radius of known stars 4U 1608-52 RX J185635-3754, SAX J1748.9-2021 and 4U 1728-34, the model parameters are estimated which led to the non-singular solutions satisfying the energy conditions.

A multimessenger model for neutron star–black hole mergers

ABSTRACT We present a semi-analytic model for predicting kilonova light curves from the mergers of neutron stars with black holes (NSBH). The model is integrated into the mosfit platform, and can generate light curves from input binary properties and nuclear equation-of-state considerations, or incorporate measurements from gravitational wave (GW) detectors to perform multimessenger parameter estimation. The rapid framework enables the generation of NSBH kilonova distributions from binary populations, light curve predictions from GW data, and statistically meaningful comparisons with an equivalent binary neutron star (BNS) model in mosfit. We investigate a sample of kilonova candidates associated with cosmological short gamma-ray bursts, and demonstrate that they are broadly consistent with being driven by NSBH systems, though most have limited data. We also perform fits to the very well sampled GW170817, and show that the inability of an NSBH merger to produce lanthanide-poor ejecta results in a significant underestimate of the early (≲2 d) optical emission. Our model indicates that NSBH-driven kilonovae may peak up to a week after merger at optical wavelengths for some observer angles. This demonstrates the need for early coverage of emergent kilonovae in cases where the GW signal is either ambiguous or absent; they likely cannot be distinguished from BNS mergers by the light curves alone from ∼2 d after the merger. We also discuss the detectability of our model kilonovae with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST).

Structural properties of a new class of stellar structures in modified teleparallel gravity

This paper explores new neutron star models based on spherically symmetric space–time. We take into account the gravitational effects of f(T,T) gravity, in which T is the torsion and T is the trace of the energy–momentum tensor. Field equations are evaluated by incorporating the off-diagonal tetrad. In this paper, we discuss the detailed properties of compact star candidates 4U1538–52, J0437–4,715, J0030 + 0451, and 4U1820–30, like energy density, pressure profiles, gradients, anisotropy, energy conditions, equation of state, speeds of sound, TOV equation, and compactification parameters. We discuss all these characteristics using the quadratic cosmological model of f(T,T) gravity. We use the well-famed junction equations to evaluate the unknown parameters. Our detailed and comprehensive graphical analysis ensures that the model containing the anisotropic nature of stellar structures is physically acceptable, regular, and stable.

NEUTRON STARS IN A UNIFORM DENSITY APPROXIMATION

Models of neutron stars are considered in the case of a uniform density distribution. A universal algebraic equation is obtained that is valid for any equation of state. This equation allows us to find the approximate mass of a star of a given density without resorting to the integration of differential equations. Solutions for various equations of state, including more realistic ones, are presented in the paper and differ from the exact solutions obtained by numerical integration of differential equations by no more than 20%.

Neutron star mass–radius constraints using the high-frequency quasi-periodic oscillations of GRB 200415A

Context. Quasi-periodic oscillations (QPOs) observed in a giant flare of a strongly magnetized neutron star (magnetar) carry crucial information for extracting the properties od neutron stars. Aims. The aim of this study is to constrain the mass and radius of the neutron star model for GRB 200415A by identifying the observed QPOs with crustal torsional oscillations and comparing these with experimental constraints on the nuclear matter properties. The frequencies of the crustal torsional oscillations are determined by solving the eigenvalue problem with the Cowling approximation, assuming a magnetic field of about 1015 G. Methods. We find that the observed QPOs can be identified with several overtones of crustal oscillations for carefully selected combinations of the nuclear saturation parameters. Thus, we can inversely constrain the neutron star mass and radius for GRB 200415A by comparing them to the values of nuclear saturation parameters obtained from terrestrial experiments. Results. We impose further constraints on the neutron star mass and radius while the candidate neutron star models examined here are consistent with the constraints obtained from other available astronomical and experimental observations.

Probing strong field f(R) gravity and ultradense matter with the structure and thermal evolution of neutron stars

Thermal evolution of neutron stars is studied in the $f(R)=R+\ensuremath{\alpha}{R}^{2}$ theory of gravity. We first review the equations of stellar structure and evolution for a spherically symmetric spacetime plus a perfect fluid at rest. We then present numerical results for the structure of neutron stars using four nucleonic dense matter equations of state and a series of gravity theories for $\ensuremath{\alpha}$ ranging from zero, i.e., general relativity, up to $\ensuremath{\alpha}\ensuremath{\approx}{10}^{16}\text{ }\text{ }{\mathrm{cm}}^{2}$. We emphasize properties of these neutron star models that are of relevance for their thermal evolution as the threshold masses for enhanced neutrino emission by the direct Urca process, the proper volume of the stellar cores where this neutrino emission is allowed, the crust thickness, and the surface gravitational acceleration that directly impact the observable effective temperature. Finally, we numerically solve the equations of thermal evolution and explicitly analyze the effects of altering gravity. We find that uncertainties in the dense matter microphysics, such as the core chemical composition and superfluidity/superconductivity properties, as well as the astrophysical uncertainties on the chemical composition of the surface layers, have a much stronger impact than possible modifications of gravity within the studied family of $f(R)$ theories. We conclude that within this family of gravity theories, conclusions from previous studies of neutron star thermal evolution are not significantly altered by modification of gravity theory. Conversely, this implies that neutron star cooling modeling may not be a useful tool to constrain deviations of gravity from Einstein theory unless these are much more radical than in the $f(R)=R+\ensuremath{\alpha}{R}^{2}$ framework.

Quakes of compact stars

ABSTRACTGlitches are commonly observed for pulsars, which are explained by various mechanisms. One hypothesis attributes the glitch effect to the instantaneous moment of inertia change of the whole star caused by a starquake, which is similar to earthquakes caused by fast dislocation occurring on planar faults for the static stress, though the quake-induced dynamics responsible for glitch (superfluid vortex versus pure starquake) remains still unknown. However, a theoretical model to quantitatively explain the stress loading, types of starquakes, and co-seismic change of moment of inertia is rarely discussed. In this study, we incorporate elastic deformation theories of earthquakes into the starquake problems. We compute the field of stress loading associated with rotation deceleration and determine the optimal type of starquakes at various locations. Two types of pulsar structure models, i.e. neutron and strangeon star models, are included in the computation, and their differences are notable. Our calculation shows that the observed glitch amplitude can be explained by the starquakes in the strangeon star model, though the required scaled starquake magnitude is much larger than that occurred on Earth. We further discuss the possibility to compute the energy budget and other glitch phenomena using the starquake model in the elastic medium framework.

The influence of deceleration and internal structure on the braking index of pulsars: Simplest model for the star

AbstractPulsars are rotating neutron stars whose electromagnetic radiation is observed to pulsate and, despite the extremely stable rates, the frequencies of the pulses decay with time, as quantified by the braking index (n). It is extremely difficult to obtain this index from observations due to different factors. Theoretically, in the canonical model for neutron stars, one finds n = 3, but observational data shows that n is less than three. In this work, the canonical model is modified, incorporating the influence of the deceleration of the neutron star. As the star's rotation decelerates, the deformation of the star due to the centrifugal force decreases thereby reducing its moment of inertia. Consequently, the star decelerates less, reducing the braking index. In this work, the star is modeled as a very hard, compressible spherical shell filled with neutrons and protons superfluid that do not interacts with the shell. Using this toy model as a first approach, the braking index is found as a function of rotation, yielding a theoretical braking index closer to the one derived from observational data.

Extended Tolman III and VII solutions in f(R,T) gravity: Models for neutron stars and supermassive stars

In the context of linear $f(\mathcal{R},T)=\mathcal{R}+\chi T$ gravity, where $\mathcal{R}$ is the Ricci scalar, $T$ is the trace of the energy-momentum tensor, and $\chi$ is a dimensionless parameter, we have obtained exact analytical and numerical solutions for isotropic perfect-fluid spheres in hydrostatic equilibrium. Our solutions correspond to two-parametric extensions of the Tolman III (T-III) and Tolman VII (T-VII) models, in terms of the compactness $\beta$ and $\chi$. By requiring configurations that exhibit monotonically decreasing radial profiles for both the energy density and pressure, compliance with the energy conditions, as well as subluminal speed of sound, we have constrained the parametric space of our solutions. We have also obtained analytically a parametric deformation of the T-VII solution that continuously interpolates between the T-III and T-VII models for any $\chi$, and in the appropriate limits, provides an analytic approximation for the uniform density configuration in linear $f(\mathcal{R},T)$ gravity. Finally, by integrating numerically the TOV equations, we have obtained a numerical solution for the uniform-density configuration and subsequently, using the mass-radius relations, we have obtained the maximum mass that can be supported by such configurations. We have found that in the appropriate parametric regime our solution is in very good agreement with the observational bounds for the masses and radii of neutron stars.

Follow-up analyses of the binary-neutron-star signals GW170817 and GW190425 by using post-Newtonian waveform models

We reanalyze the binary-neutron-star signals, GW170817 and GW190425, focusing on the inspiral regime to avoid uncertainties on waveform modeling in the postinspiral regime. We use post-Newtonian waveform models as templates, which are theoretically rigid and efficiently describe the inspiral regime. We study potential systematic difference in estimates of the binary tidal deformability $\tilde{\Lambda}$ by using different descriptions for the point-particle dynamics and tidal effects. We find that the estimates of $\tilde{\Lambda}$ show no significant systematic difference among three models for the point-particle parts: TF2, TF2g, and TF2+, when they employ the same tidal model. We compare different tidal descriptions given by different post-Newtonian orders in the tidal phase. Our results indicate that the estimates of $\tilde{\Lambda}$ slightly depend on the post-Newtonian order in the tidal phase and an increase in the tidal post-Newtonian order does not lead to a monotonic change in the estimate of $\tilde{\Lambda}$. We also compare the estimate of $\tilde{\Lambda}$ obtained by the post-Newtonian tidal model and numerical-relativity calibrated tidal models. We find that the post-Newtonian model gives slightly larger estimate of $\tilde{\Lambda}$ and wider posterior distribution than the numerical-relativity calibrated models. According to Bayesian model comparison, it is difficult to identify a preference among the post-Newtonian orders by relying on the GW170817 and GW190425 data. Our results indicate no preference among numerical-relativity calibrated tidal models over the post-Newtonian model. Additionally, we present constraints on equation-of-state models for neutron stars with the post-Newtonian model, which show that the GW170817 data disfavor less compact models, though they are slightly weaker constraints than the numerical-relativity calibrated tidal models.

Self-similarity relations for torsional oscillations of neutron stars

ABSTRACT Self-similarity relations for torsional oscillation frequencies of neutron star crust are discussed. For any neutron star model, the frequencies of fundamental torsional oscillations (with no nodes of radial wavefunction, i.e. at n = 0, and at all possible angular wave numbers ℓ ≥ 2) is determined by a single constant. Frequencies of ordinary torsional oscillations (at any n > 0 with ℓ ≥ 2) are determined by two constants. These constants are easily calculated through radial integrals over the neutron star crust, giving the simplest method to determine full oscillation spectrum. All constants for a star of fixed mass can be accurately interpolated for stars of various masses (but the same equation of state). In addition, the torsional oscillations can be accurately studied in the flat space–time approximation within the crust. The results can be useful for investigating magneto-elastic oscillations of magnetars which are thought to be observed as quasi-periodic oscillations after flares of soft-gamma repeaters.