Stable Neutron Star Research Articles

Fragmentation in Gravitationally Unstable Collapsar Disks and Subsolar Neutron Star Mergers

Although stable neutron stars (NSs) can in principle exist down to masses M ns ≈ 0.1 M ⊙, standard models of stellar core-collapse predict a robust lower limit M ns ≳ 1.2 M ⊙, roughly commensurate with the Chandrasekhar mass M Ch of the progenitor’s iron core (electron fraction Y e ≈ 0.5). However, this limit may be circumvented in sufficiently dense neutron-rich environments (Y e < 0.5) for which MCh∝Ye2 is reduced to ≲1 M ⊙. Such physical conditions could arise in the black hole accretion disks formed from the collapse of rapidly rotating stars (“collapsars”), as a result of gravitational instabilities and cooling-induced fragmentation, similar to models for planet formation in protostellar disks. We confirm that the conditions to form subsolar-mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNSs, their subsequent coalescence offers a channel for precipitating subsolar mass LIGO/Virgo gravitational-wave mergers that does not implicate primordial black holes. The model makes several additional predictions: (1) ∼Hz frequency Doppler modulation of the ssNS-merger gravitational-wave signals due to the binary’s orbital motion in the disk; (2) at least one additional gravitational-wave event (coincident within ≲hours), from the coalescence of the ssNS-merger remnant(s) with the central black hole; (3) an associated gamma-ray burst and supernova counterpart, the latter boosted in energy and enriched with r-process elements from the NS merger(s) embedded within the exploding stellar envelope (“kilonovae inside a supernova”).

Impact of the hot inner crust on compact stars at finite temperature

We conducted a study on the thermal properties of stellar matter with the nuclear energy density functional BCPM. This functional is based on microscopic Brueckner–Hartree–Fock calculations and has demonstrated success in describing cold neutron stars. To enhance its applicability in astrophysics, we extended the BCPM equation of state to finite temperature for β-stable neutrino-free matter, taking into consideration the hot inner crust. Such an equation of state holds significant importance for hot compact objects, particularly those resulting from a binary neutron star merger event. Our exploration has shown that with increasing temperature, there is a fast decrease in the crust-core transition density, suggesting that for hot stars it is not realistic to assume a fixed value of this density. The microscopic calculations also reveal that the presence of nuclear clusters persists up to T = 7.21 MeV, identified as the limiting temperature of the crust. Above this threshold, the manifestation of clusters is not anticipated. Below this temperature, clusters within the inner crust are surrounded by uniform matter with varying densities, allowing for the distinction between the upper and lower transition density branches. Moreover, we computed mass–radius relations of neutron stars, assuming an isothermal profile for β-stable neutron star matter at various temperature values. Our findings highlight the significant influence of the hot inner crust on the mass–radius relationship, leading to the formation of larger and more inflated neutron stars. Consequently, under our prescription, the final outcome is a unified equation of state at finite temperature.

Revisiting the Constraint on the Equation of State of Neutron Stars Based on Binary Neutron Star Mergers

The merger of a neutron star (NS)–NS binary can form different productions of compact remnants, among which a supramassive NS (SMNS) can create an internal plateau, and the following steep decay marks the collapse of the SMNS. The proportion of the SMNS and the corresponding collapse time are often used to constrain the NS equation of state (EOS). This paper revisits this topic by considering the effect of an accretion disk on a compact remnant, which is not considered in previous works. Compared with previous works, the collapse-time distribution (peaks ∼100 s) of SMNSs formed from an NS–NS merger is almost unaffected by the initial surface magnetic field (B s,i ) of the NS, but the total energy output of the magnetic dipole radiation from the SMNSs depends on B s,i significantly. Coupling the constraints from the SMNS fraction, we exclude some EOSs and obtain three candidate EOSs, i.e., DD2, ENG, and MPA1. By comparing the distributions of the collapse time and the luminosity of the internal plateau (in the short gamma-ray bursts) for observations obtained based on the three candidate EOSs, it is shown that only the EOS of ENG is favored. Our sample, based on the ENG EOS and a mass distribution motivated by Galactic systems, suggests that approximately 99% of NS–NS mergers collapse to form a black hole (BH) within 107s. This includes scenarios promptly forming a BH (36.5%), an SMNS (60.7%), or a stable NS that transitions into a BH or an SMNS following accretion (2.1%). It also indicates that the remnants for GW170817 and GW190425, and the second object of GW190814, are more likely to be BHs.

Normal oscillation modes and radial stability of neutron stars with a dark-energy core from the Chaplygin gas

Normal oscillation modes and radial stability of neutron stars with a dark-energy core from the Chaplygin gas

Neutron stars in the Witten-Sakai-Sugimoto model

We utilize the top-down holographic QCD model, the Witten-Sakai-Sugimoto model, in a hybrid setting with the SLy4, soft chiral EFT and stiff chiral EFT equations of state to describe neutron stars with high precision. In particular, we employ a calibration that bootstraps the nuclear matter by fitting the Kaluza-Klein scale and the ’t Hooft coupling such that the physical saturation density and physical symmetry energy are achieved. We obtain static stable neutron star mass-radius data via the Tolman-Oppenheimer-Volkov equations that yield sufficiently large maximal masses of neutron stars to be compatible with the recently observed PSR-J0952-0607 data as well as all other known radius and tidal deformation constraints.

Neutron star mass in dark matter clumps

ABSTRACT This paper investigates a hypothesis proposed in previous research relating neutron star (NS) mass and its dark matter (DM) accumulation. As DM accumulates, NS mass decreases, predicting lower NS masses toward the Galactic centre. Due to limited NSs data near the Galactic centre, we examine NSs located within DM clumps. Using the CLUMPY code simulations, we determine the DM clumps distribution, with masses from 10 to 108 M⊙ and scales from 10−3 to 10 kpc. These clumps’ DM exhibit a peak at the centre, tapering toward the outskirts, resembling our Galaxy’s DM distribution. We analyse these DM clumps’ NS mass variations, considering diverse DM particle masses and galaxy types. We find relatively stable NS mass within 0.01 – 5 kpc from the clump centre. This stability supports the initial hypothesis, particularly for NSs located beyond 0.01 kpc from the clump centre, where NS mass reaches a plateau around 0.1 kpc. Nevertheless, NS mass near the clump’s periphery reveals spatial dependence: NS position within DM clumps influences its mass in Milky Way-type galaxies. Moreover, this dependence varies with the DM model considered. In summary, our study investigates the proposed link between NS mass and DM accumulation by examining NSs within DM clumps. While NS mass remains stable at certain distances from the clump centre, spatial dependencies arise near the clump’s outer regions, contingent on the specific DM model.

Constraints on Strong Phase Transitions in Neutron Stars

We study current bounds on strong first-order phase transitions (PTs) along the equation of state (EOS) of dense strongly interacting matter in neutron stars, under the simplifying assumption that on either side of the PT, the EOS can be approximated by a simple polytropic form. We construct a large ensemble of possible EOSs of this form, anchor them to chiral effective field theory calculations at nuclear density and perturbative Quantum Chromodynamics at high densities, and subject them to astrophysical constraints from high-mass pulsars and gravitational-wave observations. Within this setup, we find that a PT permits neutron-star solutions with larger radii, but only if the transition begins below twice nuclear saturation density. We also identify a large parameter space of allowed PTs currently unexplored by numerical-relativity studies. Additionally, we locate a small region of parameter space allowing twin-star solutions, though we find them to only marginally pass the current astrophysical constraints. Finally, we find that sizeable cores of high-density matter beyond the PT may be located in the centers of some stable neutron stars, primarily those with larger masses.

Axial and polar stability of neutron stars in scalar-tensor theories with disformal coupling

Axial and polar stability of neutron stars in scalar-tensor theories with disformal coupling

Neutron Star Binaries Produced by Binary-Driven Hypernovae, Their Mergers, and the Link between Long and Short GRBs

The binary-driven hypernova (BdHN) model explains long gamma-ray bursts (GRBs) associated with supernovae (SNe) Ic through physical episodes that occur in a binary composed of a carbon-oxygen (CO) star and a neutron star (NS) companion in close orbit. The CO core collapse triggers the cataclysmic event, originating the SN and a newborn NS (hereafter νNS) at its center. The νNS and the NS accrete SN matter. BdHNe are classified based on the NS companion fate and the GRB energetics, mainly determined by the orbital period. In BdHNe I, the orbital period is of a few minutes, so the accretion causes the NS to collapse into a Kerr black hole (BH), explaining GRBs of energies &gt;1052 erg. BdHN II, with longer periods of tens of minutes, yields a more massive but stable NS, accounting for GRBs of 1050–1052 erg. BdHNe III have still longer orbital periods (e.g., hours), so the NS companion has a negligible role, which explains GRBs with a lower energy release of &lt;1050 erg. BdHN I and II might remain bound after the SN, so they could form NS-BH and binary NS (BNS), respectively. In BdHN III, the SN likely disrupts the system. We perform numerical simulations of BdHN II to compute the characteristic parameters of the BNS left by them, their mergers, and the associated short GRBs. We obtain the mass of the central remnant, whether it is likely to be a massive NS or a BH, the conditions for disk formation and its mass, and the event’s energy release. The role of the NS nuclear equation of state is outlined.

Stability of neutron stars with dark matter core using three crustal types and the impact on mass–radius relations

We investigate the effects of dark matter (DM) on the nuclear equation of state (EoS) and neutron star structure, in the relativistic mean field theory, both in the absence and presence of a crust. The σ−ω model is modified by adding a WIMP-DM component, which interacts with nucleonic matter through the Higgs portal. This model agrees well with previous studies which utilized either a more complicated nuclear model or higher-order terms of the Higgs potential, in that DM softens the EoS, resulting in stars with lower maximum masses. However, instabilities corresponding to negative pressure values in the low-energy density regime of the DM-admixed EoS are present, and this effect becomes more prominent as we increase the DM Fermi momentum. We resolve this by confining DM in the star’s core. The regions of instability were replaced by three types of crust: first by the Friedman-Pandharipande-Skyrme (FPS), Skyrme-Lyon (SLy) and BSk19 EoS from the Brussels-Montreal Group, which can be represented by analytical approximations. For a fixed value of the DM Fermi momentum pFDM, the DM-admixed neutron star does not have significant changes in its mass with the addition of the crusts. However, the entire mass–radius relation of the neutron star is significantly affected, with an observed increase in the radius of the star corresponding to the mass. The effect of DM is to reduce the mass of the star, while the crust does not affect the radius significantly, as the value of the pFDM increases.

Spontaneous scalarization in proto-neutron stars

Proto-neutron stars are born when a highly evolved and massive star collapses under gravity. In this paper, we investigate the spontaneous scalarization in proto-neutron stars. Based on the scalar tensor theory of gravity as well as the physical conditions in proto-neutron star, we examine the structure of proto-neutron star. To describe the fluid in proto-neutron star, we utilize SU(2) chiral sigma model and the finite temperature extension of the Brueckner–Bethe–Goldstone quantum many-body theory in the Brueckner–Hartree–Fock approximation. Here, we apply the equation of state of proto-neutron stars considering different cases i.e. hot pure neutron matter and hot beta -stable neutron star matter without neutrino trapping as well as with neutrino trapping. The effects of temperature and entropy of proto-neutron stars on the star structure are also studied. Our results confirm that the spontaneous scalarization is affected by different physical conditions in proto-neutron stars.

Long-range Multiparticle Interactions Induced by Neutrino Exchange in Neutron Star Matter

Forces with a large radius of interaction can have a significant impact on the equation of state of matter. Low-mass neutrinos generate a long-range potential due to the exchange of neutrino pairs. We discuss a possible relationship between the neutrino masses, which determine the interaction radius of the neutrino-pair exchange potential, and the equation of state of neutron matter. Contrary to previous statements, the thermodynamic potential, when decomposed into the number of neutrino interactions, vanishes in any decomposition order, except for the interaction of two neutrons. In the one-loop approximation, long-range multiparticle neutrino interactions are stable in the infrared region for all neutrino masses and do not affect the equation of state of neutron matter or the stability of neutron stars.

Stability of neutron stars in Horndeski theories with Gauss-Bonnet couplings

In Horndeski theories containing a scalar coupling with the Gauss-Bonnet (GB) curvature invariant ${R}_{\mathrm{GB}}^{2}$, we study the existence and linear stability of neutron star (NS) solutions on a static and spherically symmetric background. For a scalar-GB coupling of the form $\ensuremath{\alpha}\ensuremath{\xi}(\ensuremath{\phi}){R}_{\mathrm{GB}}^{2}$, where $\ensuremath{\xi}$ is a function of the scalar field $\ensuremath{\phi}$, the existence of linearly stable stars with a nontrivial scalar profile without instabilities puts an upper bound on the strength of the dimensionless coupling constant $|\ensuremath{\alpha}|$. To realize maximum masses of NSs for a linear (or dilatonic) GB coupling ${\ensuremath{\alpha}}_{\mathrm{GB}}\ensuremath{\phi}{R}_{\mathrm{GB}}^{2}$ with typical nuclear equations of state, we obtain the theoretical upper limit $\sqrt{|{\ensuremath{\alpha}}_{\mathrm{GB}}|}&lt;0.7\text{ }\text{ }\mathrm{km}$. This is tighter than those obtained by the observations of gravitational waves emitted from binaries containing NSs. We also incorporate cubic-order scalar derivative interactions, quartic derivative couplings with nonminimal couplings to a Ricci scalar besides the scalar-GB coupling, and show that NS solutions with a nontrivial scalar profile satisfying all the linear stability conditions are present for certain ranges of the coupling constants. In regularized four-dimensional Einstein-GB gravity obtained from a Kaluza-Klein reduction with an appropriate rescaling of the GB coupling constant, we find that NSs in this theory suffer from a strong coupling problem as well as Laplacian instability of even-parity perturbations. We also study NS solutions with a nontrivial scalar profile in power-law $F({R}_{\mathrm{GB}}^{2})$ models, and show that they are pathological in the interior of stars and plagued by ghost instability together with the asymptotic strong coupling problem in the exterior of stars.

Neutron star stability with equations of state breaking the conformal QCD limit

We explore the structural stability of neutron stars against radial pulsations obtained by a set of equations of state calculated from successive interpolations between perturbative QCD at high densities and chiral effective field theory at low densities being constrained also by observational data at intermediate densities. In order to do this, we solve a pair of first-order coupled differential equations equivalent to the original Sturm-Liouville problem to obtain the zero-mode frequencies characterizing stable stars. Besides, we study the effects of mild and large breakings of the conformal limit put for the speed of sound, , in stars that can possibly contain cold quark matter at their cores. We find that some neutron-star families display an unusual humps in the mass-radius diagram which are associated to large non-linear oscillation amplitudes near the core.

Relativistic mean field model parametrizations in the light of GW170817, GW190814, and PSR J0740+6620

Three parametrizations DOPS1, DOPS2, and DOPS3 (named after the Department of Physics Shimla) of the relativistic mean field model have been proposed with the inclusion of all possible self and mixed interactions between the scalar-isoscalar ($\ensuremath{\sigma}$), vector-isoscalar ($\ensuremath{\omega}$), and vector-isovector ($\ensuremath{\rho}$) mesons up to quartic order. The generated parameter sets are in harmony with the finite and bulk nuclear matter properties. A set of equations of state (EOSs) composed of pure hadronic (nucleonic) matter and nucleonic with quark matter (hybrid EOSs) for superdense hadron-quark matter in $\ensuremath{\beta}$ equilibrium is obtained. The quark matter phase is calculated by using the three-flavor Nambu-Jona-Lasinio (NJL) model. The maximum mass of a nonrotating neutron star with DOPS1 parametrization is found to be around $2.6M\ensuremath{\bigodot}$ for the pure nucleonic matter, which satisfies the recent gravitational wave analysis of GW190814 [Abbott et al., Astrophys. J. Lett. 896, L44 (2020)] with possible maximum mass constraint indicating that the secondary component of GW190814 could be a nonrotating heaviest neutron star composed of pure nucleonic matter. EOSs computed with the DOPS2 and DOPS3 parametrizations satisfy the x-ray observational data [Steiner et al., Astrophys. J. 722, 33 (2010)] and the recent observations of GW170817 maximum mass constraint of a stable nonrotating neutron star in the range $2.01\ifmmode\pm\else\textpm\fi{}0.04--2.16\ifmmode\pm\else\textpm\fi{}0.03M\ensuremath{\bigodot}$ [Rezzolla et al., Astrophys. J. Lett. 852, L25 (2018)] and also in good agreement with constraints on mass and radius measurement for PSR $\mathrm{J}0740+6620$ (NICER) [Riley et al., Astrophys. J. Lett. 918, L27 (2021); Miller et al., Astrophys. J. Lett. 918, L28 (2021)]. The hybrid EOSs obtained with the NJL model also satisfy astrophysical constraints on the maximum mass of a neutron star from PSR J1614-2230 [Demorest et al., Nature (London) 467, 1081 (2010)]. We also present the results for dimensionless tidal deformability, $\mathrm{\ensuremath{\Lambda}}$ which are consistent with the waveform models analysis of GW170817.

Editorial

Editorial

Antony Hewish. 11 May 1924—13 September 2021

Antony (Tony) Hewish was a pioneer radio astronomer who will always be remembered as the leader of the team in 1967 that discovered the pulsars, which proved to be rapidly rotating, magnetized neutron stars. The discovery resulted from Tony's programme of systematic all-sky surveys to detect the scintillation, or flickering, of small angular diameter radio sources due to electron density fluctuations in the solar wind flowing out from the Sun. The large low-frequency 4.5-acre array was designed by Tony to find radio quasars, which often display radio scintillations, to estimate the angular sizes of the sources and to study the physics of the interplanetary medium. In the course of commissioning the telescope, his research student, Jocelyn Bell (Jocelyn Bell Burnell, FRS 2003), noted a strange 100% scintillating source unlike anything seen before. Tony and the team soon established that this source was a pulsating radio source, Jocelyn first observing the pulsations with period 1.33 s in November 1967. The discovery paved the way for the rapid development of high energy astrophysics and an appreciation that general relativity plays a key role in the stability of neutron stars. Tony's contributions spanned a very wide range of pioneering studies in the new discipline of radio astronomy, including telescope and electronic design, cosmological studies of distant radio sources and the physics of the ionospheric, interplanetary and interstellar plasmas. He was awarded the 1974 Nobel Prize in physics for ‘his decisive role in the discovery of pulsars’.

Fate of twin stars on the unstable branch: Implications for the formation of twin stars

Hybrid hadron-quark equations of state that give rise a third family of stable compact stars have been shown to be compatible with the LIGO-Virgo event GW170817. Stable configurations in the third family are called hybrid hadron-quark stars. The equilibrium stable hybrid hadron-quark star branch is separated by the stable neutron star branch with a branch of unstable hybrid hadron-quark stars. The end-state of these unstable configurations has not been studied, yet, and it could have implications for the formation and existence of twin stars -- hybrid stars with the same mass as neutron stars but different radii. We modify existing hybrid hadron-quark equations of state with a first-order phase transition in order to guarantee a well-posed initial value problem of the equations of general relativistic hydrodynamics, and study the dynamics of non-rotating or rotating unstable twin stars via 3-dimensional simulations in full general relativity. We find that unstable twin stars naturally migrate toward the hadronic branch. Before settling into the hadronic regime, these stars undergo (quasi)radial oscillations on a dynamical timescale while the core bounces between the two phases. Our study suggests that it may be difficult to form stable twin stars if the phase transition is sustained over a large jump in energy density, and hence it may be more likely that astrophysical hybrid hadron-quark stars have masses above the twin star regime. We also study the minimum-mass instability for hybrid stars, and find that these configurations do not explode, unlike the minimum-mass instability for neutron stars. Additionally, our results suggest that oscillations between the two Quantum Chromodynamic phases could provide gravitational wave signals associated with such phase transitions in core-collapse supernovae and white dwarf-neutron star mergers.

Stability of Neutron Stars with Dark Matter Core Using Three Crustal Types and the Impact on Mass-Radius Relations

Stability of Neutron Stars with Dark Matter Core Using Three Crustal Types and the Impact on Mass-Radius Relations

Survival Times of Supramassive Neutron Stars Resulting from Binary Neutron Star Mergers

Abstract A binary neutron star (BNS) merger can lead to various outcomes, from indefinitely stable neutron stars, through supramassive neutron stars (SMNSs) or hypermassive neutron stars supported only temporarily against gravity, to black holes formed promptly after the merger. Up-to-date constraints on the BNS total mass and the neutron star equation of state suggest that a long-lived SMNS may form in ∼0.45–0.9 of BNS mergers. A maximally rotating SMNS needs to lose ∼(3–6) × 1052 erg of its rotational energy before it collapses, on a fraction of the spin-down timescale. An SMNS formation imprints on the electromagnetic counterparts to the BNS merger. However, a comparison with observations reveals tensions. First, the distribution of collapse times is too wide and that of released energies too narrow (and the energy itself too large) to explain the observed distributions of internal X-ray plateaus, invoked as evidence for SMNS-powered energy injection. Second, the immense energy injection into the blast wave should lead to extremely bright radio transients, which previous studies found to be inconsistent with deep radio observations of short gamma-ray bursts (GRBs). Furthermore, we show that upcoming all-sky radio surveys will constrain the extracted energy distribution, independently of a GRB jet formation. Our results can be self-consistently understood, provided that most BNS merger remnants collapse shortly after formation (even if their masses are low enough to allow for SMNS formation). This naturally occurs if the remnant retains half or less of its initial energy by the time it enters solid-body rotation.