Neutron dark decay in neutron stars: The role of the symmetry energy

Gravitational wave interferometers have studied compact object mergers and solidified our understanding of strong gravity. Their increasing precision raises the possibility of detecting new physics, especially in a neutron star binary system that may contain hidden-sector particles. In particular, a new vector force between binary constituents, giving rise to dark electromagnetic phenomena, could measurably alter the inspiral waveforms and thus be constrained by gravitational wave observations. In this work, we critically examine the mechanisms for neutron stars to acquire enough hidden-sector particles with requisite couplings to furnish a detectable signature from dark electromagnetism. In particular, we explore three plausible mechanisms for achieving a sufficient dark mass fraction, and we derive quite general limitations on the amount of charge that can be accumulated by any of these mechanisms. We demonstrate that the repulsive nature of vector forces imposes stringent constraints on any putative particle physics model or astrophysical environment which could give rise to such gravitational signatures. We argue that absent an extreme fine-tuning of parameters, such signatures are well out of reach of any current or near-future gravitational wave observatory.

We perform a comparative Bayesian analysis of fermionic and bosonic dark matter admixed neutron stars (DMANS) by incorporating a comprehensive set of theoretical, experimental, and astrophysical constraints. The hadronic matter equation of state (EoS) is modeled using a relativistic mean-field approach, constrained by chiral effective field theory (χEFT) calculations at low densities, finite nuclei and heavy-ion collision data at intermediate densities, and neutron star (NS) observations at high densities. For the dark sector, we consider fermionic dark matter (FDM) interacting via a dark vector meson, and two bosonic dark matter models (BDM1 and BDM2) characterized by self-interacting scalar fields. Bayesian inference is employed to constrain the model parameters, including the dark matter mass, coupling strength, and dark matter fraction within NSs. Our analysis finds that all models yield consistent nuclear matter parameters, allowing a small dark matter fraction under 10%. The presence of dark matter slightly softens the EoS, leading to a modest reduction in NS mass, radius, and tidal deformability, though all models remain compatible with NICER and GW170817 observations. The log-evidence and likelihood analyses reveal no statistical preference among the FDM and BDM models, indicating that current astrophysical data cannot decisively distinguish between fermionic and bosonic dark matter scenarios. This study provides a unified statistical framework to constrain dark matter properties using NS observables.

The 2024 outburst of the neutron star LMXB EXO 0748–676: An investigation of bursts and eclipses with astrosat

Tidal deformability and compactness of neutron stars and massive pulsars from semi-microscopic equations of state

The role of nuclear symmetry energy and neutron skin thickness of 208Pb in controlling the underlying physics of neutron star

Black hole X-ray binaries (BH XRBs) launch powerful relativistic jets during bright outburst phases. The properties of these outflows change dramatically between different spectral-accretion states. Compact jets are observed during the hard state and are quenched during the soft state, while discrete ejecta are mainly launched during the hard-to-soft state transition. Currently, we do not understand what triggers the formation and/or destruction of compact jets or the launch of discrete ejecta. In this context, finding a unique link between the jet evolution and the properties of the X-ray emission, such as its fast variability, would imply major progress in our understanding of the fundamental mechanisms that drive relativistic outflows in BH XRBs. Here we show that a brief, strong radio re-brightening during a predominantly soft state of the BH XRB MAXI J1348–630 was contemporaneous with a significant increase in the X-ray root-mean-square (rms) variability observed with the Neutron Star Interior Composition Explorer (NICER) in 2019. During this phase, the variability displayed significant changes and, at the same time, MAXI J1348–630 launched two relativistic discrete ejecta that we detected with the MeerKAT and ATCA radio-interferometers. We propose that short-lived compact jets were reactivated during this excursion to the hard-intermediate state and were switched off before the ejecta launch, a behavior that has been very rarely observed in these systems. Interestingly, with the caveat of gaps in our radio and X-ray coverage, we suggest a tentative correspondence between the launch of ejecta and the drop in X-ray rms variability in this source, while other typical X-ray signatures associated with discrete ejections are not detected. We discuss how these results provide us with insights into the complex dynamic coupling between the jets and hot corona in BH XRBs.

Stellar winds in high-mass X-ray binaries (HMXBs) are strongly modified by the presence of an accreting neutron star, yet the impact of X-ray photoionisation on the wind–acceleration profile remains difficult to quantify observationally. In this work, we combine the measured wind velocities at the neutron star orbital radius with spectroscopic terminal velocities in order to infer empirical β-law parameters for six well-studied HMXBs. By inverting the β-law, we reconstruct the individual acceleration curves v(r) and obtain revised estimates of the wind-acceleration parameters b and β for each system. A suggestive trend emerges from the reconstructed profiles: systems with lower terminal velocities tend to exhibit systematically larger acceleration indices β, consistent with the interpretation that dense, slowly accelerating winds may be more strongly affected by X-ray photoionisation. A secondary, weaker pattern is suggested between the orbital separation a/R* and β, although, for our small sample, it is not statistically significant, suggesting that compact systems experience a more pronounced suppression of wind acceleration in the vicinity of the neutron star. Taken together, these indicative relations provide a coherent observational picture linking the global wind–velocity scale to the local radiative environment. The resulting acceleration profiles and system-to-system correlations offer a practical empirical foundation for modelling wind-fed accretion in HMXBs. The parameter set derived here can be directly incorporated into studies of quasi-spherical accretion, torque evolution, and the dynamical influence of X-ray photoionisation in massive binaries.

Abstract The Kelvin–Helmholtz (KH) instability occurs in multiple heliospheric (solar-wind stream interfaces, planetary magnetospheres, cometary tails, heliopause flanks) and interstellar (protoplanetary disks, relativistic jets, neutron star accretion disks) environments. While the KH instability has been well studied in the magnetohydrodynamic (MHD) limit, only limited studies were performed in the collisionless regime, which is conducive to the development of anisotropic pressures. Collisionless plasmas are often described using the Chew, Goldberger, and Low (CGL) equations, which feature an anisotropic pressure tensor. This paper presents a comprehensive analysis of the CGL version of the KH instability using linearized and numerical techniques. We find that the largest growth rates and the greatest incidence of magnetic effects occur in the MHD limit. In the large relaxation time CGL limit, part of the energy goes into the formation of pressure anisotropies, resulting in smaller amounts of energy being available for bending the field lines. Consequently, when we cross-compare CGL and MHD simulations that are otherwise identical, the current densities are largest in the MHD limit, and the largest magnetic islands also form in that limit. Early time and late-time formations of pressure anisotropies have also been studied. We also find that the strongest trend for forming intermittencies in the flow also occurs in the MHD limit. The paper also discusses possible consequences of our results for turbulence and reconnection in the heliosheath (the layer between the solar wind termination shock and the heliopause).

Abstract Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are believed to be manifestations of magnetars. Typically, AXPs exhibit higher X-ray luminosities, whereas SGRs are generally fainter and display significantly high signal-to-noise ratios only during their outburst phases. In this work, we report the long-term timing evolution of four AXPs: 1E 2259+586, 4U 0142+61, 1RXS J170849.0–400910, and 1E 1841-045, which were regularly monitored with Neutron Star Interior Composition Explorer from 2017 to 2024. Over this period, we identify a total of 10 timing events. In addition to one glitch and one antiglitch in 1E 2259+586 reported in the literature, we detect another eight new timing events: five glitches, two antiglitches, and one unusual state transition event. Notably, both antiglitches were observed in 4U 0142+61, making it the most frequent source of such events, and there is a hint of regular evolution in its pulse profile. In the case of 1RXS J170849.0–400910, it continues to exhibit pronounced high-frequency timing anomalies and undergoes a state transition event. Finally, we study the evolution of the pulse profiles and find that the profiles of 1E 2259+586 and 4U 0142+61 both evolve. This is consistent with the earlier finding that pulse profile evolution is a generic feature of magnetars.

The symmetry energy is a key quantity for the structure of finite nuclei and the bulk properties of neutron stars. Therefore, its investigation has special significance in nuclear astrophysics, especially given the uncertainty that presents in the high density region and the large error in data from corresponding experiments. A way to get an indication about the behavior of symmetry energy in high densities is to examine it in the context of neutron stars. The recent observations of gravitational waves emitted from binary neutron star mergers provide useful information on characteristics such as the radius and the tidal deformability, i.e. two quantities that are in direct relation to the symmetry energy. Our work aims to examine the symmetry energy under this point of view and specifically obtain constraints on the structure of finite nuclei. In this effort, we deploy a methodology that is based on parameterization of the equation of state of asymmetric and symmetric nuclear matter through the introduction of a parameter called η=(K0L2)1/3, which combines the incompressibility K0 and the slope parameter L. In fact, the parameter η serves as a regulator of the stiffness of the equation of state. This quantity affects both the properties of finite nuclei and the properties of neutron stars, where the isovector interaction plays a significant role. Hence, we expect that the obtained constraints, through the values of η, will provide insights on the properties of neutron stars and finite nuclei vice versa. Our investigation proposes a simple and self-consistent method to examine the effects of η on both kind of properties, which led us to derive constraints on the latter systems by using recent experiments (PREX-2) and astrophysical observables (observations from LIGO/VIRGO collaboration).

Recent measurements of the compact star XTE J1814-338, with a mass of M=1.2-0.05+0.05 Mø and a radius of R=7-0.4+0.4 Km alongside those of HESS J1731-347, which has a mass of M=0.77-0.17+0.20 Mø and a radius of R=10.4-0.78+0.86 Km, provide compelling evidence for the potential existence of exotic matter in neutron star cores. These observations offer important insights into the equation of state of dense nuclear matter. In this study, we explore the presence of negatively charged kaons and neutral anti-kaons (K- and \bar{K0}) within neutron stars (NSs) using a Relativistic Mean Field (RMF) model with first order kaon condensate. To our knowledge, this is the first alternative approach aiming to simultaneously explain the observed properties of both XTE J1814-338 and HESS J1731-347 by invoking kaon condensation in dense matter. Furthermore, we compare our model with recent data from the pulsars PSR J0437-4715 andPSR J1231-1411, and argue that a two-branch scenario, each representing a distinct form of nuclear matter, may be necessary to account for the diverse range of compact astrophysical objects.

Insights into the properties of dense, neutron-rich matter emerge from the interplay between nuclear experiments and astrophysical observations. Measurements of parity-violating electron scattering on 48Ca (CREX) and 208Pb (PREX-2), together with electric dipole polarizability data, offer stringent probes of isovector dynamics in nuclei. In this study, a set of relativistic energy density functionals is employed to investigate how these nuclear signatures correlate with neutron star observables, such as stellar radii and tidal deformabilities. By confronting the theoretical predictions with data from both terrestrial experiments and multimessenger observations—including the gravitational wave event GW170817—constraints are derived on the symmetry energy and the high-density behavior of the equation of state. The analysis highlights the influence of including the fourth-order term in the isospin-asymmetry expansion of the energy density on neutron star radius and tidal deformability predictions. At the same time, discrepancies between constraints from CREX and PREX-2 underscore the need for improved experimental precision and additional astrophysical input to refine our understanding of dense matter.

Impact of crust-core connection procedures on the tidal deformability of neutron stars

Notoriously hard to detect and study, isolated neutron stars (NSs) might provide valuable answers to fundamental questions about stellar evolution and explosion physics. With the upcoming Space Telescope, scheduled for launch in 2026, a new and powerful channel for their detection will become available: astrometric microlensing. Roman We set out to create a realistic sample of simulated gravitational microlensing events as observed by with the Galactic Bulge Time Domain Survey. We focus in particular on the population of NS lenses, which has until now been largely understudied. Roman We used dedicated Galactic models tailored for application to microlensing by compact objects. In addition to populations of stars, white dwarfs, and black holes, we simulated four different NS populations with Maxwellian natal kick distributions: bar v = (150, 250, 350, 450) km/s. For each simulation, we applied projected precision, cadence, and detectability criteria. Roman We found that the parameter space log_ 10 t_ ̊m E -- log_ 10 ̊m E , which will be accessible to observations, is efficient for the classification of stellar remnants. We found a feature in this space that is characteristic of NSs; using this feature, optimal samples of NS candidates can be constructed from -like datasets. We describe the dependence of the observable parameter distributions on the assumed mean kick velocities. As the effects of natal kicks are very complex and mutually counteracting, we suggest that more detailed studies focused on the dynamics of NSs are needed in anticipation of and future surveys. We estimate that will observe approximately 11000 microlensing events, including ∼100 with NS lenses, whose photometric and astrometric signals are detectable; the event yield decreases by 38% when gap-filling low-cadence observations are not included. We make all simulated microlensing event datasets publicly available in preparation for ̊omanst data. Roman Roman Roman Roman

Abstract Light-mass nuclear reactions can play a significant role in r -process nucleosynthesis in core-collapse supernovae (SNe) and collapsars. We investigate the sensitivity of both weak and main r -process nucleosyntheses, which are responsible for the production of the first and second plus third r -process peak nuclei, respectively, to the light-mass nuclear reactions up to oxygen isotopes. We extend the reaction network to include more neutron-rich isotopes and update reaction rates using recent experimental results. For the explosion mechanisms, the sensitivity studies have previously been done in ν -driven wind SNe assuming relatively high initial neutron-to-proton ratio with high entropy. We here consider magnetohydrodynamic jet SNe and collapsars in addition to the ν -driven winds. We use a simple exponential decay model for slowly expanding disk outflows, which manifest themselves in various disk winds from collapsars, magnetohydrodynamic jets, binary neutron star mergers, as well as ν -driven winds. We study the competition between the dynamical expansion timescale of the system and the collision timescales for various nuclear reactions and β- decays in order to identify the main reaction flow paths. We find the sensitivities of the main r -process to 14 C( n , γ ) 15 C, 18 O( n , γ ) 19 O, and 8 Li( α , n ) 11 B, which are as large as 16%, 24%, and 0.6%, respectively, for the magenetohydrodynamic jet SN environment, and for the weak r -process to ( n , γ ) and ( α , n ) reactions, which depend on different astrophysical sites.

Chiral symmetry restoration and hyperon suppression in neutron stars

Binaries in the cores of globular clusters are known to prevent the gravitational collapse of the cluster, and simulations predict that the core of NGC 6397 contains a high number of white dwarfs (WDs), of which many are expected to be part of a binary system. In this work, we report the discovery of a compact binary system consisting of two WDs in the centre of the Galactic globular cluster NGC 6397. The system, known in the literature as NF1, was observed as part of a MUSE radial-velocity survey aiming at characterizing the binary population in the centre of NGC 6397. The spectral analysis of NF1 provides an effective temperature of 16,000 K and a surface gravity (log g) of 5.72 (cgs), which is consistent with the characteristics of an extremely low-mass He-core WD. This is further supported by the mass of 0.23 ± 0.03 obtained from fitting the star's spectral energy distribution using its HST magnitude in various filters. The system has a circular orbit with a period of 0.54 days. The radial velocities show a large semi-amplitude of 200 km/s, implying a minimum mass of 0.78 for the invisible companion, which is likely another WD, or a neutron star if the inclination of the system is smaller than about 50̧irc. Some significant residuals in radial velocity remain with our best orbital solution, and we tested whether a model with a third body can explain these deviations. While this possibility seems promising, additional measurements are needed to confirm whether the star is actually part of a triple system.

Scalar and vector dark matter admixed neutron stars with linear and quadratic couplings

Abstract We search for nearly monochromatic gravitational-wave signals with frequencies 800.0 Hz ≤ f ≤ 1686.0 Hz and spin-down − 2.7 × 1 0 − 9 Hz s − 1 ≤ f ̇ ≤ 0.2 × 1 0 − 9 Hz s − 1 . We use LIGO O3 public data from the Hanford and Livingston detectors, and deploy this search on the Einstein@Home volunteer-computing project. This is the most sensitive search carried out to date in this parameter space. Our results are consistent with a nondetection. We set upper limits on the gravitational-wave amplitude h 0 and translate these to upper limits on neutron star ellipticity and on r -mode amplitude. The most stringent upper limits are at 800 Hz, with h 0 = 1.32 × 10 −25 at the 90% confidence level. Searching in the high-frequency bands allows us to probe astrophysically interesting ellipticities with our results, excluding isolated neutron stars rotating faster than 2.5 ms with ellipticities ϵ ≥ 1.96 × 1 0 − 8 d 100 pc within a distance d from Earth. Our results also exclude r -mode amplitudes α ≥ 7 × 1 0 − 7 d 100 pc for neutron stars spinning faster than 400 Hz.