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Neutron stars provide a unique opportunity to study strongly interacting matter under extreme density conditions. The intricacies of matter inside neutron stars and their equation of state are not directly visible, but determine bulk properties, such as mass and radius, which affect the star's thermal X-ray emissions. However, the telescope spectra of these emissions are also affected by the stellar distance, hydrogen column, and effective surface temperature, which are not always well-constrained. Uncertainties on these nuisance parameters must be accounted for when making a robust estimation of the equation of state. In this study, we develop a novel methodology that, for the first time, can infer the full posterior distribution of both the equation of state and nuisance parameters directly from telescope observations. This method relies on the use of neural likelihood estimation, in which normalizing flows use samples of simulated telescope data to learn the likelihood of the neutron star spectra as a function of these parameters, coupled with Hamiltonian Monte Carlo methods to efficiently sample from the corresponding posterior distribution. Our approach surpasses the accuracy of previous methods, improves the interpretability of the results by providing access to the full posterior distribution, and naturally scales to a growing number of neutron star observations expected in the coming years.

Recent constraints on neutron star mass and radius have advanced our understanding of the equation of state (EOS) of cold dense matter. Some of them have been obtained by modeling the pulses of three millisecond X-ray pulsars observed by the Neutron Star Interior Composition Explorer (NICER). Here, we present a Bayesian parameter inference for a fourth pulsar, PSR J1231−1411, using the same technique with NICER and XMM-Newton data. When applying a broad mass-inclination prior from radio timing measurements and the emission region geometry model that can best explain the data, we find likely converged results only when using a limited radius prior. If limiting the radius to be consistent with the previous observational constraints and EOS analyses, we infer the radius to be 12.6 ± 0.3 km and the mass to be 1.04 − 0.03 + 0.05 M ⊙, each reported as the posterior credible interval bounded by the 16% and 84% quantiles. If using an uninformative prior but limited between 10 and 14 km, we find otherwise similar results, but R eq = 13.5 − 0.5 + 0.3 km for the radius. In both cases, we find a nonantipodal hot region geometry where one emitting spot is at the equator or slightly above, surrounded by a large colder region, and where a noncircular hot region lies close to southern rotational pole. If using a wider radius prior, we only find solutions that fit the data significantly worse. We discuss the challenges in finding the better fitting solutions, possibly related to the weak interpulse feature in the pulse profile. [ABSTRACT FROM AUTHOR]

Magnetar-like activity has been observed in a large variety of neutron stars. PSR J1846−0258 is a young 327 ms radio-quiet pulsar with a large rotational power (∼8 × 1036 erg s−1), and resides at the center of the supernova remnant Kes 75. It is one of the rare examples of a high-magnetic-field pulsar showing characteristics both of magnetars and radio pulsars, and can thus provide important clues on the differences in the emission mechanisms between these two classes. In 2006, PSR J1846−0258 was detected to undergo an outburst for the first time, accompanied by a large flux increase, millisecond X-ray bursts, significant spectral changes, and a large timing glitch. In the period between 2020 May and June, after 14 yr of quiescent stable emission, the source underwent a second magnetar-like outburst, which was followed up with several observations by Neutron Star Interior Composition Explorer, XMM-Newton, NuSTAR, and Swift. In this work, we report on the long-term timing and X-ray spectral properties of the source following the 2020 outburst, and place upper limits on any source activity at radio wavelengths. We demonstrate that the pulsed flux increased by a factor >6 during the outburst, followed by nontrivial variability in the spin-down rate. Our timing analysis shows that the spin frequency and its derivative are clearly affected by magnetospheric activity due to the outburst. We find hints for an oscillation in the frequency derivative with a timescale of 50–60 days, recovering later on to stable quiescence. [ABSTRACT FROM AUTHOR]

The accretion flow onto a neutron star will be impacted by irradiation from a type I X-ray burst. The burst radiation exerts Poynting–Robertson (PR) drag on the accretion disk, leading to an enhanced mass accretion rate. Observations of X-ray bursts often find evidence that the normalization of the disk-generated persistent emission (commonly denoted by the factor f a ) increases during a burst, and changes in f a have been used to infer the evolution of the mass accretion rate due to PR drag. Here, we examine this proposed relationship between f a and mass accretion rate enhancement using time-resolved data from simulations of accretion disks impacted by type I X-ray bursts. We consider bursts from both spinning and nonspinning neutron stars and track both the change in accretion rate due to PR drag and the disk emission spectra during the burst. Regardless of the neutron star spin, we find that f a strongly correlates with the disk temperature and only weakly follows the mass accretion rate (the Pearson correlation coefficients are ≤0.63 in the latter case). Additionally, heating causes the disk to emit at higher energies, reducing its contribution to a soft excess. We conclude that f a cannot accurately capture the mass accretion rate enhancement and is rather a tracer of the disk temperature. [ABSTRACT FROM AUTHOR]

The electromagnetic emission and the afterglow observations of the binary neutron star merger event GW170817A confirmed the association of the merger with a short gamma-ray burst (GRB) harboring a narrow (5°–10°) and powerful (1049–1050 erg) jet. Using the 1 s long neutrino-radiation general relativistic MHD simulation of coalescing neutron stars of K. Kiuchi et al., and following the semi-analytical estimates of M. Pais et al., we inject a narrow, powerful, unmagnetized jet into the post-merger phase. We explore different opening angles, luminosities, central engine durations, and times after the merger. We explore early (0.1 s following the merger) and late (1 s) jet launches; the latter is consistent with the time delay of ≈1.74 s observed between GW170817 and GRB 170817A. We demonstrate that the semi-analytical estimates correctly predict the jets' breakout and collimation conditions. When comparing our synthetic afterglow light curves to the observed radio data of GW170807, we find a good agreement for a 3 × 1049 erg jet launched late with an opening angle in the range ≃5°–7°. [ABSTRACT FROM AUTHOR]

PSR J0737−3039A/B is unique among double neutron star systems. Its near-perfect edge-on orbit causes the fast spinning pulsar A to be eclipsed by the magnetic field of the slow spinning pulsar B. Using high-sensitivity MeerKAT radio observations combined with updated constraints on the system geometry, we studied the impact of these eclipses on the incident polarization properties of pulsar A. Averaging light curves together after correcting for the rotation of pulsar B revealed enormous amounts of circular polarization and rapid changes in the linear polarization position angle, which occur at phases where emission from pulsar A is partially transmitted through the magnetosphere of pulsar B. These behaviours confirm that the eclipse mechanism is the result of synchrotron absorption in a relativistic pair-plasma confined to the closed-field region of pulsar B's truncated dipolar magnetic field. We demonstrate that changes in circular polarization handedness throughout the eclipses are directly tied to the average line of sight magnetic field direction of pulsar B, from which we unambiguously determine the complete magnetic and viewing geometry of the pulsar. [ABSTRACT FROM AUTHOR]

By applying the Efron–Petrosian method to the fluxes S and distances D of the magnetars listed in the Magnetar Outburst Online Catalogue, we show that the observational data are consistent with the dependence |$S\propto D^{-3/2}$|⁠ , which characterizes the emission from the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star, at substantially higher levels of significance than they are with the dependence |$S\propto D^{-2}$|⁠. This result agrees with that previously obtained by an analysis of the data in the McGill Online Magnetar Catalog and confirms that, contrary to the currently prevalent view, magnetars' X-ray luminosities do not exceed their spin-down luminosities. The X-ray spectra of magnetars, moreover, are congruous with the spectral energy distribution (SED) of a broad-band non-thermal emission mechanism identical to that at play in rotation-powered pulsars: we show that the SED of the caustics that are generated in certain privileged directions by the magnetospheric current sheet single-handedly fits the observed spectra of 4U 0142+61, 1E 1841−045, and XTE J1810−197 over their entire breadths. Magnetars' outbursts and their associated radio bursts are predicted to occur when, as a result of large-scale timing anomalies (such as glitches, quakes, or precession), one of the privileged directions along which the radiation from the current sheet decays more slowly than predicted by the inverse-square law either swings past or oscillates across the line of sight. [ABSTRACT FROM AUTHOR]

Tellurium is a primary candidate for the identification of the 2.1  |$\, \mu$| m emission line in kilonovae (KNe) spectra AT2017gfo and GRB230307A. Despite this, there is currently an insufficient amount of atomic data available for this species. We calculate the required atomic structure and collisional data, particularly the data required for accurate non-local-thermodynamic-equilibrium (NLTE) modelling of the low temperatures and densities in KNe. We use a multiconfigurational Dirac–Hartree–Fock method to produce optimized one-electron orbitals for Te i - iii. As a result energy levels and Einstein A-coefficients for Te i - iii have been calculated. These orbitals are then employed within Dirac R -matrix collision calculations to provide electron-impact-excitation collision strengths that were subsequently averaged according to a thermal Maxwellian distribution. Subsequent tardis simulations using this new atomic data reveal no significant changes to the synthetic spectra due to the very minor contribution of Te at early epochs. NLTE simulations with the colradpy package reveal optically thin spectra consistent with the increasing prominence of the Te iii 2.1 |$\, \mu$| m line as the KNe ejecta cools. This is reinforced by the estimation of luminosities at nebular KNe conditions. New line ratios for both observation and laboratory benchmarks of the atomic data are proposed. [ABSTRACT FROM AUTHOR]

Strongly magnetized neutron stars are popular candidates for producing detectable electromagnetic and gravitational-wave signals. Gravitational collapses of neutron stars triggered by a phase transition from hadrons to deconfined quarks in the cores could also release a considerable amount of energy in the form of gravitational waves and neutrinos. Hence, the formation of a magnetized hybrid star from such a phase-transition-induced collapse is an interesting scenario for detecting all these signals. These detections may provide essential probes for the magnetic field and composition of such stars. Thus far, a dynamical study of the formation of a magnetized hybrid star from a phase-transition-induced collapse has yet to be realized. Here, we investigate the formation of a magnetized hybrid star with a purely toroidal field and its properties through dynamical simulations. We find that the maximum values of rest-mass density and magnetic field strength increase slightly and these two quantities are coupled in phase during the formation. We then demonstrate that all microscopic and macroscopic quantities of the resulting hybrid star vary drastically when the maximum magnetic field strength goes beyond a threshold of |$\sim 5 \times 10^{17}$| G, but they are insensitive to the magnetic field below this threshold. Specifically, the magnetic deformation makes the rest-mass density drop significantly, suppressing the matter fraction in the mixed phase. These behaviours agree with those in the equilibrium models of previous studies. Therefore, this work provides a solid support for the magnetic effects on a hybrid star. [ABSTRACT FROM AUTHOR]

The Small and Large Magellanic Clouds are the only galaxies outside our own in which radio pulsars have been discovered to date. The sensitivity of the MeerKAT radio interferometer offers an opportunity to search for a population of more distant extragalactic pulsars. The TRAPUM (TRansients And PUlsars with MeerKAT) collaboration has performed a radio-domain search for pulsars and transients in the dwarf star-forming galaxies Sextans A and B, situated at the edge of the Local Group 1.4 Mpc away. We conducted three 2-h multibeam observations at L band (856–1712 MHz) with the full array of MeerKAT. No pulsars were found down to a radio pseudo-luminosity upper limit of 7.9 |$\pm$| 0.4 Jy kpc |$^{2}$| at 1400 MHz, which is 28 times more sensitive than the previous limit from the Murriyang telescope. This luminosity is 30 per cent greater than that of the brightest known radio pulsar and sets a cut-off on the luminosity distributions of the entire Sextans A and B galaxies for unobscured radio pulsars beamed in our direction. A fast radio burst was detected in one of the Sextans A observations at a dispersion measure (DM) of 737 pc cm |$^{-3}$|⁠. We believe this is a background event not associated with the dwarf galaxy due to its large DM and its signal-to-noise ratio being strongest in the wide-field incoherent beam of MeerKAT. [ABSTRACT FROM AUTHOR]

In modelling a relativistic disc around a compact object, the self-gravity of the disc is often neglected while it needs to be incorporated for more accurate descriptions in several circumstances. Extending the Komatsu–Eriguchi–Hachisu self-consistent field method, we present numerical models of a rapidly rotating neutron star with a self-gravitating disc in stationary equilibrium. In particular, our approach allows us to obtain numerical solutions involving a massive disc with the rest mass |$\mathcal {O}(10^{-1})-\mathcal {O}(10^0)\, \mathrm{ M}_\odot$| closely attached to a rotating neutron star, given that the disc is mainly supported by the relativistic electron degeneracy pressure. We also assess the impact of self-gravity on the internal structure of the disc and the neutron star. These axisymmetric, stationary solutions can be employed for simulations involving the neutron star–disc system in the context of high-energy transients and gravitational-wave emissions. [ABSTRACT FROM AUTHOR]

Comprehensive spectrotemporal analyses of the Z-type neutron star low-mass X-ray binary GX 5−1 were performed using 10 broad-band observations from AstroSat /Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) instruments. The LAXPC-20 hardness–intensity diagram showed horizontal and normal branches (HBs and NBs) of the Z track which exhibited secular motion. The time-averaged spectra in the energy range 0.7–25.0 keV could be fitted with the model combination – |$\tt {constant}\, \times \, \tt {tbabs}\, \times \, \tt {edge}\, \times \, \tt {edge}\, \times \, \tt {thcomp}\, \times \, \tt {diskbb}$|⁠. This yielded |$\Gamma \, \sim$| 2, |$kT_{\mathrm{ e}}\, \sim$| 3.3 keV, and |$F_{\mathrm{ disc}}$| / |$F_{\mathrm{ total}}\, \sim$| 0.8 indicating the soft/intermediate spectral state of the source during the observations. Flux-resolved spectral analysis revealed a positive correlation between |$kT_{\mathrm{ in}}$| and |$F_{\mathrm{ bol}}$|⁠. However, a negative correlation was observed between them in one of the NBs. Time-averaged temporal analysis revealed multiple HB oscillations (HBOs) and NB oscillations (NBOs), and peaked noise components in the |$\sim$| 5–50 Hz range. Furthermore, flux-resolved temporal analysis showed that the frequency of the HBOs correlates positively whereas the strength of HBOs correlates negatively with |$F_{\mathrm{ bol}}$|⁠ , indicating their probable origin from the accretion disc. In contrast, the frequency and strength of NBOs remain fairly constant with |$F_{\mathrm{ bol}}$|⁠ , suggesting that they originate from a different region in the system. Using the relativistic precession model along with highest frequency of the HBO, the upper limits of the magnetic dipole moment (⁠|$\mu$|⁠) and field strength (B) at the poles of the neutron star in the system were found to be 25.60 |$\times \, 10^{25}$| G cm3 and 3.64 |$\times \, 10^{8}$| G, respectively, for |$k_{\mathrm{ A}}$| = 1. [ABSTRACT FROM AUTHOR]

The macroscopic model is formulated for a neutron star (NS) as a perfect liquid drop at the equilibrium. We use the leptodermic approximation a∕R≪1, where a is the crust thickness of the effective surface (ES) of NS, and R is the mean radius of the ES curvature. Within the approximate Schwarzschild metric solution to the general relativity theory equations for the spherically symmetric systems, the macroscopic gravitation is taken into account in terms of the total separation particle energy and incompressibility. Density distribution ρ across the ES in the normal direction to the ES was obtained analytically for a general form of the energy density ℰ(ρ). For the typical crust thickness, and effective radius, one finds the leading expression for the density ρ. NS masses are analytically calculated as a sum of the volume and surface terms, taking into account the radial curvature of the metric space, in reasonable agreement with the recently measured masses for several NSs. We derive the simple macroscopic equation of state (EoS) with the surface correction. The analytical and numerical solutions to Tolman–Oppenheimer–Volkoff equations for the pressure are in good agreement with the volume part of our EoS. [ABSTRACT FROM AUTHOR]

Recent gravitational wave (GW) observations include possible detections of black hole—neutron star binary mergers. As with binary black hole mergers, numerical simulations help characterize the sources. For binary systems with neutron star components, the simulations help to predict the imprint of tidal deformations and disruptions on the GW signals. In a previous study, we investigated how the mass of the black hole has an impact on the disruption of the neutron star and, as a consequence, on the shape of the GWs emitted. We extend these results to study the effects of varying the compactness of the neutron star. We consider neutron star compactness in the 0.113–0.2 range for binaries with mass ratios of 3 and 5. As the compactness and the mass ratio increase, the binary system behaves during the late inspiral and merger more like a black hole binary. For the cases with the least compact neutron star, the GWs emitted, in terms of mismatches, are the most distinguishable from those by a binary black hole. The disruption of the star significantly suppresses the kicks on the final black hole. The disruption also affects, although not dramatically, the spin of the final black hole. Lastly, for neutron stars with low compactness, the quasi-normal ringing of the black hole after the merger does not show a clean quasi-normal ringing because of the late accretion of debris from the neutron star. [ABSTRACT FROM AUTHOR]

Introduction: This 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. Methods: Utilizing 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. Results: The 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. Discussion: These 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. [ABSTRACT FROM AUTHOR]

We present the results obtained from X-ray and optical analysis of the Be/X-ray binary IGR J06074+2205, focusing on before, during, and after the X-ray outbursts in 2023 October and December. The properties of the neutron star in the binary are investigated using NICER and NuSTAR observations during the X-ray outbursts. The pulse profiles across a broad energy range, are found to be strongly dependent on luminosity and energy, revealing the complex nature of the emitting region. An absorbed power law can describe each NICER spectrum in the 1–7 keV band. The 3–79 keV NuSTAR spectrum can be well described by a negative and positive power law with an exponential cut-off model. Utilizing the MAXI/GSC long-term light curve, we estimate the probable orbital period to be 80 or 80/n (n = 2, 3, 4) d. We investigate the evolution of the circumstellar disc around the Be star by using optical spectroscopic observations of the system between 2022 and 2024. We observe variable H  |$\alpha$| and Fe  ii emission lines with an increase in equivalent width, indicating the presence of a dynamic circumstellar disc. A distinct variation in the V/R value for H  |$\alpha$| and Fe  ii lines is also observed. The appearance of additional emission lines, such as He  i (5875.72 Å), He  i (6678 Å), and He  i (7065 Å), during the post-outburst observation in 2024 February suggests the growing of a larger or denser circumstellar disc. The disc continues to grow without any noticeable mass-loss, even during the 2023 X-ray outbursts, which may lead to a future giant X-ray outburst. [ABSTRACT FROM AUTHOR]

This study utilizes the simultaneous broad-band observations of 4U 1735−44 from AstroSat , offering enhanced spectral and temporal resolution, to investigate its spectral properties, temporal behaviour, and burst characteristics. Spectral, type I X-ray burst, and temporal analyses on 4U 1735−44 were performed using AstroSat /Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) observations. The hardness–intensity diagram from LAXPC-20 showed a positive correlation between hardness and intensity, with a pattern resembling the banana branch typical of atoll sources. Spectral analysis carried out in the 0.7–20.0 keV energy range, using the model combination – |$\tt {constant}$| |$\times$| |$\tt {tbabs}$| (⁠|$\tt {nthcomp}$| |$+$| |$\tt {diskbb}$| |$+$| |$\tt {bbodyrad}$|⁠), suggested a cool accretion disc truncated at a large distance from the neutron star in the system. Time-resolved spectral studies of two type I X-ray bursts detected from the source revealed evidence of photospheric radius expansion, allowing for an estimation of the source distance. Temporal analysis showed the presence of low-frequency quasi-periodic oscillation at |$\sim$| 69 Hz (3.3 |$\sigma$| significance with more than 99 per cent confidence) and prominent noise features below 30 Hz. [ABSTRACT FROM AUTHOR]

The coalescence of binary neutron stars (BNS) is a prolific source of gravitational waves (GWs) and electromagnetic (EM) radiation, offering a dual observational window into the Universe. Lowering the signal-to-noise ratio (S/N) threshold is a simple and cost-effective way to enhance the detection probability of GWs from BNS mergers. In this study, we introduce a metric of the purity of joint GW and EM detections |$P_{\rm joint}$|⁠ , which is in analogue to |$P_{\rm astro}$| in GW-only observations. By simulating BNS merger GWs jointly detected by the Handford, LIGO Livingston, and Virgo network and EM counterparts (kilonovae and short gamma-ray bursts, sGRBs) with an assumed merger rate density of BNS, we generate catalogues of GW events and EM counterparts. Through this simulation, we analyse joint detection pairs, both correct and misidentified. We find the following: (1) For kilonovae, requiring |$P_{\rm joint}\gt $| 95 per cent instead of |$P_{\rm astro}\gt 95~{{\ \rm per\ cent}}$| reduces the S/N from 9.2 to 8.5–8.8, allowing 5–13 additional joint detections per year and increasing the GW detection volume by 9–17 per cent. (2) For sGRBs, requiring |$P_{\rm joint}\gt $| 95 per cent instead of |$P_{\rm astro}$| reduces the S/N from 9.2 to 8.1–8.5. (3) Increasing kilonova or sGRB detection capability does not improve |$P_{\rm joint}$| due to a higher rate of misidentifications. We also show that sub-threshold GW and kilonova detections can reduce the uncertainty in measuring the Hubble constant to 89–92 per cent of its original value, and sub-threshold GW and sGRB observations can enhance the precision of constraining the speed of GWs to 88 per cent of previously established values. [ABSTRACT FROM AUTHOR]

The recent observation of the GW230529 event indicates that black hole–neutron star binaries can contain low-mass black holes. Since lower mass systems are more favourable for tidal disruption, such events are promising candidates for multimessenger observations. In this study, we employ five finite-temperature, composition-dependent matter equations of state and present results from ten 3D general relativistic hydrodynamic simulations for the mass ratios |$q = 2.6$| and 5. Two of these simulations target the chirp mass and effective spin parameter of the GW230529 event, while the remaining eight contain slightly higher mass black holes, including both spinning (⁠|$a_{\mathrm{ BH}} = 0.7$|⁠) and non-spinning (⁠|$a_{\mathrm{ BH}} = 0$|⁠) models. We discuss the impact of the equation of state, spin, and mass ratio on black hole–neutron star mergers by examining both gravitational-wave and ejected matter properties. For the low-mass ratio model we do not see fast-moving ejecta for the softest equation of state model, but the stiffer model produces on the order of |$10^{-6}\,\mathrm{ M}_\odot$| of fast-moving ejecta, expected to contribute to an electromagnetic counterpart. Notably, the high-mass ratio model produces nearly the same amount of total dynamical ejecta, but yields 52 times more fast-moving ejecta than the low-mass ratio system. In addition, we observe that the black-hole spin tends to decrease the amount of fast-moving ejecta while increasing significantly the total ejected mass. Finally, we note that the disc mass tends to increase as the neutron star compactness decreases. [ABSTRACT FROM AUTHOR]

In this paper, we present rapid follow-up observations of the short GRB 201006A, consistent with being a compact binary merger, using the LOw Frequency ARray (LOFAR). We have detected a candidate 5.6 |$\sigma$|⁠ , short, coherent radio flash at 144 MHz at 76.6 min post-GRB with a 3 |$\sigma$| duration of 38 s. This radio flash is 27 arcsec offset from the GRB location, which has a probability of being co-located with the GRB of |$\sim$| 0.05 per cent (3.8 |$\sigma$|⁠) when accounting for measurement uncertainties. Despite the offset, we show that the probability of finding an unrelated transient within 40 arcsec of the GRB location is |$\lt 10^{-6}$| and conclude that this is a candidate radio counterpart to GRB 201006A. We performed image plane dedispersion and the radio flash is tentatively (2.4 |$\sigma$|⁠) shown to be highly dispersed, allowing a distance estimate, corresponding to a redshift of |$0.58\pm 0.06$|⁠. The corresponding luminosity of the event at this distance is |$6.7^{+6.6}_{-4.4} \times 10^{32}$| erg s |$^{-1}$| Hz |$^{-1}$|⁠. If associated with GRB 201006A, this emission would indicate prolonged activity from the central engine that is consistent with being a newborn, supramassive, likely highly magnetized, millisecond spin neutron star (a magnetar). [ABSTRACT FROM AUTHOR]

Current generation of ground based gamma-ray telescopes observed dozens of sources of photons above 100 TeV. Supernova remnants, pulsar wind nebulae, young stellar clusters and superbubbles are considered as possible sites of PeV-regime particles producing the radiation. Another possible source of PeV particles could be gamma-ray binary systems. In these systems, a strong relativistic outflow from a compact object (neutron star or black hole) collides with the dense wind from a massive companion early-type star. Gamma-ray binaries are observed from radio to high energy gamma-rays as luminous non-thermal sources. Apart from acceleration of very high energy leptons producing most of the non-thermal radiation, these systems may also efficiently accelerate protons. We present here the results of numerical simulation of the PeV-regime proton acceleration in gamma-ray binaries. The simulation is based on relativistic MHD modeling of local flows of magnetized plasma in the region of interaction of two colliding winds. We then inject 0.1 PeV protons into the system and directly follow their trajectories to demonstrate that they are accelerated to energies above PeV. High magnetization of the wind of the young massive star providing a Gauss range field in the winds interaction region is of paramount importance for the acceleration of protons above PeV. The maximum energies of protons accelerated by colliding winds in gamma-ray binaries can significantly exceed the energy of the pulsar potential's drop, which limits from above the energy of particles accelerated by an isolated pulsar. [ABSTRACT FROM AUTHOR]

We have investigated the evolutionary connections of the isolated neutron star (NS) populations including radio pulsars (RPs), anomalous X-ray pulsars (AXPs), soft gamma repeaters (SGRs), dim isolated NSs (XDINs), 'high-magnetic field' RPs ('HBRPs'), central compact objects (CCOs), rotating radio transients (RRATs), and long-period pulsars (LPPs) in the fallback disc model. The model can reproduce these NS families as a natural outcome of different initial conditions (initial period, disc mass, and dipole moment, μ) with a continuous μ distribution in the |$\sim 10^{27} - 5 \times 10^{30}$| G cm |$^3$| range. Results of our simulations can be summarized as follows: (1) A fraction of 'HBRPs' with relatively high μ evolve into the persistent AXP/SGR properties, and subsequently become LPPs. (2) Persistent AXP/SGRs do not have evolutionary links with CCOs, XDINs, and RRATs. (3) For a wide range of μ , most RRATs evolve passing through RP or 'HBRP' properties during their early evolutionary phases. (4) A fraction of RRATs which have the highest estimated birth rate seem to be the progenitors of XDINs. (5) LPPs, whose existence was predicted by the fallback disc model, are the sources evolving in the late stage of evolution before the discs become inactive. These results provide concrete support to the ideas proposing evolutionary connections between the NS families to account for the 'birth rate problem', the discrepancy between the cumulative birth rate estimated for these systems and the core-collapse supernova rate. [ABSTRACT FROM AUTHOR]

The Be/X-ray binary pulsar RX J0440.9+4431 went through a giant outburst in December 2022 with a peak flux of |$\sim$| 2.3 Crab in 15–50 keV. We studied the broad-band timing and spectral properties of RX J0440.9+4431 using four AstroSat observations, where the source transited between subcritical and supercritical accretion regimes. Pulsations were detected significantly above 100 keV. The pulse profiles were found to be highly luminosity- and energy-dependent. A significant evolution in the pulse profile shape near the peak of the outburst indicates a possible change in the accretion mode and beaming patterns of RX J0440.9+4431. The rms pulsed fraction was luminosity- and energy-dependent, with a concave-like feature around 20–30 keV. The depth of this feature varied with luminosity, indicating changes in the accretion column height and proportion of reflected photons. The broad-band continuum spectra were best fitted with a two-component Comptonization model with a blackbody component or a two-blackbody component model with a thermal Comptonization component. A quasi-periodic oscillation (QPO) at 60 mHz was detected at a luminosity of |$2.6 \times 10^{37}$|  erg s |$^{-1}$|⁠ , which evolved into 42 mHz at |$1.5 \times 10^{37}$|  erg s |$^{-1}$|⁠. The QPO rms were found to be energy dependent with an overall increasing trend with energy. For the first time, we found the QPO frequency varying with photon energy in an X-ray pulsar, which poses a challenge in explaining the QPO with current models such as the Keplarian and beat frequency model. Hence, more physically motivated models are required to understand the physical mechanism behind the mHz QPOs. [ABSTRACT FROM AUTHOR]

Magnetars are neutron stars that host huge, complex magnetic fields which require supporting currents to flow along the closed field lines. This makes magnetar atmospheres different from those of passively cooling neutron stars because of the heat deposited by back-flowing charges impinging on the star surface layers. This particle bombardment is expected to imprint the spectral and, even more, the polarization properties of the emitted thermal radiation. We present solutions for the radiative transfer problem for bombarded plane-parallel atmospheres in the high magnetic field regime. The temperature profile is assumed a priori, and selected in such a way to reflect the varying rate of energy deposition in the slab (from the impinging currents and/or from the cooling crust). We find that thermal X-ray emission powered entirely by the energy released in the atmosphere by the magnetospheric back bombardment is linearly polarized and X-mode dominated, but its polarization degree is significantly reduced (down to 10 per cent–50 per cent) when compared with that expected from a standard atmosphere heated only from the cooling crust below. By increasing the fraction of heat flowing in from the crust the polarization degree of the emergent radiation increases, first at higher energies (⁠|${\sim} 10\ \mathrm{keV}$|⁠) and then in the entire soft X-ray band. We use our models inside a ray-tracing code to derive the expected emission properties as measured by a distant observer and compare our results with recent IXPE observations of magnetar sources. [ABSTRACT FROM AUTHOR]

Utilizing the large quantity of hyperons and antihyperons produced by the decay of 10 billion J/ψ and 2.7 billion ψ(3686) collected at BESIII, the cross-sections of several specific elastic or inelastic (anti-)hyperon–nucleus/nucleon reactions have been measured via the scattering between the (anti-)hyperons and the nucleus in the dense objects of BESIII detector. The novel method developed in these works extends the research field and opens a new era for the experiments at e+e− colliders. The results of such measurements will definitely benefit a lot the precise probe of the (anti-)hyperon–nucleus/nucleon interactions and provide constraints for the studies of the potential of strong interaction, the origin of color confinement, the unified model for baryon–baryon interactions, and the internal structure of neutron stars. The desirable prospects of corresponding studies in the future Super Tau-Charm Factory (STCF) are also discussed in this report. [ABSTRACT FROM AUTHOR]

This study comprehensively analyzes type I X-ray bursts observed by Swift/BAT from 2005 to April 2024 to search for X-ray burst oscillations (XBOs) in neutron star low-mass X-ray binaries. XBOs, periodic signals detected within type I X-ray bursts, typically range from 11 to 620 Hz and are often observed in the soft X-ray data of these bursts. Using the high-sensitivity and precise timing capabilities of the Swift/BAT, we found 50 type I X-ray bursts from 37 neutron star low-mass X-ray binaries. We conducted a detailed timing analysis of these bursts. For sources with known burst oscillation frequencies, our findings largely corroborate previous studies. However, many sources displayed low confidence levels in the oscillation signals, with [sub [sup Z 1 2]] values between 10 and 20. For sources without known oscillation/spin frequencies, we utilized FFT analysis to search for signals across a broad frequency range. This approach revealed potential oscillation signals, with several bursts showing significance levels exceeding 3σ, including those from MAXI J1421-613, XTE J1701-407, XMM J174457-2850.3, Swift J1734.5-3027, IGR J17473-2721, Swift J174805.3-244637, Swift J181723.1-164300, and X 1832-330. [ABSTRACT FROM AUTHOR]

IC 10 X-1 is one of the close X-ray binaries containing a Wolf–Rayet donor, which can provide an evolutionary link between high-mass X-ray binaries and gravitational wave sources. It is still unclear about the precise nature of the accreting compact object in IC 10 X-1, although it looks more like a black hole (BH) than a neutron star. In this work, we use a binary population synthesis method to simulate the formation of IC 10 X-1–like binaries by assuming different common-envelope ejection efficiencies. This work represents a big step forward over previous studies since we adopt new criteria for mass-transfer stability. These criteria allow the formation of IC 10 X-1–like systems without experiencing common envelope evolution. Based on our calculations, we propose that the compact object in IC 10 X-1 is a BH with a mass of ∼10–30 M ⊙ and the progenitor evolution of this binary probably just experienced stable mass transfer. [ABSTRACT FROM AUTHOR]

We present an extensive contemporaneous X-ray and radio campaign performed on the repeating fast radio burst (FRB) source FRB 20220912A for 8 weeks immediately following the source's detection by CHIME/FRB. This includes X-ray data from XMM-Newton, NICER, and Swift, and radio detections of FRB 20220912A from CHIME/Pulsar and Effelsberg. We detect no significant X-ray emission at the time of 30 radio bursts with upper limits on a 0.5–10.0 keV X-ray fluence of (1.5–14.5) × 10−10 erg cm−2 (99.7% credible interval, unabsorbed) on a timescale of 100 ms. Translated into a fluence ratio η x/r = F X-ray/ F radio, this corresponds to η x/r < 7 × 106. For persistent emission from the location of FRB 20220912A, we derive a 99.7% 0.5–10.0 keV isotropic flux limit of 8.8 × 10−15 erg cm−2 s−1 (unabsorbed) or an isotropic luminosity limit of 1.4 × 1041 erg s−1 at a distance of 362.4 Mpc. We derive a hierarchical extension to the standard Bayesian treatment of low-count and background-contaminated X-ray data, which allows the robust combination of multiple observations. This methodology allows us to place the best (lowest) 99.7% credible interval upper limit on an FRB η x/r to date, η x/r < 2 × 106, assuming that all 30 detected radio bursts are associated with X-ray bursts with the same fluence ratio. If we instead adopt an X-ray spectrum similar to the X-ray burst observed contemporaneously with FRB-like emission from the Galactic magnetar SGR 1935+2154 detected on 2020 April 28, we derive a 99.7% credible interval upper limit on η x/r of 8 × 105, which is only 3 times the observed value of η x/r for SGR 1935+2154. [ABSTRACT FROM AUTHOR]

Recycled millisecond pulsars are susceptible to starquakes as they are continuously accreting matter from their binary companion. A starquake happens when the rotational frequency of the star crosses its breaking frequency. In this study, we perform a model analysis of an accreting neutron star suffering a starquake. We analyse two models: a spherical star with accreting mountains and a deformed star with accreting mountains. We find that as the star crosses the breaking frequency and suffers a starquake, there is a sudden change in the continuous gravitational wave signal arriving from it. The amplitude of the gravitational wave signal increases suddenly both for the spherical and deformed star. For the spherical star, the accreting matter entirely dictates the amplitude of the gravitational wave. For the deformed star, both the accreting matter and the deformation from spherical symmetry play a significant role in determining the amplitude of the gravitational wave signal. This sudden change in the continuous gravitational wave signal in recycled millisecond pulsars can be a unique signature for such pulsars undergoing a starquake. [ABSTRACT FROM AUTHOR]

Recent radio surveys have revealed pulsars with dispersion and scattering delays induced by ionized gas that are larger than the rest of the observed pulsar population, in some cases with electron column densities (or dispersion measures, DMs) larger than the maximum predictions of Galactic electron density models. By cross-matching the observed pulsar population against H ii region catalogs, we show that the majority of pulsars with DM > 600 pc cm−3 and scattering delays τ (1 GHz) > 10 ms lie behind H ii regions, and that H ii region intersections may be relevant to as much as a third of the observed pulsar population. The fraction of the full pulsar population with sightlines intersecting H ii regions is likely larger. Accounting for H ii regions resolves apparent discrepancies where Galactic electron density models place high-DM pulsars beyond the Galactic disk. By comparing emission measures inferred from recombination line observations to pulsar DMs, we show that H ii regions can contribute tens to hundreds of parsecs per cubic centimeter in electron column density along a pulsar line of sight. We find that nearly all pulsars with significant excess (and deficit) scattering from the mean τ –DM relation are spatially coincident with known discrete ionized gas structures, including H ii regions. Accounting for H ii regions is critical to the interpretation of radio dispersion and scattering measurements as electron density tracers, both in the Milky Way and in other galaxies. [ABSTRACT FROM AUTHOR]

Neutron stars (NSs) provide a unique laboratory to study matter under extreme densities. Recent observations from gravitational and electromagnetic waves have enabled constraints on NS properties, such as tidal deformability (related to the tidal Love number) and stellar compactness. Although each of these two NS observables depends strongly on the stellar internal structure, the relation between them (called the Love–C relation) is known to be equation-of-state insensitive. In this study, we investigate the effects of a possible crystalline phase in the core of hybrid stars (HSs) on the mass–radius and Love–C relations, where HSs are a subclass of NS models with a quark matter core and a nuclear matter envelope with a sharp phase transition in between. We find that both the maximum mass and the corresponding radius increase as one increases the stiffness of the quark matter core controlled by the speed of sound, while the density discontinuity at the nuclear-quark matter transition effectively softens the equations of state. Deviations of the Love–C relation for elastic HSs from that of fluid NSs become more pronounced with a larger shear modulus, lower transition pressure, and larger density gap and can be as large as 60%. These findings suggest a potential method for testing the existence of distinct phases within HSs, though deviations are not large enough to be detected with current measurements of the tidal deformability and compactness. [ABSTRACT FROM AUTHOR]

This paper was a pioneering contribution to a symplectic description of the dynamics of spinning particles in general relativity which remained somewhat unnoticed. In particular, it introduced the pre-symplectic 2-form σ describing the dynamics of spinning particles coupled to an Einsteinian curved background. The method throws light on approaches to spinning black holes and neutron stars. [ABSTRACT FROM AUTHOR]

The r-process is one of the processes that produces heavy elements in the Universe. One of its possible astrophysical sites is the neutron star–black hole (NS-BH) merger. We first show that the neutrons can degenerate before and during the r-process in these mergers. Previous studies assumed neutrons were non-degenerate and the related rates were calculated under Maxwell–Boltzmann approximations. Hence, we corrected the related rates with neutron degeneracy put in the network code and calculated with the trajectories of NS-BH mergers. We show that there are differences in the nuclei distributions. The heating rates and the temperature at most can be two times larger. The change in heating rates and temperature can affect the light curves of the kilonovae. However, this has little effect on the final abundances. [ABSTRACT FROM AUTHOR]

With advancements in gravitational wave detection technology, an increasing number of binary neutron star (BNS) merger events are expected to be detected. Due to the narrow opening angle of jet cores, many BNS merger events occur off-axis, resulting in numerous gamma-ray bursts (GRBs) going undetected. Models suggest that kilonovae, which can be observed off-axis, offer more opportunities to be detected in the optical/near-infrared band as electromagnetic counterparts of BNS merger events. In this study, we calculate kilonova emission using a three-dimensional semi-analytical code and model the GRB afterglow emission with the open-source Python package afterglowpy at various inclination angles. Our results show that it is possible to identify the kilonova signal from the observed color evolution of BNS merger events. We also deduce the optimal observing window for SVOM/VT and JWST/NIRCam, which depends on the viewing angle, jet opening angle, and circumburst density. These parameters can be cross-checked with the multi-band afterglow fitting. We suggest that kilonovae are more likely to be identified at larger inclination angles, which can also help determine whether the observed signals without accompanying GRBs originate from BNS mergers. [ABSTRACT FROM AUTHOR]

In this study, the geometrically deformed compact objects in the f(Q, T) gravity theory under an electric field through gravitational decoupling via minimal geometric deformation (MGD) technique are developed for the first time. The decoupled field equations are solved via two different mimic approaches Θ00=ρ${\Theta}_{0}^{0}=\rho $ and Θ11=pr${\Theta}_{1}^{1}={p}_{r}$ through the Karmarkar condition. Physical viability tests are conducted on our models and examine how decoupling parameters affect the physical qualities of objects. The obtained models are compared with the observational constraints for neutron stars PSR J1810+174, PSR J1959+2048, and PSR J2215+5135, including GW190814. Particularly, by modifying parameters α and n, the occurrence of a "mass gap" component is accomplished. The resulting models exhibit stable, well‐behaved mass profiles, regular behavior and no gravitational collapse, as verified by the Buchdahl–Andréasson's limit. Furthermore, a thorough physical analysis that is based on two parameters: n (f(Q, T)–coupling parameter) and α (decoupling parameter) is provided. This work extends our current understanding of compact star configurations and sheds light on the behavior of compact objects in the f(Q, T) gravity. [ABSTRACT FROM AUTHOR]

The neutrino-driven wind cooling phase of proto-neutron stars (PNSs) follows successful supernovae. Wind models without magnetic fields or rotation fail to achieve the necessary conditions for production of the third r -process peak, but robustly produce a weak r- process in neutron-rich winds. Using 2D magnetohydrodynamic simulations with magnetar-strength magnetic fields and rotation, we show that the PNS rotation rate significantly affects the thermodynamic conditions of the wind. We show that high-entropy material is quasiperiodically ejected from the closed zone of the PNS magnetosphere with the required thermodynamic conditions to produce heavy elements. We show that maximum entropy S of the material ejected depends systematically on the magnetar spin period P ⋆ and scales as S ∝ P ⋆ − 5 / 6 for sufficiently rapid rotation. We present results from simulations at a constant neutrino luminosity representative of ∼1–2 s after the onset of cooling for P ⋆ ranging from 5–200 ms and a few simulations with evolving neutrino luminosity where we follow the evolution of the magnetar wind until 10–14 s after the onset of cooling. We estimate at magnetar polar magnetic field strength B 0 = 3 × 1015 G and 1015 G that neutron-rich magnetar winds can, respectively, produce at least ∼1–5 × 10−5 M ⊙ and ∼1–4 × 10−7 M ⊙ of material with the required parameters for synthesis of the third r -process peak, within 1–2 s and 10 s, respectively, in that order after the onset of cooling. We show that proton-rich magnetar winds can have favorable conditions for production of p- nuclei, even at a modest B 0 = 5 × 1014 G. [ABSTRACT FROM AUTHOR]

A small number of pulsars are known to emit giant pulses (GPs), single pulses much brighter than average. Among these is PSR J0534+2200, also known as the Crab pulsar, a young pulsar with high GP rates. Long-term monitoring of the Crab pulsar presents an excellent opportunity to perform statistical studies of its GPs and the processes affecting them, potentially providing insight into the behavior of other neutron stars that emit bright single pulses. Here, we present an analysis of a set of 24,985 Crab GPs obtained from 88 hr of daily observations at a center frequency of 1.55 GHz by the 20 m telescope at the Green Bank Observatory, spread over 461 days. We study the effects of refractive scintillation at higher frequencies than previous studies and compare methods of correcting for this effect. We also search for deterministic patterns seen in other single-pulse sources, possible periodicities seen in several rotating radio transients and fast radio bursts, and clustering of GPs like that seen in the repeating fast radio burst FRB 121102. [ABSTRACT FROM AUTHOR]

The central engine of a gamma-ray burst (GRB) is widely believed to launch a pair of oppositely moving jets, i.e., the forward jet moving toward us and the counter jet regressing away. The forward jet generates the radiation typically observed in GRBs, while the counter jet has not been detected yet due to its dimness. GRB 170817A, a short burst associated with a binary neutron star merger event, is a nearby event (z = 0.0097) with an off-axis structured energetic forward jet and hence probably the most suitable target for searching the counter jet radiation. Assuming the same properties for the forward and counter jet components as well as the shock parameters, the fit to the multiwavelength afterglow emission of GRB 170817A suggests a peak time ∼quite a few ×103 day of the counter jet radiation, but the detection prospect of this new component is not promising. Anyhow, if the shock parameters (ϵ e and ϵ B) of the counter jet component are (a few times) higher than that of the forward shock, e.g., the posterior results of the magnetic energy fraction of the forward shock and the counter jet are log 10 ϵ B = − 4.23 − 0.69 + 1.42 and log 1 0 ϵ B , cj = − 3.65 − 2.11 + 3.06 , respectively, as still allowed by the current data, the counter jet afterglow emission will be enhanced and hence may be detected. A few hours of exposure by JWST in the F356W band will stringently test such a scenario. [ABSTRACT FROM AUTHOR]

PSR J1048 |$-$| 5832 (B1046 |$-$| 58) is a Vela-like pulsar that has exhibited multiple glitch events. In this study, we analyse the timing data spanning nearly 16 yr, acquired from both the Fermi Gamma-ray Space Telescope and the Parkes 64 m radio telescope. As a result, a total of five glitches are detected within this data set. Among them, a previously unknown small glitch is newly found at MJD 56985(9) (November 24, 2014), making it the smallest glitch recorded from this source so far. The increments of the spin frequency and its first derivative are |$\Delta \nu \approx 2.2(3) \times 10^ {-8}$|  Hz, and |$\Delta \dot{\nu } \approx 3(2) \times 10^ {-15}$|  s |$^{-2}$|⁠ , respectively. Significant changes in the integrated normalized mean pulse profile are detected following three of the five glitch events, notably in the radio band. Although no evidence of a correlation is found between the spin-down rate and profile evolution, the jump phenomenon of |$W_{55}$| (pulse width at the 55% peak amplitude) after the glitch in the narrow mode suggests that the glitch may influence the profile change. We discuss the influence of glitches on the pulsar's emission properties in terms of platelet motion by a crustquake and also put constraints on the equation of state from the moment of inertia and response time-scales of involved superfluid layers inside the neutron star. [ABSTRACT FROM AUTHOR]

We report new radio timing solutions from a 3-yr observing campaign conducted with the MeerKAT and Murriyang telescopes for nine Small Magellanic Cloud pulsars, increasing the number of characterized rotation-powered extragalactic pulsars by 40 per cent. We can infer from our determined parameters that the pulsars are seemingly all isolated, that six are ordinary pulsars, and that three of the recent MeerKAT discoveries have a young characteristic age of under 100 kyr and have undergone a spin-up glitch. Two of the sources, PSRs J0040 |$-$| 7337 and J0048 |$-$| 7317, are energetic young pulsars with spin-down luminosities of the order of 10 |$^{36}$|  erg s |$^{-1}$|⁠. They both experienced a large glitch, with a change in frequency of about 30  |$\mu$| Hz, and a frequency derivative change of order |$-10^{-14}$|  Hz s |$^{-1}$|⁠. These glitches, the inferred glitch rate, and the properties of these pulsars (including potentially high inter-glitch braking indices) suggest these neutron stars might be Vela-like repeating glitchers and should be closely monitored in the future. The position and energetics of PSR J0048 |$-$| 7317 confirm it is powering a new pulsar wind nebula (PWN) detected as a radio continuum source; and similarly the association of PSR J0040 |$-$| 7337 with the PWN of supernova remnant (SNR) DEM S5 (for which we present a new Chandra image) is strengthened. Finally, PSR J0040 |$-$| 7335 is also contained within the same SNR but is a chance superposition. It has also been seen to glitch with a change of frequency of |$10^{-2}$|   |$\mu$| Hz. This work more than doubles the characterized population of SMC radio pulsars. [ABSTRACT FROM AUTHOR]

The Sculptor Galaxy (NGC 253), located in the Southern Hemisphere, far off the Galactic Plane, has a relatively high star-formation rate of about 7 M |$_{\odot }$|  yr−1 and hosts a young and bright stellar population, including several super star clusters and supernova remnants. It is also the first galaxy, apart from the Milky Way Galaxy to be associated with two giant magnetar flares. As such, it is a potential host of pulsars and/or fast radio bursts in the nearby Universe. The instantaneous sensitivity and multibeam sky coverage offered by MeerKAT therefore make it a favourable target. We searched for pulsars, radio-emitting magnetars, and fast radio bursts in NGC 253 as part of the TRAPUM large survey project with MeerKAT. We did not find any pulsars during a 4 h observation, and derive a flux density limit of 4.4 µJy at 1400 MHz, limiting the pseudo-luminosity of the brightest putative pulsar in this galaxy to 54 Jy kpc2. Assuming universality of pulsar populations between galaxies, we estimate that detecting a pulsar as bright as this limit requires NGC 253 to contain a pulsar population of ⪞20 000. We also did not detect any single pulses, and our single pulse search flux density limit is 62 mJy at 1284 MHz. Our search is sensitive enough to have detected any fast radio bursts and radio emission similar to the brighter pulses seen from the magnetar SGR J1935+2154 if they had occurred during our observation. [ABSTRACT FROM AUTHOR]