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The given text appears to be a correction to a previous paper that examined the relationship between neutron star mass and dark matter accumulation within dark matter clumps. The authors identified an error in their code that resulted in distance measurements being given with respect to the Sun instead of the Galactic center. They corrected the distance values and found that the resulting plots changed, but the results and conclusions remained qualitatively similar. The document includes figures and graphs that illustrate the corrected analysis. [Extracted from the article]

Crystallization of dense matter in neutron star crusts and white dwarf cores may be similar to epitaxial crystal growth in terrestrial laboratories. However, in stellar crystals, the spacing between horizontal planes has to gradually increase with the outward movement of the crystallization front, tracing decrease of the electron density. This process produces Coulomb crystals with stretched rather than cubic elementary cells. We extend the analysis of the elastic and breaking properties of such crystals to the face‐centered (fc) lattice. Shear deformations orthogonal to the stretch direction have been studied for 22 crystallographic shear planes. A common property for all these planes is the reduction and eventual nulling of the breaking shear strain with deviation from the unstretched configuration. The effective shear moduli for deformations orthogonal to the stretch direction have been calculated. It is possible that the epitaxial crystallization in compact stars results in a formation of large‐scale crystallites or, at least, in growth of the whole crystallization front perpendicular to particular crystallographic planes. For fc structure growth orthogonal to the {111}$$ \left\{111\right\} $$ planes, we expect that, at any density, ∼5%$$ \sim 5\% $$ (∼0.5%$$ \sim 0.5\% $$) of crystallite height is occupied by layers one (two) orders of magnitude weaker than the bulk of the crystallite. This may be important for realistic modeling of crustquakes on neutron stars. [ABSTRACT FROM AUTHOR]

This paper used Nuclear Spectroscopic Telescope Array observations to examine the temporal and spectral features of the Be/X-ray binary pulsar MAXI J1409−619. The timing analysis of the light curve finds the pulsation of the source at |$(501.23\,\,\pm \,\,0.01)$|  s. The pulse profile of the source in various energy bands was analysed and showed weak dependence on energy exhibiting asymmetric character and generally suggests a source accretion in the subcritical regime. The variation of pulse fraction with photon energy in general shows an increasing trend. Assuming a distance of 14.5 kpc, we calculated the 3–30 keV source luminosity to be |$\sim 6.13\,\,\times \,\,10^{34}\,\,$|  erg s |$^{-1}$|⁠. The long-term spin evolution of the source was carried out, where the source underwent torque reversal. [ABSTRACT FROM AUTHOR]

We present results from fully general relativistic (GR), three-dimensional (3D), neutrino-radiation magneto-hydrodynamic (MHD) simulations of stellar core collapse of a 20 M⊙ star with spectral neutrino transport. Our focus is to study the gravitational-wave (GW) signatures from the magnetorotationally (MR)-driven models. By parametrically changing the initial angular velocity and the strength of the magnetic fields in the core, we compute four models. Among our models, only those with cores having an initial magnetic field strength of 1012 G and rotation rates of 1 or 2 rad s−1 produce MHD jets. Seen from the direction perpendicular to the rotational axis, a characteristic waveform is obtained exhibiting a monotonic time increase in the wave amplitude. As previously identified, this stems from the propagating MHD outflows along the axis. We show that the GW amplitude from anisotropic neutrino emission becomes more than one order-of-magnitude bigger than that from the matter contribution, whereas seen from the rotational axis, both of the two components are in the same order-of-magnitudes. Due to the memory effect, the frequency of the neutrino GW from our full-fledged 3D-MHD models is in the range less than ∼10 Hz. Toward the future GW detection for a Galactic core-collapse supernova, if driven by the MR mechanism, the planned next-generation detector as DECIGO is urgently needed to catch the low-frequency signals. [ABSTRACT FROM AUTHOR]

While much has been learned in recent decades about the X-ray emission of the extragalactic ultraluminous X-ray sources (ULXs), their radiative output in the ultraviolet (UV) band remains poorly constrained. Understanding of the full ULX spectral energy distribution (SED) is imperative to constrain the accretion flow geometry powering them, as well as their radiative power. Here we present constraints on the UV emission of the pulsating ULX (PULX) NGC 1313 X-2 based on the absence of nebular He  ii  λ4686 emission in its immediate environment. To this end, we first perform multiband spectroscopy of the ULX to derive three realistic extrapolations of the SED into the inaccessible UV, each predicting varying levels of UV luminosity. We then perform photoionization modelling of the bubble nebula and predict the He  ii  λ4686 fluxes that should have been observed based on each of the derived SEDs. We then compare these predictions with the derived upper limit on He  ii  λ4686 from the Multi-Unit Spectroscopic Explorer data, which allows us to infer a UV luminosity L UV ≲ 1 × 1039 erg s−1 in the PULX NGC 1313 X-2. Comparing the UV luminosity inferred with other ULXs, our work suggests there may be an intrinsic difference between hard and soft ULXs, either related to different mass-transfer rates and/or the nature of the accretor. However, a statistical sample of ULXs with inferred UV luminosities is needed to fully determine the distinguishing features between hard and soft ULXs. Finally, we discuss ULXs ionizing role in the context of the nebular He  ii  λ4686 line observed in star-forming metal-poor galaxies. [ABSTRACT FROM AUTHOR]

Binary neutron star mergers are expected to produce fast dynamical ejecta, with mildly relativistic velocities extending to β = v / c > 0.6. In a preceding paper, we derived an analytic description of the time-dependent radio to X-ray synchrotron flux produced by collision-less shocks driven by such fast ejecta into the interstellar medium, for spherical ejecta with broken power-law mass (or energy) distributions, M (> γβ)∝(γβ)− s with s = s KN at γβ < γ0β0 and s = s ft at γβ > γ0β0 (where γ is the Lorentz factor). Here, we extend our analysis and provide analytic expressions for the self-absorption frequency, the cooling frequency, and the observed angular size of the emitting region (which appears as a ring in the sky). For parameter values characteristic of merger calculation results – a 'shallow' mass distribution, 1 < s KN < 3, for the bulk of the ejecta (at γβ ≈ 0.2), and a steep, s ft > 5, 'fast tail' mass distribution – the analytic results reproduce well (to tens of per cent accuracy) the results of detailed numeric calculations, a significant improvement over earlier order-of-magnitude estimates (based on extrapolations of results valid for γβ ≪ 1). [ABSTRACT FROM AUTHOR]

The sensitivity of the MeerKAT radio interferometer is an opportunity to probe deeper into the population of rare and faint extragalactic pulsars. The TRAPUM (TRAnsients and PUlsars with MeerKAT) collaboration has conducted a radio-domain search for accelerated pulsars and transients in the Small Magellanic Cloud (SMC). This partially targeted survey, performed at L band (856–1712 MHz) with the core array of the MeerKAT telescope in 2-h integrations, is twice as sensitive as the latest SMC radio pulsar survey. We report the discovery of seven new SMC pulsars, doubling this galaxy's radio pulsar population and increasing the total extragalactic population by nearly a quarter. We also carried out a search for accelerated millisecond pulsars in the SMC globular cluster NGC 121 using the full array of MeerKAT. This improved the previous upper limit on pulsed radio emission from this cluster by a factor of 6. Our discoveries reveal the first radio pulsar-PWN systems in the SMC, with only one such system previously known outside our Galaxy (the 'Crab pulsar twin' in the Large Magellanic Cloud, PSR J0540−6919). We associate the 59 ms pulsar discovery PSR J0040−7337, now the fastest spinning radio pulsar in the SMC, with the bow-shock Pulsar Wind Nebula (PWN) of supernova remnant DEM S5. We also present a new young pulsar with a 79 ms period, PSR J0048−7317, in a PWN recently discovered in a MeerKAT radio continuum image. Using the multibeam capability of MeerKAT, we localized our pulsar discoveries, and two previous Murriyang discoveries, to a positional uncertainty of a few arcseconds. [ABSTRACT FROM AUTHOR]

Isolated black holes (BHs) and neutron stars (NSs) are largely undetectable across the electromagnetic spectrum. For this reason, our only real prospect of observing these isolated compact remnants is via microlensing; a feat recently performed for the first time. However, characterization of the microlensing events caused by BHs and NSs is still in its infancy. In this work, we perform N -body simulations to explore the frequency and physical characteristics of microlensing events across the entire sky. Our simulations find that every year we can expect |$88_{-6}^{+6}$| BH, |$6.8_{-1.6}^{+1.7}$| NS, and |$20^{+30}_{-20}$| stellar microlensing events which cause an astrometric shift larger than 2 mas. Similarly, we can expect |$21_{-3}^{+3}$| BH, |$18_{-3}^{+3}$| NS, and |$7500_{-500}^{+500}$| stellar microlensing events which cause a bump magnitude larger than 1 mag. Leveraging a more comprehensive dynamical model than prior work, we predict the fraction of microlensing events caused by BHs as a function of Einstein time to be smaller than previously thought. Comparison of our microlensing simulations to events in Gaia finds good agreement. Finally, we predict that in the combination of Gaia and Gaia NIR data there will be |$14700_{-900}^{+600}$| BH and |$1600_{-200}^{+300}$| NS events creating a centroid shift larger than 1 mas and |$330_{-120}^{+100}$| BH and |$310_{-100}^{+110}$| NS events causing bump magnitudes >1. Of these, <10 BH and |$5_{-5}^{+10}$| NS events should be detectable using current analysis techniques. These results inform future astrometric mission design, such as Gaia NIR, as they indicate that, compared to stellar events, there are fewer observable BH events than previously thought. [ABSTRACT FROM AUTHOR]

The spectral properties of the transient neutron star low-mass X-ray binary XTE J1701−462 were studied using the data obtained from FPMA/B detectors onboard NuSTAR during its second known outburst (2022 September). The physical parameters of the system were derived from the analysis of the data in the 3.0−30.0 keV energy range. The patterns displayed on the hardness–intensity diagram of the three observations closely resembled the banana branch/normal branch, a vertex of horizontal and normal branch of the Z-track and a transition from normal branch to flaring branch. Spectral analysis of the source revealed the presence of Fe K emission complex. The source spectra were fitted with a multitemperature blackbody (⁠|$\tt {diskbb}$|⁠) component in conjunction with the reflection model (⁠|$\tt {relxillNS}$|⁠). The values of temperature (kT in) and radius (R in) of the inner accretion disc obtained from the spectral fitting with the model combination – |$\tt {constant} \times \tt {tbabs} (\tt {diskbb}$|  +  |$\tt {relxillNS})$| showed the source to be in its soft spectral state during the observations. The inclination angle (θ) of the source was estimated to be between 19° and 33° and the inner disc radius (R in) was found to be 17.4 km. Assuming the case of magnetic truncation of accretion disc, the upper limits for the magnetic dipole moment (μ) and the magnetic field strength (B) at the poles of the neutron star in the system were found to be 5.78 × 1026 G cm3 and 8.23 × 108 G, respectively, for kA  = 1. [ABSTRACT FROM AUTHOR]

Core-collapse supernovae (CCSNe) are considered the primary magnetar formation channel, with 15 magnetars associated with supernova remnants (SNRs). A large fraction of these should occur in massive stellar binaries that are disrupted by the explosion, meaning that |$\sim 45~{{\ \rm per\ cent}}$| of magnetars should be nearby high-velocity stars. Here, we conduct a multiwavelength search for unbound stars, magnetar binaries, and SNR shells using public optical (uvgrizy bands), infrared (J, H, K , and Ks bands), and radio (888 MHz, 1.4 GHz, and 3 GHz) catalogues. We use Monte Carlo analyses of candidates to estimate the probability of association with a given magnetar based on their proximity, distance, proper motion, and magnitude. In addition to recovering a proposed magnetar binary, a proposed unbound binary, and 13 of 15 magnetar SNRs, we identify two new candidate unbound systems: an OB star from the Gaia catalogue we associate with SGR J1822.3−1606, and an X-ray pulsar we associate with 3XMM J185246.6 + 003317. Using a Markov Chain Monte Carlo simulation that assumes all magnetars descend from CCSNe, we constrain the fraction of magnetars with unbound companions to |$5\lesssim f_u \lesssim 24~{{\ \rm per\ cent}}$|⁠ , which disagrees with neutron star population synthesis results. Alternate formation channels are unlikely to wholly account for the lack of unbound binaries as this would require |$31\lesssim f_{nc} \lesssim 66~{{\ \rm per\ cent}}$| of magnetars to descend from such channels. Our results support a high fraction (⁠|$48\lesssim f_m \lesssim 86~{{\ \rm per\ cent}}$|⁠) of pre-CCSN mergers, which can amplify fossil magnetic fields to preferentially form magnetars. [ABSTRACT FROM AUTHOR]

The Laser Interferometer Space Antenna (LISA) will detect ∼104 Galactic binaries, the majority being double white dwarfs. However, approximately |$1 \!-\! 5~{{\ \rm per\ cent}}$| of these systems will contain neutron stars which, if they can be correctly identified, will provide new opportunities for studying binary evolution pathways involving mass reversal and supernovae as well as being promising targets for multimessenger observations. Eccentricity, expected from neutron star natal kicks, will be a key identifying signature for binaries containing a neutron star. Eccentric binaries radiate at widely spaced frequency harmonics that must first be identified as originating from a single source and then analysed coherently. A multiharmonic heterodyning approach for this type of data analysis is used to perform Bayesian parameter estimation on a range of simulated eccentric LISA signals. This is used to: (i) investigate LISA 's ability to measure orbital eccentricity and to quantify the minimum detectable eccentricity; (ii) demonstrate how eccentricity and periastron precession help to break the mass degeneracy allowing the individual component masses to be inferred, potentially confirming the presence of a neutron star; (iii) investigate the possibility of source misidentification when the individual harmonics of an eccentric binary masquerade as separate circular binaries; and (iv) investigate the possibility of source reclassification, where parameter estimation results of multiple circular analyses are combined in post-processing to quickly infer the parameters of an eccentric source. The broader implications of this for the ongoing design of the LISA global fit are also discussed. [ABSTRACT FROM AUTHOR]

During thermonuclear bursts, it is suspected that cooling of the corona by the burst emission may be the cause of hard X-ray deficits. Although such a deficit has been observed in nine sources, it has not been observed from 4U 1608–52, a nearby prolific burster. Therefore, the authenticity and universality of the hard X-ray deficit may be in question. To investigate this suspicion, Insight–HXMT performed cadence observations during the low/hard state of 4U 1608–52 in 2022 September and detected 10 thermonuclear X-ray bursts. Two of these bursts show a double-peaked structure in the soft X-ray band, which could be caused by the high temperature of the burst emission and a marginal photospheric radius expansion (PRE) around the burst peak time. This is indicated by their peak fluxes being up to the Eddington limit and having a large colour factor at the peak of the bursts. A hard X-ray deficit is observed at a significant level during bursts at > 30 keV. Furthermore, the fraction of this deficit shows saturation at 50 per cent for the first eight bursts. This saturation may indicate that the corona is layered and only part of the corona is cooled by the bursts: for example, the part close to the neutron star surface is cooled while the rest remains intact during bursts. This result provides a clue to the geometry of the corona. For example, a possible scenario is that the corona has two forms: a quasi-spheric corona between the neutron star and the disc and a disc corona on both surfaces of the disc. [ABSTRACT FROM AUTHOR]

Mature neutron stars are expected to exhibit gravity g modes due to stratification caused by a varying matter composition. These modes are affected by nuclear reactions, leading to complex (damped) mode frequencies and the suppression of high-order g modes. This is in contrast with the common non-dissipative analysis which leads to an infinite g-mode spectrum. Focusing on the transition between the fast- and slow-reaction regimes, we examine the precise impact of nuclear reactions on the g-mode spectrum. The general framework for the analysis is presented along with sample numerical results for a matter model based on the BSk21 equation of state with a suitable parametrization for the reaction rates. [ABSTRACT FROM AUTHOR]

Low-mass X-ray binaries (LMXBs) with neutron stars show quite different features that depend on the rate of mass transfer from the donor star. With a high transfer rate, the Z sources are in a persistent soft spectral state, and with a moderate transfer rate the transient Atoll sources have outburst cycles like the black hole X-ray binaries. The observations document very long outburst recurrence times for quite a number of sources. We follow with our computations the evolution of the accretion disc until the onset of the ionization instability. For sources with a low mass transfer rate, the accumulation of matter in the disc is essentially reduced due to the continuous evaporation of matter from the disc to the coronal flow. Different mass transfer rates result in nearly the same amount of matter accumulated for the outburst, which means that the outburst properties are similar for sources with short outburst cycles and sources with long outburst cycles, contrary to some expectations. Then, for systems with long recurrence time, less sources will be detected and the total population of LMXBs could be larger than it appears. This would relieve the apparent problem that the observed number of LMXBs as progenitors of millisecond pulsars (MSPs) is too small compared to the number of MSPs. Concerning the few quasi-persistent sources with year-long soft states, we argue that these states are not outbursts, but quasi-stationary hot states as in Z sources. [ABSTRACT FROM AUTHOR]

The detection of the short gamma-ray burst (SGRB) 050709 by the HETE-2 satellite opened the door to understanding the nature of SGRBs, offering clues about their emission mechanism and progenitors, with the crucial aid of optical follow-up observations. Here, we revisit the prompt emission of GRB 050709. Our analysis reveals an initial hard spike ∼200 ms long, followed by a subsequent soft-tail emission lasting ∼300 ms. These components could be common among other SGRBs originating from binary neutron merger events, such as GW 170817 and GRB 170817A. Detailed temporal and spectral analyses indicate that the soft-tail emission might be attributed to the cocoon formed by the relativistic jet depositing energy into the surrounding material. We find the necessary cocoon parameters at the breakout, as consistent with numerical simulation results. We compared the physical parameters of this cocoon with those of other SGRBs. The relatively higher cocoon pressure and temperature in GRB 050709 may indicate a more on-axis jet compared with GRB 170817A and GRB 150101B. [ABSTRACT FROM AUTHOR]

Some short gamma-ray bursts (SGRBs) exhibit a short-duration and spectral hard emission (referred to as a "hard spike") followed by a slightly longer soft emission (known as a "soft tail"). We identified nine SGRBs with the known redshift in the Swift/BAT gamma-ray burst catalog by specifically searching for the soft tail. We found that the spectra of these SGRBs can be described as a cutoff power-law model for the hard spike and the soft tail, and both show a time variation keeping the E peak– L iso correlation. This suggests that the emission mechanisms of both phenomena are identical. Furthermore, we found a trend of luminosity evolution as a function of redshift. This phenomenon suggests that these bursts originate from sources that are intrinsically bright and/or energy-density-concentrated within a narrower jet at higher redshift. We demonstrate that the average jet opening angle, derived from the jet break, can be explained by considering a model based on a strongly redshift-dependent jet opening angle. [ABSTRACT FROM AUTHOR]

The Vela pulsar is a young neutron star with spin period of P = 89 ms and a measured low braking index (˜1.4) that is much less than the standard value of 3 predicted by the magnetic dipole radiation (MDR) model; however, its spin period evolution has been a mystery. In this article, we assume that the spin‐down of the Vela pulsar is attributed to both MDR and wind flow (hereafter MDRW), and find that the ratio of wind flow to the magnetic dipole radiation is about 80%, which is higher than that of the Crab pulsar (25%). In other words, the spin‐down torque of the Vela pulsar is dominated by the wind flow. The spin period (P) evolution of the Vela pulsar depends on its real age, where its supernova remnant age is assumed to be an indicator of its true age, estimated from 10 to 20 kyr, and then we obtain their initial spin periods of ˜53.89 and ˜20.90 ms, respectively, which are consistent with the observed initial spin period ranges of young pulsars. Furthermore, we find that the Vela‐like pulsar by MDRW can evolve to the long spin period of a thousand of seconds in less than million years, which can conveniently help the astronomers understand the recently observed ultra‐long period radio pulsars like GPM J1839‐10 (P = 21 min), GLEAM‐X J1627 (P = 18 min), as well as PSR J0901+4046 (P = 76 s). [ABSTRACT FROM AUTHOR]

In the standard two-component crust-superfluid model of a neutron star, timing noise can arise when the two components are perturbed by stochastic torques. Here it is demonstrated how to analyse fluctuations in radio pulse times of arrival with a Kalman filter to measure physical properties of the two-component model, including the crust-superfluid coupling time-scale and the variances of the crust and superfluid torques. The analysis technique, validated previously on synthetic data, is applied to observations with the Molonglo Observatory Synthesis Telescope of the representative pulsar PSR J1359−6038. It is shown that the two-component model is preferred to a one-component model, with log Bayes factor 6.81 ± 0.02. The coupling time-scale and the torque variances on the crust and superfluid are measured with 90 per cent confidence to be |$10^{7.1^{+0.8}_{-0.5}}$| |$\rm {s}$| and |$10^{-24.0^{+0.4}_{-5.6}}$| |$\rm {rad^2~s^{-3}}$| and |$10^{-21.7^{+3.5}_{-0.9}}$| |$\rm {rad^2~s^{-3}}$|⁠ , respectively. [ABSTRACT FROM AUTHOR]

Fast radio bursts (FRBs) are cosmological radio transients with millisecond durations and extremely high brightness temperatures. One FRB repeater, FRB 180916.J0158+65 (FRB 180916B), was confirmed to appear 16.35-day periodic activities with 5-day activity window. Another FRB repeater, FRB 121102, and two soft gamma-ray repeaters (SGRs), SGR 1935+2154 and SGR 1806−20, also show possible periodic activities. These periodicities might originate from the precession process of young magnetars due to the anisotropic pressure from the inner magnetic fields as proposed in the literature. In this work, we analyse a self-consistent model for the rotation evolution of magnetars and obtain the evolutions of magnetar precession and obliquity. We find that if the FRB repeaters and the SGRs with (possible) periodic activities originate from the magnetar precession, their ages would be constrained to be hundreds to tens of thousands of years, which is consistent with the typical ages of magnetars. Assuming that the FRB emission is beaming in the magnetosphere as proposed in the literature, we calculate the evolution of the observable probability and the duty cycle of the active window period. We find that for a given magnetar the observable probability increases with the magnetar age in the early stage and decreases with the magnetar age in the later stage; meanwhile, there are one or two active windows in one precession period if the emission is not perfectly axisymmetric with respect to the deformation axis of a magnetar, which could be tested by the future observation for repeating FRB sources. [ABSTRACT FROM AUTHOR]

Cyclotron resonant scattering features (CRSFs) are the absorption features in the X-ray spectra of strongly magnetized accretion neutron stars (NSs), which are probably the most reliable probe to the surface magnetic fields of NSs. The high-mass X-ray binary GX 301–2 exhibits a very wide, variable, and complicated CRSF in the average spectra, which should be two absorption lines based on Nuclear Spectroscopic Telescope Array (NuStar) and Hard X-ray Modulation Telescope (Insight - HXMT) observations. With the Insight - HXMT frequent observations, we performed the phase-resolved spectroscopy and confirmed two cyclotron absorption lines in the phase-resolved spectra, with their centroid energy ratio ∼1.6–1.7 in the supercritical luminosity case. A major hindrance in understanding those CRSFs is the very poorly constrained magnetic inclination angle, which is also a fundamental property of an NS and key to understanding the emission characteristics of a pulsar. Comparing the phase-resolved CRSF with simulated X-ray spectra, the magnetic inclination angle is found to be ≳70°, i.e. nearly orthogonal between the NS's spin and magnetic axis. The implications of an orthogonal rotator and magnetic structure evolution in the accreting X-ray binary are also discussed. [ABSTRACT FROM AUTHOR]

It is expected that the extreme mass accretion rate onto strongly magnetized neutron stars results in the appearance of accretion columns above the stellar surface. For a distant observer, rotation of a star results in periodic variations of X-ray flux. Because the mass accretion rate fluctuates around the average value, the pulse profiles are not stable and demonstrate fluctuations as well. In the case of bright X-ray pulsars, however, pulse fluctuations are not solely attributed to variations in the mass accretion rate. They are also influenced by the variable height of the columns, which is dependent on the mass accretion rate. This study delves into the process of pulse profile formation in bright X-ray pulsars, taking into account stochastic fluctuations in the mass accretion rate, the corresponding variations in accretion column geometry, and gravitational bending. Our analysis reveals that potential eclipses of accretion columns by a neutron star during their spin period should manifest specific features in pulse profile variability. Applying a novel pulse profile analysis technique, we successfully detect these features in the bright X-ray transient V 0332+53 at luminosities |$\gtrsim 2\times 10^{38}\, {\rm erg\ \rm s^{-1}}$|⁠. This detection serves as compelling evidence for the eclipse of an accretion column by a neutron star. Detection of the eclipse places constraints on the relation between neutron star mass, radius, and accretion column height. Specifically, we can establish an upper limit on the accretion column height, which is crucial for refining theoretical models of extreme accretion. [ABSTRACT FROM AUTHOR]

Oscillating neutron stars are sources of continuous gravitational waves. We study analytically the excitation of stellar oscillations by the mechanical impact on the stellar surface of 'clumps' of stochastically accreted matter. We calculate the waveform and spectrum of the gravitational wave signal emitted by the accretion-driven pulsations. Results are generated for an idealized model of a non-rotating, unmagnetized, one-component star with uniform polytropic index n poly assuming Newtonian gravity and the Cowling approximation. We find that the excited mode amplitudes grow with increasing n poly and mode order n. The gravitational wave signal forms a sequence of amplitude-modulated packets for n poly = 1, lasting ∼10−3 s after each impact. The gravitational wave strain increases with increasing n poly, but decreases with increasing n and increasing multipole order l for n poly = 1. In the observing band of current long-baseline interferometers, g modes emit higher, narrower peaks in the amplitude spectral density than f and p modes, with the highest peaks reaching ∼10−26 Hz−1/2 for modes with damping time τ nl ∼ 108 yr. The root-mean-square strain h rms, calculated by summing over modes with 2 ≤ l ≤ 4 and τ nl ≤ 108 yr, spans the range 10−33 ≤ h rms ≤ 10−32 for 1 ≤ n poly ≤ 2. [ABSTRACT FROM AUTHOR]

It was recently discovered that the time correlations of repeating fast radio bursts (FRBs) are similar to earthquake aftershocks. Motivated by the association between FRBs and magnetars, here we report correlation function analyses in the time-energy space for the 563 periodic radio pulses and the 579 X-ray short bursts from the magnetar SGR 1935+2154, which is known to have generated FRBs. Although radio pulses are concentrated near the fixed phase of the rotational cycle, we find that when multiple pulses occur within a single cycle, their correlation properties (aftershock production probability, aftershock rate decaying in power of time, and more) are similar to those of extragalactic FRBs and earthquakes. A possible interpretation is that the radio pulses are produced by rupture of the neutron star crust, and the first pulse within one cycle is triggered by external force periodically exerted on the crust. The periodic external force may be from the interaction of the magnetosphere with material ejected in an outburst. For X-ray bursts, we found no significant correlation signal, though correlation on the same time-scale as radio pulses may be hidden due to the long event duration. The aftershock similarity between the periodic radio pulsation and FRBs is surprising, given that the two are energetically very different, and therefore the energy sources would be different. This suggests that the essence of FRB-like phenomena is starquakes, regardless of the energy source, and it is important to search for FRB-like bursts from neutron stars with various properties or environments. [ABSTRACT FROM AUTHOR]

Double neutron star (DNS) systems offer excellent opportunities to test gravity theories. We report the timing results of PSR J1901+0658, the first pulsar discovered in the FAST Galactic Plane Pulsar Snapshot (GPPS) Survey. Based on timing observations by FAST over 5 yr, we obtain the phase-coherent timing solution and derive the precise measurements of its position, spin parameters, orbital parameters, and dispersion measure. It has a period of 75.7 ms, a period derivative of 2.169(6) × 10−19 s s−1, and a characteristic age of 5.5 Gyr. This pulsar is in an orbit with a period of 14.45 d and an eccentricity of 0.366. One post-Keplerian parameter, periastron advance, has been well-measured as being 0.00531(9) deg yr−1, from which the total mass of this system is derived to be 2.79(7) M⊙. The pulsar has the mass upper limit of 1.68 M⊙, so the lower limit for the companion mass is 1.11 M⊙. Because PSR J1901+0658 is a partially recycled pulsar in an eccentric binary orbit with such a large companion mass, it should be in a DNS system according to the evolution history of the binary system. [ABSTRACT FROM AUTHOR]

Using MeerKAT , we have discovered three new millisecond pulsars (MSPs) in the bulge globular cluster M62: M62H, M62I, and M62J. All three are in binary systems, which means all ten known pulsars in the cluster are in binaries. M62H has a planetary-mass companion with a median mass M c, med ∼ 3 MJ and a mean density of ρ ∼ 11 g cm−3. M62I has an orbital period of 0.51 d and a M c, med ∼ 0.15 M⊙. Neither of these low-mass systems exhibit eclipses. M62J has only been detected in the two Ultra High Frequency band (816 MHz) observations with a flux density S 816 = 0.08 mJy. The non-detection in the L-band (1284 MHz) indicates it has a relatively steep spectrum (β < −3.1). We also present 23-yr-long timing solutions obtained using data from the Parkes 'Murriyang', Effelsberg , and MeerKAT telescopes for the six previously known pulsars. For all these pulsars, we measured the second spin-period derivatives and the rate of change of orbital period caused by the gravitational field of the cluster, and their proper motions. From these measurements, we conclude that the pulsars' maximum accelerations are consistent with the maximum cluster acceleration assuming a core-collapsed mass distribution. Studies of the eclipses of the redback M62B and the black widow M62E at four and two different frequency bands, respectively, reveal a frequency dependence with longer and asymmetric eclipses at lower frequencies. The presence of only binary MSPs in this cluster challenges models which suggest that the MSP population of core-collapsed clusters should be dominated by isolated MSPs. [ABSTRACT FROM AUTHOR]

Recent studies of the neutrino-driven wind from proto-neutron stars have indicated that the wind is likely proton rich for much of its lifetime, and the high flux of neutrinos can induce νp-process nucleosynthesis allowing for the formation of heavy elements. It has also been shown that gravito-acoustic waves, generated by convection within the proto-neutron star, can significantly alter the dynamics and nucleosynthesis in the wind. Therefore, we present a study of the effects of convection-driven waves on the nucleosynthesis in proton-rich neutrino-driven winds, focusing on the νp-process. We find that wave effects can strongly impact νp-process nucleosynthesis even at wave luminosities a factor of 10−5 smaller than the total neutrino luminosity. The momentum flux of the waves accelerates the wind, reducing the net neutrino heating and the persistent neutron abundance created by p (⁠|$\bar{\nu }_\mathrm{ e},e^+$|⁠), which impedes νp-process nucleosynthesis. However, this effect is generally counteracted by the effects of waves on seed nucleus formation, as the acceleration of the wind and the heating that occurs as these waves shock both favour a more α-rich environment with very little heavy seed nucleus formation. Overall, higher wave luminosities correlate (albeit non-monotonically) with heavier element νp-processing, up to A ≈ 200 in some cases. At very high wave luminosities (≳10−3 L ν), early shock heating by the waves disrupts α recombination, and drives a suppressed, fast-outflow r-process proceeding up to A ≈ 200. This occurs despite an assumed neutrino spectrum that predicts a proton-rich wind with equilibrium Y e = 0.6. [ABSTRACT FROM AUTHOR]

Recently, there has been renewed interest in the Be X-ray binary (Be/XRB) SAX J1324.4−6200 because of its spatial coincidence with a variable γ-ray source detected by Fermi/LAT. To explore more thoroughly its properties, new X-ray observations were carried out in 2023 by NuSTAR, XMM-Newton, and Swift satellites, jointly covering the energy range from 0.2 − 79 keV. SAX J1324.4−6200 was caught at an X-ray flux of ∼10−11 erg cm−2 s−1. The X-ray spectrum fits well with an absorbed power law with a high energy cutoff. Other acceptable fits require an additional blackbody component (kTbb ≈ 1.1 keV) or a Gaussian in absorption (Egabs ≈ 6.9 keV). We measured a NuSTAR spin period of 175.8127 ± 0.0036 s and an XMM-Newton spin period of 175.862 ± 0.025 s. We show that all the available spin period measurements of SAX J1324.4−6200, spanning 29 yr, are highly correlated with time, resulting in a remarkably stable spin-down of Ṗ = 6.09 ± 0.06 × 10−9 s s−1. We find that if SAX J1324.4−6200 hosts an accretion-powered pulsar, accretion torque models indicate a surface magnetic field of ∼1012 − 13 G. The X-ray properties emerging from our analysis strenghten the hypothesis that SAX J1324.4−6200 belongs to the small group of persistent Be/XRBs. We also performed radio observations with the Parkes Murriyang telescope, to search for radio pulsations. However, no radio pulsations compatible with the rotational ephemeris of SAX J1324.4−6200 were detected. We rule out the hypothesis that SAX J1324.4−6200 is a γ-ray binary where the emission is produced by interactions between the pulsar and the companion winds. Other models commonly used to account for the production of γ-rays in accreting pulsars cannot reproduce the bright emission from SAX J1324.4−6200. We examined other possible mechanisms behind the γ-ray emission and note that there is a ∼0.5% chance probability that an unknown extragalactic active galactic nucleus (AGN) observed through the Galactic plane may coincidentally fall within the Fermi/LAT error circle of the source and be responsible for the γ-ray emission. [ABSTRACT FROM AUTHOR]

Context. The radio-quiet γ-ray pulsar PSR J2021+4026 is a peculiar Fermi-LAT pulsar showing repeated and quasi-periodic mode changes. Its γ-ray flux shows repeated variations between two states at intervals of ∼3.5 years. These events occur over timescales < 100 days and are correlated with sudden changes in the spin-down rate. Multiwavelength observations also revealed an X-ray phase shift relative to the γ-ray profile for one of the events. PSR J2021+4026 is currently the only known isolated γ-ray pulsar showing significant variability, and thus it has been the object of thorough investigations. Aims. The goal of our work is to study the mode changes of PSR J2021+4026 with improved detail. By accurately characterizing variations in the γ-ray spectrum and pulse profile, we aim to relate the Fermi-LAT observations to theoretical models. We also aim to interpret the mode changes in terms of variations in the structure of a multipolar dissipative magnetosphere. Methods. We continually monitored the rotational evolution and the γ-ray flux of PSR J2021+4026 using more than 13 years of Fermi-LAT data with a binned likelihood approach. We investigated the features of the phase-resolved spectrum and pulse profile, and from these we inferred the macroscopic conductivity, the electric field parallel to the magnetic field, and the curvature radiation cutoff energy. These physical quantities are related to the spin-down rate and the γ-ray flux and therefore are relevant to the theoretical interpretation of the mode changes. We introduced a simple magnetosphere model that combines a dipole field with a strong quadrupole component. We simulated magnetic field configurations to determine the positions of the polar caps for different sets of parameters. Results. We clearly detect the previous mode changes and confirm a more recent mode change that occurred around June 2020. We provide a full set of best-fit parameters for the phase-resolved γ-ray spectrum and the pulse profile obtained in five distinct time intervals. We computed the relative variations in the best-fit parameters, finding typical flux changes between 13% and 20%. Correlations appear between the γ-ray flux and the spectral parameters, as the peak of the spectrum shifts by ∼10% toward lower energies when the flux decreases. The analysis of the pulse profile reveals that the pulsed fraction of the light curve is larger when the flux is low. Finally, the magnetosphere simulations show that some configurations could explain the observed multiwavelength variability. However, self-consistent models are required to reproduce the observed magnitudes of the mode changes. [ABSTRACT FROM AUTHOR]

Fast radio bursts (FRBs) can present a variety of polarization properties and some of them are characterized by narrow spectra. In this work, we study spectral properties from the perspective of intrinsic radiation mechanisms and absorption through the waves propagating in the magnetosphere. The intrinsic radiation mechanisms are considered by invoking quasi-periodic bunch distribution and perturbations on charged bunches moving on curved trajectories. The narrowband emission is likely to reflect some quasi-periodic structure on the bulk of bunches, which may be due to quasi-periodically sparking in a "gap" or quasi-monochromatic Langmuir waves. A sharp spike would appear in the spectrum if the perturbations were to induce a monochromatic oscillation of bunches; however, it is difficult to create a narrow spectrum because the Lorentz factor has large fluctuations, so the spike disappears. Both the bunching mechanism and perturbations scenarios share the same polarization properties, with a uniformly distributed bulk of bunches. We investigated the absorption effects, including Landau damping and curvature self-absorption in the magnetosphere, which are significant at low frequencies. Subluminous O-mode photons cannot escape from the magnetosphere due to the Landau damping, leading to a height-dependent lower frequency cut-off. The spectra can be narrow when the frequency cut-off is close to the characteristic frequency of curvature radiation, however, such conditions cannot always be met. The spectral index is 5/3 at low-frequency bands due to the curvature self-absorption is not as steep as what is seen in observations. The intrinsic radiation mechanisms are more likely to generate the observed narrow spectra of FRBs, rather than the absorption effects. [ABSTRACT FROM AUTHOR]

Context. Magnetic neutron stars (NSs) often exhibit a cyclotron resonant scattering feature (CRSF) in their X-ray spectra. Accretion onto their magnetic poles is responsible for the emergence of X-rays, but the site of the CRSF formation is still a puzzle. A promising candidate for high-luminosity sources has always been the radiative shock in the accretion column. Nevertheless, no quantitative calculations of spectral formation at the radiative shock have been performed so far. Aims. It is well accepted that, in the accretion column of a high-luminosity, accreting magnetic NS, a radiative shock is formed. Here, we aim to explore the scenario where the shock is the site of the cyclotron-line formation. We studied spectral formation at the radiative shock and the emergent spectral shape across a wide range of the parameter space and determined which parameters hold an important role in shaping a prominent CRSF. Methods. We developed a Monte Carlo (MC) code based on the forced first collision numerical scheme to conduct radiation transfer simulations at the radiative shock. The seed photons were due to bremsstrahlung and were emitted in the post-shock region. We properly treated bulk-motion Comptonization in the pre-shock region, thermal Comptonization in the post-shock region, and resonant Compton scattering in both regions. We adopted a fully relativistic scheme for the interaction between radiation and electrons, employing an appropriate polarization-averaged differential cross-section. As a result, we calculated the angle- and energy-dependent emergent X-ray spectrum from the radiative shock, focusing on both the CRSF and the X-ray continuum, under diverse conditions. The accretion column was characterized by cylindrical symmetry, and the radiative shock was treated as a mathematical discontinuity. Results. We find that a power law, hard X-ray continuum, and a CRSF are naturally produced by the first-order Fermi energization as the photons criss-cross the shock. The depth and the width of the CRSF depend mainly on the transverse optical depth and the post-shock temperature. We show that the cyclotron-line energy centroid is shifted by ∼(20 − 30)% to lower energies compared to the classical cyclotron energy; this is due to the Doppler boosting between the shock reference frame and the bulk-motion frame. We demonstrate that a "bump" feature arises in the right wing of the CRSF due to the up-scattering of photons by the accreting plasma and extends to higher energies for larger optical depths and post-shock temperatures. Conclusions. We conclude that resonant Compton scattering of photons by electrons in a radiative shock is efficient in producing a power-law X-ray continuum with a high-energy cutoff accompanied by a prominent CRSF. The implications of the Doppler effect on the centroid of the emergent absorption feature must be considered if an accurate determination of the magnetic field strength is desired. [ABSTRACT FROM AUTHOR]

Massive stars are predominantly found in binaries and higher order multiples. While the period and eccentricity distributions of OB stars are now well established across different metallicity regimes, the determination of mass-ratios has been mostly limited to double-lined spectroscopic binaries. As a consequence, the mass-ratio distribution remains subject to significant uncertainties. Open questions include the shape and extent of the companion mass-function towards its low-mass end and the nature of undetected companions in single-lined spectroscopic binaries. In this contribution, we present the results of a large and systematic analysis of a sample of over 80 single-lined O-type spectroscopic binaries (SB1s) in the Milky Way and in the Large Magellanic Cloud (LMC). We report on the developed methodology, the constraints obtained on the nature of SB1 companions, the distribution of O star mass-ratios at LMC metallicity and the occurrence of quiescent OB+black hole binaries. [ABSTRACT FROM AUTHOR]

Neutron star–white dwarf (NS + WD) binaries offer a unique opportunity for studying NS-specific phenomena with gravitational waves. In this paper, we employ the binary population synthesis technique to study the Galactic population of NS + WD binaries with the future Laser Interferometer Space Antenna (LISA). We anticipate approximately |$\mathcal {O}(10^2)$| detectable NS + WD binaries by LISA , encompassing both circular and eccentric ones formed via different pathways. Despite the challenge of distinguishing these binaries from more prevalent double white dwarfs (especially at frequencies below 2 mHz), we show that their eccentricity and chirp mass distributions may provide avenues to explore the NS natal kicks and common envelope evolution. Additionally, we investigate the spatial distribution of detectable NS + WD binaries relative to the Galactic plane and discuss prospects for identifying electromagnetic counterparts at radio wavelengths. Our results emphasise LISA 's capability to detect and characterize NS + WD binaries and to offer insights into the properties of the underlying population. Our conclusions carry significant implications for shaping LISA data analysis strategies and future data interpretation. [ABSTRACT FROM AUTHOR]

The massive stars end their lives by supernova explosions leaving central compact objects that may evolve into neutron stars. Initially, after birth, the star remains hot and gradually cools down. We explore the matter and star properties during this initial stage of the compact stars considering the possibility of the appearance of deconfined quark matter in the core of the star. At the initial stage after the supernova explosion, the occurrence of non-radial oscillation in the newly born compact object is highly possible. Non-radial oscillations are an important source of gravitational waves (GWs). There is a high chance for GWs from these oscillations, especially the nodeless fundamental (f) mode to be detected by next-generation GW detectors. We study the evolution in frequencies of non-radial oscillation after birth considering phase transition and predicting the possible signature for different possibilities of theoretical compact star models. [ABSTRACT FROM AUTHOR]

Within the magnetospheric radius, the geometry of accretion flow in X-ray pulsars is shaped by a strong magnetic field of a neutron star. Starting at the magnetospheric radius, accretion flow follows field lines and reaches the stellar surface in small regions located close to the magnetic poles of a star. At low mass accretion rates, the dynamics of the flow is determined by gravitational attraction and rotation of the magnetosphere due to the centrifugal force. At the luminosity range close to the Eddington limit and above it, the flow is additionally affected by the radiative force. We construct a model simulating accretion flow dynamics over the magnetosphere, assuming that the flow strictly follows field lines and is affected by gravity, radiative, and centrifugal forces only. The magnetic field of a neutron star is taken to be dominated by the dipole component of arbitrary inclination with respect to the accretion disc plane. We show that accretion flow becomes unstable at high mass accretion rates and tends to fluctuate quasi-periodically with a typical period comparable to the free-fall time from the inner disc radius. The inclination of a magnetic dipole with respect to the disc plane and strong anisotropy of X-ray radiation stabilize the mass accretion rate at the poles of a star, but the surface density of material covering the magnetosphere fluctuates even in this case. [ABSTRACT FROM AUTHOR]

We have determined positions, proper motions, and parallaxes of 77 millisecond pulsars (MSPs) from ∼3 yr of MeerKAT radio telescope observations. Our timing and noise analyses enable us to measure 35 significant parallaxes (12 of them for the first time) and 69 significant proper motions. Eight pulsars near the ecliptic have an accurate proper motion in ecliptic longitude only. PSR J0955−6150 has a good upper limit on its very small proper motion (<0.4 mas yr−1). We used pulsars with accurate parallaxes to study the MSP velocities. This yields 39 MSP transverse velocities, and combined with MSPs in the literature (excluding those in Globular Clusters) we analyse 66 MSPs in total. We find that MSPs have, on average, much lower velocities than normal pulsars, with a mean transverse velocity of only 78(8) km s−1 (MSPs) compared with 246(21) km s−1 (normal pulsars). We found no statistical differences between the velocity distributions of isolated and binary MSPs. From Galactocentric cylindrical velocities of the MSPs, we derive 3D velocity dispersions of σρ, σϕ, σ z  = 63(11), 48(8), 19(3) km s−1. We measure a mean asymmetric drift with amplitude 38(11) km s−1, consistent with expectation for MSPs, given their velocity dispersions and ages. The MSP velocity distribution is consistent with binary evolution models that predict very few MSPs with velocities >300 km s−1 and a mild anticorrelation of transverse velocity with orbital period. [ABSTRACT FROM AUTHOR]

Measurements of neutron star mass and radius or tidal deformability deliver unique insight into the equation of state (EOS) of cold dense matter. EOS inference is very often done using generalized parametric or non-parametric models, which deliver no information on composition. In this paper, we consider a microscopic nuclear EOS model based on a field theoretical approach. We show that current measurements from NICER and gravitational wave observations constrain primarily the symmetric nuclear matter EOS. We then explore what could be delivered by measurements of mass and radius at the level anticipated for future large-area X-ray timing telescopes. These should be able to place very strong limits on the symmetric nuclear matter EOS, in addition to constraining the nuclear symmetry energy that determines the proton fraction inside the neutron star. [ABSTRACT FROM AUTHOR]

In their catalogue of pulsars' radio spectra, Swainston et al. distinguish between five different forms of these spectra: those that can be fitted with (i) a simple power law, (ii) a broken power law, (iii) a low-frequency turn-over, (iv) a high-frequency turn-over or (v) a double turn-over spectrum. Here, we choose two examples from each of these categories and fit them with the spectral distribution function of the caustics that are generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star. In contrast to the prevailing view that the curved features of pulsars' radio spectra arise from the absorption of the observed radiation in high-density environments, our results imply that these features are intrinsic to the emission mechanism. We find that all observed features of pulsar spectra (including those that are normally fitted with simple or broken power laws) can be described by a single spectral distribution function and regarded as manifestations of a single emission mechanism. From the results of an earlier analysis of the emission from a pulsar's current sheet and the values of the fit parameters for each spectrum, we also determine the physical characteristics of the central neutron star of each considered example and its magnetosphere. [ABSTRACT FROM AUTHOR]

In eclipsing X-ray binary systems, the direct X-ray emission is blocked by the companion star during the eclipse. We observe only reprocessed emission that contains clues about the environment of the compact object and its chemical composition, ionization levels, etc. We have found flares in some X-ray binaries during their eclipses. The study of eclipse flares provides additional clues regarding the size of the reprocessing region and helps distinguish between different components of the X-ray spectrum observed during the eclipse. In the archival data, we searched for flares during eclipses of high-mass X-ray binaries and found flares in three sources: Vela X-1, LMC X-4, and 4U 1700−37. Comparing spectral properties of the eclipse flare and non-flare data, we found changes in the power-law photon index in all three sources and multiple emission lines in Vela X-1 and 4U 1700-37. The fluxes of prominent emission lines showed a similar increase as the overall X-ray flux during the eclipse flare, suggesting the lines originate in the binary environment and not in the interstellar medium. We also observed a soft excess in 4U 1700-37 that remains unchanged during both eclipse flare and non-flare states. Our analysis suggests that this emission originates from the extremely thin shell of the stellar wind surrounding the photosphere of its companion star. The detection of short (100–200 s) count-rate doubling time-scale in 4U 1700−37 and LMC X-4 indicates that the eclipse reprocessing occurs in a region larger than, but comparable to the size of the companion star. [ABSTRACT FROM AUTHOR]

We perform a comprehensive spectral study of a carefully selected sample (total exposure ∼50.5 ks) of NICER observations of the atoll neutron star low-mass X-ray binary 4U 1702−429. Our sample encompasses nearly all classical spectral states found within the NICER data set. We require two thermal emission components, originating from the accretion disc and the boundary layer, to describe the soft state spectra in the energy band 0.3–10.0 keV. In contrast, in our model, only the disc component directly contributes to the intermediate/hard state. Additionally, we use a thermally Comptonized component (or a power law with pegged normalization) to represent the hard coronal emission in the soft and intermediate/hard state spectra. The boundary layer emerges as the principal source providing soft seed photons for Comptonization across all spectral states. In contrast to a previously held assertion regarding this source, our analyses reveal a decrease in the inner disc temperature coupled with the retreat of the inner disc from the neutron star surface as the source evolves from the soft to the intermediate/hard state. The reflection features are either absent or weak (∼3σ–4σ) in all these observations. Further investigation using broad-band NuSTAR (3.0–50.0 keV) and AstroSat spectra (1.3–25.0 keV) shows a slightly stronger iron emission line (∼5.8σ) in the NuSTAR spectra. However, this feature is not significantly detected in the AstroSat observation. The AstroSat data suggest a highly ionized disc, explaining the absence of reflection features. In the case of NuSTAR , a truncated disc is likely responsible for the weak reflection features. [ABSTRACT FROM AUTHOR]

4U 1702−429, an atoll-type neutron star low-mass X-ray binary, was observed twice by the AstroSat /Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC-20) on 2018 April 27 and 2019 August 8. Persistent emission spectra of the source were well fitted with the model combination - constant × tbabs (thcomp × diskbb+powerlaw). The parameters obtained from the spectral analysis revealed the source to be in a hard spectral state during the observations. Time-resolved spectral analyses were performed on the three type I X-ray bursts detected from the source. Burst analysis showed that the source underwent a photospheric radius expansion. Consequently, the radius of the neutron star and distance to the source (with isotropic and anisotropic burst emission) were obtained as 12.65 |$\substack{+0.90\\-0.86}$| km and 6.92 |$\substack{+0.16\\-0.09}$| and 8.43 |$\substack{+0.20\\-0.10}$| kpc, respectively. [ABSTRACT FROM AUTHOR]

Binary neutron star mergers are the first confirmed site of element nucleosynthesis by the rapid neutron-capture process (r -process). The kilonova AT2017gfo is the only electromagnetic counterpart of a neutron star merger spectroscopically observed. We analyse the entire spectral sequence of AT2017gfo (from merger to +10.4 d) and identify seven emission-like features. We confirm that the prominent 1.08  |$\mu{\text{m}}$| feature can be explained by the Sr  ii near-infrared triplet evolving from a P-Cygni profile through to pure emission. We calculate the expected strength of the [Sr  ii ] doublet and show that its absence requires highly clumped ejecta. Near-infrared features at 1.58 and 2.07  |$\mu {\text{m}}$| emerge after three days and become more prominent as the spectra evolve. We model these as optically thick P-Cygni profiles and alternatively as pure emission features (with FWHM ≃ 35 600 ± 6600 km s−1) and favour the latter interpretation. The profile of the strong 2.07  |$\mu {\text{m}}$| emission feature is best reproduced with two lines, centred at 2.059 and 2.135  |$\mu {\text{m}}$|⁠. We search for candidate ions for all prominent features in the spectra. Strong, permitted transitions of La  iii , Ce  iii , Gd  iii , Ra  ii , and Ac  i are plausible candidates for the emission features. If any of these features are produced by intrinsically weak, forbidden transitions, we highlight candidate ions spanning the three r -process peaks. The second r -process peak elements Te and I have plausible matches to multiple features. We highlight the need for more detailed and quantitative atomic line transition data. [ABSTRACT FROM AUTHOR]

Neutron star–white dwarf (NSWD) binaries are one of the most abundant sources of gravitational waves (GWs) in the Milky Way. These GW sources are the evolutionary products of primordial binaries that experienced many processes of binary interaction. We employ a binary population synthesis method to investigate the properties of Galactic NSWD binaries detectable by the Laser Interferometer Space Antenna (LISA). In this paper, only the NSWD systems with a COWD or ONeWD component are included. We consider various models related to mass-transfer efficiencies during primordial binary evolution, supernova explosion mechanisms at NS formation, common envelope ejection efficiencies, and critical WD masses that determine the stability of mass transfer between WDs and NSs. Based on our calculations, we estimate that tens to hundreds of LISA NSWD binaries exist in the Milky Way. We find that the detection of LISA NSWD binaries is able to provide profound insights into mass-transfer efficiencies during the evolution of primordial binaries and critical WD masses during mass transfer from a WD to an NS. [ABSTRACT FROM AUTHOR]

The accretion flow within the magnetospheric radius of bright X-ray pulsars can form an optically thick envelope, concealing the central neutron star from the distant observer. Most photons are emitted at the surface of a neutron star and leave the system after multiple reflections by the accretion material covering the magnetosphere. Reflections cause momentum to be transferred between photons and the accretion flow, which contributes to the radiative force and should thus influence the dynamics of accretion. We employ Monte Carlo simulations and estimate the acceleration along magnetic field lines due to the radiative force as well as the radiation pressure across magnetic field lines. We demonstrate that the radiative acceleration can exceed gravitational acceleration along the field lines, and similarly, radiation pressure can exceed magnetic field pressure. Multiple reflections of X-ray photons back into the envelope tend to amplify both radiative force along the field lines and radiative pressure. We analyse the average photon escape time from the magnetosphere of a star and show that its absolute value is weakly dependent on the magnetic field strength of a star and roughly linearly dependent on the mass accretion rate being |$\sim 0.1\, {\rm s}$| at |$\dot{M}\sim 10^{20}\, {\rm g\, s^{-1}}$|⁠. At high mass accretion rates, the escape time can be longer than free-fall time from the inner disc radius. [ABSTRACT FROM AUTHOR]

We conducted an analysis of 45 bursts observed from 4U 1636−53. To investigate the mechanism behind the light-curve profiles and the impact of thermonuclear X-ray bursts on the accretion environment in accreting neutron star low-mass X-ray binaries. This analysis employed both light-curve and time-resolved spectroscopy methodologies, with data collected by the Insight - Hard X-ray Modulation Telescope instrument. We found that 30 bursts exhibited similar light-curve profiles and were predominantly in the hard state, and two photospheric radius expansion (PRE) bursts were in the soft state. The light curves of most bursts did not follow a single exponential decay but displayed a dual-exponential behaviour. The initial exponent had a duration of approximately 6 s. We utilized both the standard method and the ' f a' method to fit the burst spectra. The majority of the ' f a' values exceeded 1, indicating an enhancement of the persistent emission during the burst. Under the two Comptonization components assumption, we suggest that the scattering of burst photons by the inner corona may mainly contribute to the persistent emission enhancement. We also observed an inverse correlation between the maximum f a and the persistent emission flux in the non-PRE burst. This anticorrelation suggests that when the accretion rate is lower, there is a greater enhancement of persistent emission during the burst peak. The prediction based on Poynting–Robertson drag (P–R drag) aligns with this observed anticorrelation. [ABSTRACT FROM AUTHOR]

After GW170817, kilonovae have become of great interest for the astronomical, astrophysics, and nuclear physics communities due to their potential in revealing key information on the compact binary merger from which they emerge, such as the fate of the central remnant or the composition of the expelled material. Therefore, the landscape of models employed for their analysis is rapidly evolving, with multiple approaches being used for different purposes. In this paper, we present xkn , a semi-analytic framework that predicts and interprets the bolometric luminosity and the broad-band light curves of such transients. xkn models the merger ejecta structure accounting for different ejecta components and non-spherical geometries. In addition to light-curve models from the literature based on time-scale and random-walk arguments, it implements a new model, xkn-diff , which is grounded on a solution of the radiative transfer equation for homologously expanding material. In order to characterize the variety of the ejecta conditions, it employs time- and composition-dependent heating rates, thermalization efficiencies, and opacities. We compare xkn light curves with reference radiative transfer calculations, and we find that xkn-diff significantly improves over previous semi-analytic prescriptions. We view xkn as an ideal tool for extensive parameter estimation data analysis applications. [ABSTRACT FROM AUTHOR]

Radio emission from pulsars can be used to map out their distances through dispersion measure (DM), which quantifies the amount of radio pulse dispersion. However, this method relies on accurately modelling the free electron density in the line of sight. Here, we present a detailed study of the multiwavelength emission from PSR J1720−0534, a black widow compact binary millisecond pulsar discovered in 2021, which the latest electron density model of the Galaxy places at only 191 pc. We obtained and analysed deep multiwavelength observations in the γ-ray (Fermi -Large Area Telescope, 2008–2022), optical (Las Cumbres Observatory, 2.7 h), near-infrared (Nordic Optical Telescope, 3.5 h), and X-ray (Swift -X-Ray Telescope, 10 ks) bands. We found no significant detection of γ-ray, optical, near-infrared, or X-ray counterparts around the radio-timing position of PSR J1720−0534, which we thus nickname 'the invisible black widow'. Employing the most constraining near-infrared limit (J > 23.4 mag), we established a lower limit on the source distance, d > 1.1 kpc, assuming conservative properties for the black widow companion star. This distance lower limit differs drastically (by a factor of more than 5) from the Yao et al. DM distance estimate. We attribute this difference to the inclusion in the Yao et al. model of a large and dense component towards the North Polar Spur. Considering our results and recent parallax distances to other pulsars in this direction, we argue that such a local and large component in the electron density model of the Galaxy is unnecessary. [ABSTRACT FROM AUTHOR]

A number of pulsars are known to have profile evolution on time-scales of months, often correlated with spin-down rate changes. Here, we present the first result from 3 yr of monitoring observations from MeerKAT as part of the Thousand Pulsar Array programme. This programme obtains high-fidelity pulse profiles for ∼ 500 pulsars, which enabled the detection of subtle changes in seven sources not previously known to exhibit long-term profile evolution. A 2D Gaussian convolution is used to highlight correlated emission variability in both the pulse phase and observing epoch direction. Simulations show that for one additional source the observed profile variability is likely to originate from stochastic single-pulse shape variability (jitter). We find that it is common for long-term profile variability to be associated with changes in polarization fractions, but not with polarization position angle (PA) changes. PA changes are expected if emission height changes or precession is responsible for the profile variability. PSR J1741−3927 is the only pulsar in our sample that shows correlated PA variability, and this is associated with orthogonal polarization mode activity. For the six other pulsars limits on possible emission height changes and impact angle changes are derived. These limits are consistent with the small changes in the total intensity profile shape. None of the sources show detectable spin-down variability correlated with the emission changes, which are thought to be driven by magnetospheric current fluctuations. Therefore, the absence of correlated spin-down rate variability allows upper limits to be placed on changes in the magnetospheric charge density. [ABSTRACT FROM AUTHOR]

We reported the observations of nulling, subpulse drifting, and moding of radio radiation in pulsar (PSR) B1918+19 at 1250 MHz with the Five-hundred-meter Aperture Spherical radio Telescope. The nulling fraction is 2.6 ± 0.1 per cent and no periodicity of nulling can be seen. We confirmed the existence of three different drift modes (A, B, C) and a disordered mode (N) at 1250 MHz. Unlike the other three modes where the second component of the average pulse profile dominates, the first component of the average pulse profile in mode C is brighter than the other components, the second component shifts forward and the fourth component shifts backward. The bidrifting phenomenon is observed in PSR B1918+19, the drifting of the first and second components is positive, and the drift direction of the fourth component is negative. The drifting rate of the drift bands composed of the first and second components has an obvious evolutionary trend. Mode B has a short duration and no clear trend can be seen. But for mode A, the drift rate of each drift band starts relatively fast, then slows down to a steady state, and finally increases slowly until it enters either null or mode N. Further analysis shows that the emergence and significant variation in the drifting period of multidrifting subpulse emission modes for PSR B1918+19 may be due to the aliasing effect. The interesting subpulse emission phenomenon of PSR B1918+19 at different frequencies provides a unique opportunity to understand the switching mechanism of the multidrift mode of the pulsars. [ABSTRACT FROM AUTHOR]

It is widely believed that the binary neutron star merger GW190425 produced a black hole promptly upon merger. Motivated by the potential association with the fast radio burst FRB 20190425A, which took place 2.5 h after the merger, we revisit the question of the outcome of GW190425 by means of numerical relativity simulations. We show that current laboratory and astrophysical constraints on the equation of state of dense matter do not rule out the formation of a long-lived remnant. However, the formation of a stable remnant would have produced a bright kilonova, in tension with upper limits by ZTF at the location and time of FRB 20190425A. Moreover, the ejecta would have been optically thick to radio emission for days to months, preventing a putative FRB from propagating out. The predicted dispersion measure is also several orders of magnitude larger than that observed for FRB 20190425A. Our results indicate that FRB 20190425A and GW190425 are not associated. However, we cannot completely rule out the formation of a long-lived remnant, due to the incomplete coverage of the relevant sky regions. More observations of GW190425-like events, including potential upper limit, have the potential to constrain nuclear physics. To this aim, it is important that follow-up observational campaigns of gravitational wave events are informed by the properties of the source, such as their chirp mass, and we urge the LIGO-Virgo-KAGRA collaboration to promptly release them publicly. [ABSTRACT FROM AUTHOR]

After a successful supernova, a proto-neutron star (PNS) cools by emitting neutrinos on ∼1–100 s time-scales. Provided that there are neutrino emission 'hotspots' or 'cold-spots' on the surface of the rotating PNS, we can expect a periodic modulation in the number of neutrinos observable by detectors. We show that Fourier transform techniques can be used to determine the PNS rotation rate from the neutrino arrival times. Provided there is no spin-down, a 1-parameter Discrete Fourier Transform (DFT) is sufficient to determine the spin period of the PNS. If the PNS is born as a magnetar with polar magnetic field strength B 0 ≳ 1015 G and is 'slowly' rotating with an initial spin period ≳100 ms, then it can spin-down to periods of the order of seconds during the cooling phase. We propose a modified DFT technique with three frequency parameters to detect spin-down. Due to lack of neutrino data from a nearby supernova except the ∼20 neutrinos detected from SN1987A, we use toy models and one physically motivated modulating function to generate neutrino arrival times. We use the false alarm rate (FAR) to quantify the significance of the Fourier power spectrum peaks. We show that PNS rotation and spin-down are detected with |$\rm FAR\,\lt\, 2~{{\ \rm per\ cent}}$| (2σ) for periodic signal content |$\rm M\gtrsim 13-15~{{\ \rm per\ cent}}$| if 5 × 103 neutrinos are detected in ∼3 s and with |$\rm FAR\,\lt\, 1{{\ \rm per\ cent}}$| for |$\rm M\,\ge 5{{\ \rm per\ cent}}$| if 5 × 104 neutrinos are detected in ∼3 s. Since we can expect ∼104−105 neutrino detections from a supernova at 10 kpc, detection of PNS rotation and spin-down is possible using the neutrinos from the next Galactic supernova. [ABSTRACT FROM AUTHOR]