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The maximum mass of neutron stars (M TOV) plays a crucial role in understanding their equation of state (EoS). Previous studies have used the measurements for the compactness of massive pulsars and the tidal deformability of neutron stars in binary neutron star (BNS) mergers to constrain the EoS and thus the M TOV. The discovery of the most massive pulsar, PSR J0952−0607, with a mass |$\sim 2.35\, {\rm M}_{\odot }$|⁠ , has provided a valuable lower limit for M TOV. Another efficient method to constrain M TOV is by examining the type of central remnant formed after a BNS merger. Gravitational wave (GW) data can provide the total mass of the system, while accompanying electromagnetic signals can help infer the remnant type. In this study, we combine all the previous constraints and utilize the observational facts that about 24 per cent of the short gamma-ray bursts are followed by an X-ray internal plateau, which indicate that roughly this fraction of BNS mergers yield supermassive neutron stars, to perform (Markov Chain) Monte Carlo simulations. These simulations allow us to explore the probability density distribution of M TOV and other parameters related to BNS mergers. Our findings suggest that M TOV is likely around |$2.49\!-\!2.52\, {\rm M}_{\odot }$|⁠ , with an uncertainty range of approximately [ |$-0.16$|⁠ , |$0.15\, {\rm M}_{\odot }$| ] ([ |$-0.28$|⁠ , |$0.26\, {\rm M}_{\odot }$| ]) at 1σ (2σ) confidence level. Furthermore, we examine the type of merger remnants in specific events like GW170817 and GW190425 to further constrain M TOV and other relevant parameters, which can help to understand the physical processes involved in BNS mergers. [ABSTRACT FROM AUTHOR]

As the most energetic explosions in the Universe, gamma-ray bursts (GRBs) are commonly believed to be generated by relativistic jets. Recent observational evidence suggests that the jets producing GRBs are likely to have a structured nature. Some studies have suggested that non-axisymmetric structured jets may be formed through internal non-uniform magnetic dissipation processes or the precession of the central engine. In this study, we analyze the potential characteristics of GRB afterglows within the framework of non-axisymmetric structured jets. We simplify the profile of the asymmetric jet as a step function of the azimuth angle, dividing the entire jet into individual elements. By considering specific cases, we demonstrate that the velocity, energy, and line-of-sight direction of each jet element can greatly affect the behaviour of the overall light curve. The radiative contributions from multiple elements may lead to the appearance of multiple distinct peaks or plateaus in the light curve. Furthermore, fluctuations in the rising and declining segments of each peak can be observed. These findings establish a theoretical foundation for future investigations into the structural characteristics of GRBs by leveraging GRB afterglow data. [ABSTRACT FROM AUTHOR]

The giant X-ray bump of GRB 121027A observed by Swift is different from the typical X-ray flares in gamma-ray bursts. The observed structural variability in the rise and decay phases of the bump has four components. Of these four components, only the data in the bump from about 5300 to about 6100 s is of good enough quality to be analysed using the stepwise filter correlation method. A $86^{+5.9}_{-9.4} \rm s$ periodic oscillation is postulated, which is confirmed by the Lomb–Scargle method. A jet precession model is proposed to account for this variability. [ABSTRACT FROM AUTHOR]

Strong gravitational lensing effect is a powerful tool to probe cosmological models and gravity theories. Recently, the time-delay cosmography from strong lensing and the stellar kinematics of the deflector, which encode the Hubble constant and the post-Newtonian parameter via two distance ratios reflecting the lensing mass and dynamical mass, respectively, have been proposed to investigate these two parameters simultaneously. Among strong-lensing systems with different sources, strongly lensed fast radio bursts (FRBs) have been proposed as precision probes of the universe since the time-delay ∼ 10 d between images could be measured extremely precisely because of their short duration of a few milliseconds. In this work, we investigate the ability of strongly lensed FRBs on simultaneously estimating these two parameters via simulations. Take the expected FRB detection rate of upcoming facilities and lensing probability into consideration, it is likely to accumulate 10 lensed FRBs in several years and we find that H 0 could be determined to an |$\sim 1.5{{\,\rm per\,cent}}$| precision and γPPN could be constrained to an |$\sim 8.7{{\,\rm per\,cent}}$| precision simultaneously from them. These simultaneous estimations will be helpful for properly reflecting the possible correlation between these two fundamental parameters. [ABSTRACT FROM AUTHOR]

The detection of a secular post-merger gravitational wave (GW) signal in a binary neutron star (BNS) merger serves as strong evidence for the formation of a long-lived post-merger neutron star (NS), which can help constrain the maximum mass of NSs and differentiate NS equations of state. We specifically focus on the detection of GW emissions from rigidly rotating NSs formed through BNS mergers, using several kilohertz GW detectors that have been designed. We simulate the BNS mergers within the detecting limit of LIGO-Virgo-KARGA O4 and attempt to find out on what fraction the simulated sources may have a detectable secular post-merger GW signal. For kilohertz detectors designed in the same configuration of LIGO A+, we find that the design with peak sensitivity at approximately 2 kHz is most appropriate for such signals. The fraction of sources that have a detectable secular post-merger GW signal would be approximately |$0.94{\!-\!}11~{{ \rm per\ cent}}$| when the spin-downs of the post-merger rigidly rotating NSs are dominated by GW radiation, while it would be approximately |$0.46{\!-\!}1.6~{{ \rm per\ cent}}$| when the contribution of electromagnetic (EM) radiation to the spin-down processes is non-negligible. We also estimate this fraction based on other well-known proposed kilohertz GW detectors and find that, with advanced design, it can reach approximately |$12{\!-\!}45~{{ \rm per\ cent}}$| for the GW-dominated spin-down case and |$4.7{\!-\!}16~{{\rm per\ cent}}$| when both the GW and EM radiations are considered. [ABSTRACT FROM AUTHOR]

For non-spinning, charged (Reissner–Nordström) black holes, the particles with an opposite sign of charge with respect to that of the black hole will be pulled into the black hole by the extra electromagnetic force. Such a hole will be quickly neutralized so that there should not exist significantly charged, non-spinning black holes in the universe. The case of spinning, charged (Kerr–Newmann, KN) black holes is more complicated. For a given initial position and initial velocity of the particle, an oppositely charged particle does not always more easily fall into the black hole than a neutral particle. The possible existence of a magnetosphere further complicate the picture. One therefore cannot straightforwardly conclude that a charged spinning black hole will be neutralized. In this paper, we make the first step to investigate the neutralization of KN black holes without introducing a magnetosphere. We track the particle trajectories under the influence of the curved space–time and the electromagnetic field carried by the spinning, charged black hole. A statistical method is used to investigate the neutralization problem. We find a universal dependence of the falling probability into the black hole on the charge of the test particle, with the oppositely charged particles having a higher probability of falling. We therefore conclude that charged, spinning black holes without a magnetosphere should be quickly neutralized, consistent with people's intuition. The neutralization problem of KN black holes with a corotating force-free magnetosphere is subject to further studies. [ABSTRACT FROM AUTHOR]

Spectral lag of the low-energy photons with respect to the high-energy ones is a common astrophysical phenomenon (such as gamma-ray bursts and the Crab Pulsar) and may serve as a key probe to the underlying radiation mechanism. However, spectral lag in keV range of the magnetar bursts has not been systematically studied yet. In this work, we perform a detailed spectral lag analysis with the Li et al.'s Cross-Correlation Function (Li-CCF) method for SGR J1935+2154 bursts observed by Insight - Hard X-ray Modulation Telescope (HXMT), Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM), and Fermi /Gamma-ray Burst Monitor (GBM) from 2014 July to 2022 January. We discover that the spectral lags of about 61 per cent (non-zero significance >1σ) bursts from SGR J1935+2154 are linearly dependent on the photon energy (E) with t lag(E) = α(E /keV) + C , which may be explained by a linear change of the temperature of the blackbody-emitting plasma with time. The distribution of the slope (α) approximately follows a Gaussian function with mean and standard deviation of 0.02 ms keV−1 (i.e. high-energy photons arrive earlier) and 0.02 ms keV−1, respectively. We also find that the distribution can be well fitted with three Gaussians with mean values of ∼−10.009, 0.013, and 0.039 ms keV−1, which may correspond to different origins of the bursts. These spectral lag features may have important implications on the magnetar bursts. [ABSTRACT FROM AUTHOR]

The multimessenger joint observations of GW170817 and GRB170817A shed new light on the study of short-duration gamma-ray bursts (SGRBs). Not only did it substantiate the assumption that SGRBs originate from binary neutron star (BNS) mergers, but it also confirms that the jet generated by this type of merger must be structured, hence the observed energy of an SGRB depends on the viewing angle from the observer. However, the precise structure of the jet is still subject to debate. Moreover, whether a single unified jet model can be applied to all SGRBs is not known. Another uncertainty is the delay time-scale of BNS mergers with respect to star-formation history of the Universe. In this paper, we conduct a global test of both delay and jet models of BNS mergers across a wide parameter space with simulated SGRBs. We compare the simulated peak flux, redshift, and luminosity distributions with the observed ones and test the goodness-of-fit for a set of models and parameter combinations. Our simulations suggest that GW170817/GRB 170817A and all SGRBs can be understood within the framework of a universal structured jet viewed at different viewing angles. Furthermore, model invoking a jet plus cocoon structure with a lognormal delay time-scale is most favoured. Some other combinations (e.g. a Gaussian delay with a power-law jet model) are also acceptable. However, the Gaussian delay with Gaussian jet model and the entire set of power-law delay models are disfavoured. [ABSTRACT FROM AUTHOR]

Primordial black holes (PBHs) has been considered to form a part of dark matter for a long time but the possibility has been poorly constrained over a wide mass range, including the stellar mass range (⁠|$1{-}100\ {\rm M}_{\odot }$|⁠). However, due to the discovery of merger events of black hole binaries by LIGO-Virgo gravitational wave observatories, the interest for PBHs in the stellar mass window has been aroused again. Fast radio bursts (FRBs) are bright radio transients with millisecond duration and very high all-sky occurrence rate. Lensing effect of these bursts has been proposed as one of the optimal probes for constraining the abundance of PBHs in the stellar mass range. In this paper, we first investigate constraints on the abundance of PBHs from the latest 593 FRB observations for both the monochromatic mass distribution and three other popular extended mass distributions related to different formation mechanisms of PBHs. It is found that constraints from currently public FRB observations are relatively weaker than those from existing gravitational wave detections. Furthermore, we forecast constraining power of future FRB observations on the abundance of PBHs with different mass distributions of PBHs and different redshift distributions of FRBs taken into account. Finally, We find that constraints of parameter space on extended mass distributions from ∼105 FRBs with |$\overline{\Delta t}\le 1 ~\rm ms$| would be comparable with what can be constrained from gravitational wave events. It is foreseen that upcoming complementary multimessenger observations will yield considerable constraints on the possibilities of PBHs in this intriguing mass window. [ABSTRACT FROM AUTHOR]