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The milestone discovery of GW170817-GRB 170817A-AT 2017gfo has shown that gravitational waves (GWs) could be produced during the merger of a neutron star–neutron star/black hole and that in electromagnetic (EM) waves, a gamma-ray burst (GRB) and a kilonova (KN) are generated in sequence after the merger. Observationally, however, EM properties before the merger phase are still unclear. Here we report a peculiar precursor in a KN-associated long-duration GRB 211211A, providing evidence of the EM before the merger. This precursor lasts ∼0.2 s, and the waiting time between the precursor and the main burst is ∼1 s, comparable to that between GW170817 and GRB 170817A. The spectrum of the precursor could be well fit with a nonthermal cutoff power-law model instead of a blackbody. In particular, a ∼22 Hz quasiperiodic oscillation candidate (∼3 σ) is detected in the precursor. These temporal and spectral properties indicate that this precursor is probably produced by a catastrophic flare accompanied with magnetoelastic or crustal oscillations of a magnetar in a binary compact merger. The strong magnetic field of the magnetar can also account for the prolonged duration of GRB 211211A. However, it poses a challenge to reconcile the rather short lifetime of a magnetar with the rather long spiraling time of a binary neutron star system only by the GW radiation before the merger. [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]

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]

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]

A newly born millisecond magnetar is thought to be the central engine of some gamma-ray bursts (GRBs), especially those that present long-lasting X-ray plateau emissions. By solving the field equations, we find that when the rotational speed of the magnetar is approaching the breakup limit, its radius R and moment of inertia I undergo an obvious evolution as the magnetar spins down. Meanwhile, the values of R and I sensitively depend on the adoption of a neutron star (NS) equation of state (EoS) and the NS baryonic mass. With different EoSs and baryonic mass considered, the magnetic dipole radiation luminosity (Ldip) could be variant within one to two orders of magnitude. We thus suggest that when using the X-ray plateau data of GRBs to diagnose the properties of the nascent NSs, EoS and NS mass information should be invoked as simultaneously constrained parameters. On the other hand, due to the evolution of R and I, the temporal behavior of Ldip becomes more complicated. For instance, if the spin-down process is dominated by gravitational wave emission due to the NS asymmetry caused by magnetic field distortion (), the segment Ldip ∝ t0 could be followed by Ldip ∝ t−γ with γ larger than 3. This case could naturally interpret the so-called internal X-ray plateau feature shown in some GRB afterglows, which means the sharp decay following the plateau unnecessarily corresponds to the collapse of the NS. This may explain why some internal X-ray plateaus are followed by late-time central engine activity, manifested through flares and second shallow plateaus. [ABSTRACT FROM AUTHOR]

Gravitational wave (GW) events, produced by the coalescence of binary neutron stars (BNSs), can be treated as the standard sirens to probe the expansion history of the universe, if their redshifts can be determined from electromagnetic (EM) observations. For the high-redshift (z ≳ 0.1) events, the short γ-ray bursts (sGRBs) and the afterglows are always considered as the primary EM counterparts. In this paper, by investigating various models of sGRBs and afterglows, we discuss the rates and distributions of the multimessenger observations of BNS mergers using GW detectors in the second-generation (2G), 2.5G, and 3G era with detectable sGRBs and afterglows. For instance, for the Cosmic Explorer GW detector, the rate is about 300–3500 yr−1 with a GECAM-like detector for γ-ray emissions and an LSST/WFST detector for optical afterglows. In addition, we find that these events have redshifts z ≲ 2 and inclination angles ι ≲ 20°. These results justify the rough estimation in previous works. Considering these events as standard sirens to constrain the equation-of-state parameters of dark energy w0 and wa, we obtain the potential constraints of Δw0 ≃ 0.02–0.05 and Δwa ≃ 0.1–0.4. [ABSTRACT FROM AUTHOR]