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A tiny fraction of observed gamma-ray bursts (GRBs) may be lensed. The time delays induced by the gravitational lensing are milliseconds to seconds if the point lenses are intermediate-mass black holes. The prompt emission of the lensed GRBs, in principle, should have repeated pulses with identical light curves and spectra but different fluxes and slightly offset positions. In this work, we search for such candidates within the GRBs detected by Fermi/GBM, Swift/BAT, and HXMT/HE and report the identification of an attractive event GRB 200716C that consists of two pulses. Both the autocorrelation analysis and the Bayesian inference of the prompt emission light curve are in favor of the gravitational lensing scenario. Moreover, the spectral properties of the two pulses are rather similar and follow the so-called Amati relation of short GRBs rather than long duration bursts. The measured flux ratios between the two pulses are nearly constant in all channels, as expected from gravitational lensing. We therefore suggest that the long duration burst GRB 200716C was a short event being lensed. The redshifted mass of the lens was estimated to be $4.25^{+2.46}_{-1.36}$ $times$ $10^5$ $M_{odot}$ (90$%$ credibility). If correct, this could point towards the existence of an intermediate-mass black hole along the line of sight of GRB 200716C.
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Observationally, there are a small fraction GRBs prompt emission observed by Fermi/GBM that are composed of two pulses. Occasionally, the cosmological distance of GRB may be lensed when a high mass astrophysical object reside in path between GRB source and observer. In this paper, we are lucky to find out GRB 200716C with two-pulse emission which duration is a few seconds. We present a Bayesian analysis identifying gravitational lensing in both temporal and spectral properties, and calculate the time decay ($Delta tsim 1.92$ s) and magnification ($gammasim 1.5$) between those two pulses based on the temporal fits. One can roughly estimate the lens mass is about $2.38times 10^{5}~M_{odot}$ in the rest frame. If the first pulse of this GRB near triggered time is indeed gravitationally echoed by a second pulse, GRB 200716C may be a short GRB candidate with extended emission.
The giant flares of soft gamma-ray repeaters (SGRs) have long been proposed to contribute to at least a subsample of the observed short gamma-ray bursts (GRBs). In this paper, we perform a comprehensive analysis of the high-energy data of the recent bright short GRB 200415A, which was located close to the Sculptor galaxy. Our results suggest that a magnetar giant flare provides the most natural explanation for most observational properties of GRB 200415A, including its location, temporal and spectral features, energy, statistical correlations, and high-energy emissions. On the other hand, the compact star merger GRB model is found to have difficulty reproducing such an event in a nearby distance. Future detections and follow-up observations of similar events are essential to firmly establish the connection between SGR giant flares and a subsample of nearby short GRBs.
The coincident detection of GW170817 in gravitational waves and electromagnetic radiation spanning the radio to MeV gamma-ray bands provided the first direct evidence that short gamma-ray bursts (GRBs) can originate from binary neutron star (BNS) mergers. On the other hand, the properties of short GRBs in high-energy gamma rays are still poorly constrained, with only $sim$20 events detected in the GeV band, and none in the TeV band. GRB~160821B is one of the nearest short GRBs known at $z=0.162$. Recent analyses of the multiwavelength observational data of its afterglow emission revealed an optical-infrared kilonova component, characteristic of heavy-element nucleosynthesis in a BNS merger. Aiming to better clarify the nature of short GRBs, this burst was automatically followed up with the MAGIC telescopes, starting from 24 seconds after the burst trigger. Evidence of a gamma-ray signal is found above $sim$0.5 TeV at a significance of $sim3,sigma$ during observations that lasted until 4 hours after the burst. Assuming that the observed excess events correspond to gamma-ray emission from GRB 160821B, in conjunction with data at other wavelengths, we investigate its origin in the framework of GRB afterglow models. The simplest interpretation with one-zone models of synchrotron-self-Compton emission from the external forward shock has difficulty accounting for the putative TeV flux. Alternative scenarios are discussed where the TeV emission can be relatively enhanced. The role of future GeV-TeV observations of short GRBs in advancing our understanding of BNS mergers and related topics is briefly addressed.
We present a generic theoretical model for the structuring of a relativistic jet propagating through the ejecta of a binary neutron star merger event, introducing the effects of the neutron conversion-diffusion, which provides a baryon flux propagating transversely from the ejecta towards the jet axis. This results naturally in an increased baryon load structure of the outer jet with the approximate isotropic energy distribution $E_{iso}(theta) propto theta^{-4}$, which is compatible with the first gravitational wave and short gamma-ray burst event GW170817/GRB 170817A observed at an off-axis angle of the jet.
Short duration Gamma-Ray Bursts are thought to originate from the coalescence of neutron stars in binary systems. They are detected as a brief ($<$ 2s), intense flash of gamma-ray radiation followed by a weaker, rapidly decreasing afterglow. They are expected to be detected by Advanced LIGO and Virgo when their sensitivity will be low enough. In a recent study we identified a population of short Gamma-Ray Bursts that are intrinsically faint and nearby. Here we provide evidence in favor of the existence of this new population that can hardly be reproduced with a model of field neutron star binary coalescences. We propose that these systems may be produced dynamically in globular clusters, and may result from the merger of a black hole and a neutron star. The advanced LIGO and Virgo observation of a high rate of NSBH mergers compatible with the dynamical formation in globular clusters would be a confirmation of this hypothesis and would enable for the derivation of the mass function of black holes inside globular clusters, as well as the luminosity function of faint short GRBs.
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