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GRB 170817A, detected by Fermi Gamma-ray Burst Monitor 1.7 s after the merger of a neutron star (NS) binary, provides the first direct evidence for a link between such a merger and a short-duration gamma-ray burst. The X-ray observations after GRB 170817A indicate a possible X-ray flare with a peak luminosity |$L_{\rm peak} \sim 2\times 10^{39}\, {\rm erg\, s}^{-1}$| near day 156. Here we show that this X-ray flare may be understood based on a slim disc around a compact object. On the one hand, there exists the maximal accretion rate |$\dot{M}_{\rm max}$| for the slim disc, above which an optically thick outflow is significant and radiation from the disc is obscured. Based on the energy balance analysis, we find that |$\dot{M}_{\rm max}$| is in the range of |${\sim } 4\dot{M}_{\rm Edd}$| and |${\sim } 21\dot{M}_{\rm Edd}$| when the angular velocity of the slim disc is between |$\rm (1/5)^{1/2}\Omega _K$| and |$\rm \Omega _K$| (where |$\dot{M}_{\rm Edd}$| is the Eddington accretion rate and ΩK is the Keplerian angular velocity). With |$\dot{M}_{\rm max}$|⁠, the slim disc can provide a luminosity ∼ L peak for a compact object of |$2.5 \, \mathrm{M}_{\odot }$|⁠. On the other hand, if the merger of two NSs forms a typical neutrino-dominated accretion disc whose accretion rate |$\dot{M}$| follows a power-law decline with an index −1.8, then the system must pass through the outflow regime and enter the slim disc in ∼11–355 d. These results imply that a post-merger slim accretion disc could account for the observed late-time L peak. [ABSTRACT FROM AUTHOR]

The occurrence rates of bright X-ray flares in z  < 1 gamma-ray bursts (GRBs) with or without observed supernovae (SNe) association were compared. Our Sample I: the z  < 1 long GRBs (LGRBs) with SNe association (SN-GRBs) and with early Swift /X-Ray Telescope (XRT) observations, consists of 18 GRBs, among which only two GRBs have bright X-ray flares. Our Sample II: for comparison, all the z  < 1 LGRBs without observed SNe association and with early Swift /XRT observations, consists of 45 GRBs, among which 16 GRBs present bright X-ray flares. Thus, the study indicates a lower occurrence rate of bright X-ray flares in Sample I (11.1 per cent) than in Sample II (35.6 per cent). In addition, if dim X-ray fluctuations are included as flares, then 16.7 per cent of Sample I and 55.6 per cent of Sample II are found to have flares, again showing the discrepancy between these two samples. We examined the physical origin of these bright X-ray flares and found that most of them are probably related to the central engine reactivity. To understand the discrepancy, we propose that such a lower occurrence rate of flares in the SN-GRB sample may hint at an energy partition among the GRB, SNe, and X-ray flares under a saturated energy budget of massive star explosion. [ABSTRACT FROM AUTHOR]

Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes (BHs) are plausible candidates for the central engines of gamma-ray bursts (GRBs). NDAFs are hyperaccretion disks with accretion rates in the range of around 0.001–10 M ⊙ s − 1 , which have high density and temperature and therefore are extremely optically thick and geometrically slim or even thick. We review the theoretical progresses in studying the properties of NDAFs as well as their applications to the GRB phenomenology. The topics include: the steady radial and vertical structure of NDAFs and the implications for calculating neutrino luminosity and annihilation luminosity, jet power due to neutrino-antineutrino annihilation and Blandford–Znajek mechanism and their dependences on parameters such as BH mass, spin, and accretion rate, time evolution of NDAFs, effect of magnetic fields, applications of NDAF theories to the GRB phenomenology such as lightcurve variability, extended emission, X-ray flares, kilonovae, etc., as well as probing NDAFs using multi-messenger signals such as MeV neutrinos and gravitational waves. [ABSTRACT FROM AUTHOR]

Double gamma-ray bursts (DGRBs) have two well-separated sub-bursts in the main prompt emission and the typical time interval between them is in the hundreds of seconds. Among DGRBs, gamma-ray bursts (DGRBs) 110801A and 120716A are the ones with known redshifts. However, unlike GRB 110801A, we show that the two sub-bursts of GRB 120716A is severally similar to the short- and long-duration GRBs, thus it is difficult to explain the origin of GRB 120716A by the popular models on the central engine of GRBs. We suggest that some mechanisms of x-ray flares in GRBs, i.e., a post-merger millisecond pulsars or the jet precession in a black hole hyperaccretion system may produce the DGRB. [ABSTRACT FROM AUTHOR]