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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]

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]

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]

ABSTRACT In accretion systems, outflows may have significant influence on luminosity fluctuations. In this paper, following Lyubarskii's general scheme, we revisit the power spectral density of luminosity fluctuations by taking into account the role of outflows. Our analysis is based on the assumption that the coupling between the local outflow and inflow is weak for accretion-rate fluctuations. We find that, for inflow mass accretion rate , the power spectrum of the flicker-noise component will present a power-law distribution p( f) ∝ f−(1 + 4 s/3) for advection-dominated flows. We also obtain descriptions of p( f) for both standard thin discs and neutrino-cooled discs, which show that the power-law index of a neutrino-cooled disc is generally larger than that of a photon-cooled disc. Furthermore, the relationship obtained between p( f) and s indicates the possibility of evaluating the strength of outflows through the power spectrum in X-ray binaries and gamma-ray bursts. In addition, we discuss the possible influence of outflow-inflow coupling on our results. [ABSTRACT FROM AUTHOR]

We revisit the vertical structure of neutrino-dominated accretion flows (NDAFs) in spherical coordinates with a new boundary condition based on the mechanical equilibrium. The solutions show that NDAF is significantly thick. The Bernoulli parameter and neutrino trapping are determined by the mass accretion rate and the viscosity parameter. According to the distribution of the Bernoulli parameter, the possible outflow may appear in the outer region of the disk. The neutrino trapping can essentially affect the neutrino radiation luminosity. The vertical structure of NDAF is like a 'sandwich', and the multilayer accretion may account for the flares in gamma-ray bursts. [ABSTRACT FROM AUTHOR]

We investigate the vertical structure of neutrino-dominated accretion flows in spherical coordinates. In our calculation, the empty funnel along the rotation axis can naturally explain the neutrino annihilable ejection. The outflow is possible due to the positive Bernoulli function, and the luminosity of neutrino annihilation is enhanced by one or two orders of magnitude. [ABSTRACT FROM AUTHOR]