Possible Origin of Rapid Variability of Gamma-Ray Bursts due to Convective Energy Transfer in Hyperaccretion Disks

Abstract

We investigate the effects of the convection in the hyperaccretion disk around a stellar-mass black hole, which is considered to be the central engine of gamma-ray bursts (GRBs), with simple analytical calculations. If the convective energy transfer in the vertical direction becomes efficient compared with the inward advective energy transport, the hyperaccretion disk is expected to be hotter and the neutrino emission due to the electron-positron annihilation becomes the most efficient cooling process. We find that the sequence of the thermal equilibrium solutions for the convective hyperaccretion disk would have a viscously unstable branch, especially when the viscosity parameter is relatively small (alpha <~ 0.01). This means that the sporadic mass accretion onto a black hole would occur in this disk. We propose that this process can be the origin of the highly variable lightcurves observed in the prompt emissions of GRBs.