Transient optical emission from a nonstationary accretion disk: A model for optical afterglow of gamma ray bursts

Abstract

We consider the time-dependent behavior of nonstationary accretion disk which is formed by the capture and disruption of a main-sequence star around a massive black hole. If an instability is developed in the inner edge of the nonstationary disk and produces a strong magnetic field, a large amount of the black hole rotational energy can be extracted via the Blandford-Znajek process, which results in a Gamma-Ray Burst (GRB) with a typical duration ∼ 10-102 s. Other parameters related to a typical GRB including burst energy, rise time of the burst, number of sub-burst etc are also estimated. The vertical structure and thermal stability properties of the disk are investigated in detail. In the early stage of forming transient accretion disk, the material supplied by the tidally disrupted star is clumped in the relatively cool disk. This eventually leads to a thermal instability, which results in an increase of the viscosity of the disk. As the instability propagates across the disk, the stability of a time-dependent disk and variability of the mass-deposition rate provide a possible explanation for transient optical emission. We speculate that this transient optical emission from a nonstationary accretion disk may relate to the behavior of GRB optical afterglow, and the transient optical afterglow can be reproduced if one takes into account the effect of an evolving viscous parameter, α.