Accretion Models of Gamma‐Ray Bursts

Many models of gamma-ray bursts (GRBs) involve accretion onto a compactobject, usually a black hole, at a mass accretion rate of order a fraction of asolar mass per second. If the accretion disk is larger than a few tens orhundreds of Schwarzschild radii, the accretion will proceed via aconvection-dominated accretion flow (CDAF) in which most of the matter escapesto infinity rather than falling onto the black hole. Models involving themergers of black hole white dwarf binaries and black hole helium star binariesfall in this category. These models are unlikely to produce GRBs since verylittle mass reaches the black hole. If the accretion disk is smaller, thenaccretion will proceed via neutrino cooling in a neutrino-dominated accretiondisk (NDAF) and most of the mass will reach the center. Models involving themergers of double neutron star binaries and black hole neutron star binariesfall in this category and are capable of producing bright GRBs. If theviscosity parameter $\alpha$ in the NDAF has a standard value $\sim0.1$, thesemergers can explain short GRBs with durations under a second, but they areunlikely to produce long GRBs with durations of tens or hundred of seconds. Ifthe accretion disk is fed by fallback of material after a supernova explosion,as in the collapsar model, then the time scale of the burst is determined byfallback, not accretion. Such a model can produce long GRBs. Fallback modelsagain require that the accretion should proceed via an NDAF rather than a CDAFin order for a significant amount of mass to reach the black hole. Thiscondition imposes an upper limit on the radius of injection of the gas.Comment: 20 pages 3 figure, revised version accepted for publication in Ap. J. The revised version includes a new analysis of stability to gravitational mode