Hyper- and Suspended-Accretion States of Rotating Black Holes and the Durations of Gamma-Ray Bursts

We analyze the temporal evolution of accretion onto rotating black holes subject to large-scale magnetic torques. Wind torques alone drive a disk toward collapse in a finite time ~ t ff E k / E B , where t ff is the initial free-fall time and E k / E B is the ratio of kinetic energy to poloidal magnetic energy. Additional spin-up torques from a rapidly rotating black hole can arrest the disk's inflow. We associate short/long gamma-ray bursts with hyperaccretion/suspended-accretion onto slowly/rapidly spinning black holes. This model predicts afterglow emission from short bursts and may be tested by HETE-2 .