Magnetic Stresses at the Inner Edges of Accretion Disks Around Black Holes

Abstract

For the past twenty-five years, nearly all analyses of accretion disk dynamics have assumed that stress inside the disk is locally proportional to pressure (the "alpha-model") and that this stress goes to zero at the marginally stable orbit. Recently, it has been demonstrated that MHD turbulence accounts for the bulk of internal disk stress. In contradiction with the traditional view, the stress from this MHD turbulence does not diminish near the marginally stable orbit, and the ratio of magnetic stress to pressure rises sharply there. Examples of the consequences include: an increase in accretion efficiency that may also be time- and circumstance-dependent; a decrease in the rate of black hole spin-up by accretion; and generation of disk luminosity fluctuations. Preliminary results from numerical simulations lend support to analytic estimates of these effects.