BLACK HOLE ACCRETION DISKS ON THE EDGE

摘要

The local axisymmetric stability of hydrodynamical and magnetized, nearly Keplerian gaseous accretion disks around nonrotating black holes is examined in the vicinity of the classical marginally stable orbit (at radii ～R_(ms)). An approximate Paczynski-Wiita pseudo-Newtonian potential is used. Hydrodynamical disks are linearly unstable inside a radius that differs slightly from the classical R_(ms) value because of finite pressure and radial stratification effects. Linear stresses associated with unstable hydrodynamical modes vanish exactly at the radius of marginal stability and are generally positive inside of that radius. When a magnetic field is introduced, however, the concept of a radius of marginal stability becomes largely irrelevant because there are linearly unstable magnetorotational modes everywhere. Associated linear stresses are positive and continuous across the region of hydrodynamical marginal stability, even for large-scale " hydro-like " modes subject only to weak magnetic tension. This conclusion is valid for arbitrarily thin disks (in ideal MHD) and it does not require a large-scale " radially connecting " magnetic field. Results on hydrodynamical diskoseis-mic modes trapped in deep relativistic potential wells should be revised to account for the short-wavelength Alfven-like behavior of inertio-gravity waves in magnetized disks.