Black Hole Accretion Disks On The Edge

The local axisymmetric stability of hydrodynamical and magnetized, nearly-Keplerian gaseous accretion disks around non-rotating 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 which 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 radius of marginal stability becomes largely irrelevant because there are linearly unstable magneto-rotational 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 diskoseismic 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.