We discuss the results of three-dimensional magnetohydrodynamic simulations,
using a pseudo-Newtonian potential, of thin disk (h/r ~ 0.1) accretion onto
black holes. We find (i) that magnetic stresses persist within the marginally
stable orbit, and (ii) that the importance of those stresses for the dynamics
of the flow depends upon the strength of magnetic fields in the disk outside
the last stable orbit. Strong disk magnetic fields (alpha > 0.1) lead to a
gross violation of the zero-torque boundary condition at the last stable orbit,
while weaker fields (alpha ~ 0.01) produce results more akin to traditional
models for thin disk accretion onto black holes. Fluctuations in the magnetic
field strength in the disk could lead to changes in the radiative efficiency of
the flow on short timescales.Comment: 6 pages, to appear in proceedings, 20th Texas Symposium on
Relativistic Astrophysics, eds J.C. Wheeler and H. Marte
