We discuss the results of three-dimensional magnetohydrodynamic simulations , using a pseudo-Newtonian potential , of thin disk ( h / r \approx 0.1 ) accretion onto black holes .
We find ( i ) that magnetic stresses persist within r _ { ms } , 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 r _ { ms } .
Strong disk magnetic fields ( \alpha \gtrsim 0.1 ) lead to a gross violation of the zero-torque boundary condition at r _ { ms } , while weaker fields ( \alpha \sim 10 ^ { -2 } ) 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 .