Efficiency of accretion disks in the kerr metric

The efficiency of thin disk accretion onto black holes depends on the inner boundary condition, specifically the torque applied to the disk at the last stable orbit. This is usually assumed to vanish. I estimate the torque on a magnetized disk using a steady magnetohydrodynamic inflow model originally developed by Takahashi et al. I find that the efficiency e can depart significantly from the classical thin disk value. In some cases e > 1, i.e., energy is extracted from the black hole.