A variable efficiency for thin-disk black hole accretion

We explore the presence of torques at the inner edges of geometrically thin black hole accretion disks using three-dimensional MHD simulations in a pseudo-Newtonian potential. By varying the saturation level of the magnetorotational instability that leads to angular momentum transport, we show that the dynamics of gas inside the radius of marginal stability varies depending upon the magnetic field strength just outside that radius. Weak fields are unable to causally connect material within the plunging region to the rest of the disk, and zero torque is an approximately correct boundary condition at the radius of marginal stability. Stronger fields, which we obtain artificially but which may occur physically within more complete disk models, are able to couple at least some parts of the plunging region to the rest of the disk. In this case, angular momentum (and implicitly energy) is extracted from the material in the plunging region. Furthermore, the magnetic coupling to the plunging region can be highly time dependent with large fluctuations in the torque at the radius of marginal stability. This implies varying accretion efficiencies, both across systems and within a given system at different times. The results suggest a possible link between changes in X-ray and outflow activity, with both being driven by transitions between weak and strong field states.