Simulations of accretion flows crossing the last stable orbit

We use three-dimensional magnetohydrodynamic simulations, in a pseudo-Newtonian potential, to study geometrically thin accretion disk flows crossing r_ms, the marginally stable circular orbit around black holes. We concentrate on vertically unstratified and isothermal disk models but also consider a model that includes stratification. In all cases we find that the sonic point lies just inside r_ms, with a modest increase in the importance of magnetic field energy, relative to the thermal energy, observed inside the last stable orbit. The time-averaged gradient of the specific angular momentum of the flow, (dl/dr), is close to zero within r_ms despite the presence of large fluctuations and continuing magnetic stress in the plunging region. The result that the specific angular momentum is constant within r_ms is in general agreement with traditional disk models computed using a zero-torque boundary condition at the last stable orbit.