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 disc flows crossing rms, 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 rms, with modest magnetic field amplification observed inside the last stable orbit. The gradient of the specific angular momentum of the flow, (dl/dr), is close to zero within rms, despite the presence of continuing magnetic stress in the plunging region. These results are in general agreement with expectations based on traditional disk models, but differ from recent results obtained from simulations of geometrically thick disks. For thin disks, we find that the use of a zero-torque boundary condition, at the last stable orbit, provides a reasonable approximation to the numerical results. Subject headings: accretion, accretion disks – black hole physics – MHD – stars: neutron – hydrodynamics – instabilities