Where do Accretion Disks Around Black Holes End?

Accretion disks around (supermassive) black holes act as "machines" which extract gravitational energy. In fact, the observed radiation allows to sample the physical conditions very close to the event horizon. For a test particle, the innermost stable circular orbit (ISCO) is located at 3 r(s) for a non-rotating hole (Schwarzschild metrics; at smaller radii for a rotating black hole). This ISCO is usually identified with the inner edge of the accretion disk. For a given black hole mass, it allows, in principle, to determine the Kerr parameter. In "real life", however, we deal not with test particles but with a viscous flow, which introduces additional forces. We have calculated the location of the inner edge in a more realistic environment. The results show that the true inner edge of the disk is no longer located at the ISCO, when radial advection of energy is taken into account with a careful treatment of the transonic nature of the flow.