Numerical models of black hole accretion flows

Active galactic nuclei, black hole binaries, and gamma-ray bursts are all probably powered by an accretion flow around a rotating black hole. It has recently become possible to construct numerical, fully relativistic ideal magnetohydrodynamic models of these flows. The current generation of codes has undergone extensive testing, and remarkably, quite different codes produce quantitatively consistent results. Numerical models of thick, nonradiative accretion flows show that the matter accreted onto the black hole has a smaller specific angular momentum per baryon than anticipated in the classical thin disk model. If supermassive black holes acquire their mass through a similar series of accretion flows, then they should reach spin equilibrium at a / M similar to 0.93, much less than the 0.998 envisioned in classical disk theory. Numerical models also show low density regions over the poles of the black hole. These regions are approximately force-free, and the electromagnetic field structure there is consistent with the Blandford-Znajek force-free magnetosphere model.