Radiatively inefficient accretion flows

Black hole accretion flows could be radiatively inefficient when a mass supply rate is either much less or much larger than the Eddington rate. Because of lack of radiative cooling, such flows are hot and geometrically thick. This article will review different models and different approaches to study radiatively inefficient accretion flows. Initially, the flows were studied as purely hydrodynamic viscous flows. It was shown analytically and numerically that the hydrodynamic flows are convectively unstable. Convection significantly changes the flow structure, flattening the radial density profile and decreasing the accretion rate. More recently, numerical MHD models of the flows have been developed by several groups. In these models the problems of angular momentum transport and dissipation of binding energy can be solved self-consistently. The MHD models show the flow's strong dependence on the topology of the magnetic field, and some models show efficient convection.