Rotation-induced bistability of transonic accretion onto a black hole

The influence of angular momentum on the accretion of a polytropic gas onto a black hole is discussed. There are two physically different regimes, including quasi-spherical accretion and disklike accretion. In the first case the energy is well above the top of the effective potential barrier. The sonic point is far from the horizon, and mechanical equilibrium is not established. In the disklike accretion the energy level only slightly overcomes the top of the barrier. A disk in approximate mechanical equilibrium is formed. The sonic point lies in the potential spout, between the inner edge of the disk and the horizon. Contrary to the common opinion, viscosity is not necessary to support accretion through the inner edge. The inward motion is caused by a small violation of the mechanical equilibrium. The difference between the two regimes is qualitative.