Accretion onto Black Holes
Most energetic events or objects in the universe are explained by accretion onto black holes. However, the accretion flow onto black holes can assume diverse forms. It can be spherical or disk-like, hot or cold, optically thick or thin, highly luminous or under-luminous, efficient or inefficient in terms of energy production, even for the same physical conditions of gas from which black holes accrete. I will review each form of accretion flow with its physical characteristics. I will especially discuss the mass accretion rate and its dependence on the density, temperature, and the angular momentum of gas being accreted. The mass accretion rate is the most important factor to affect the radiation efficiency and the luminosity, and thereby, the growth history of the black holes and their coevolution with host galaxies. The ubiquity of outflow from hot accretion flow and its implications are also discussed.