Black Hole Accretion with Saturated Magnetic Pressure and Disk Wind

Abstract

We construct an analytical black hole accretion disk model that incorporates both magnetic pressure and disk wind, which are found to be important from numerical simulations. A saturated magnetic pressure that relates the Alfven velocity with local Keplerian velocity and gas sound speed is assumed in addition to radiation and gas pressures. The mass accretion rate is assumed to have a power-law form in response to mass loss in the wind. We find three sets of self-consistent solutions that are thermally stable and satisfy the model assumptions. At high accretion rates, the disk is geometrically and optically thick, resembling the slim disk solution. At relatively low accretion rates, our model predicts an accretion flow consisting of a geometrically thin and optically thick outer disk (similar to the standard disk), and a geometrically thick and optically thin inner disk (similar to the advection-dominated accretion flow or ADAF). Thus, this is a natural solution for a truncated disk connected with an inner ADAF, which has been proposed to explain some observations. The magnetic pressure plays a more important role than the outflow in shaping the disk structure. The observed disk luminosity tends to saturate around 8 times the Eddington limit, suggesting that supercritical accretion onto black holes can be used for black hole mass estimate, or a standard candle with known black hole masses.