Advection-dominated models of luminous accreting black holes

It has been found that a class of optically thin two-temperature advection-dominated accretion solutions explains many observations of low-luminosity accreting black holes. Here it is shown that these models give a satisfactory description also of higher luminosity systems, provided the viscosity parameter a is large. The models reproduce the spectra of black hole X-ray binaries in the low state and explain the transition from the low state to the high state at a critical mass accretion rate. The models also show that the X-ray/gamma-ray spectra of X-ray binaries and active galactic nuclei should be similar, as confirmed by observations.