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 $\alpha$ 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.Comment: 12 pages compressed uuencoded postscript, including 2 figures. To appear in Part 1 of The Astrophysical Journa