Advection-Dominated Accretion Model of Sagittarius A and Other Accreting Black Holes

Viscous rotating accretion ows around black holes become advection-dominated when the accretion rate _ M is su ciently low. Most of the accretion energy in such ows is stored within the gas and advected radially inward. The temperature is therefore very high, and much of the radiation comes out in hard X-rays and -rays. We have constructed an advection-dominated accretion ow model for the Galactic Center source Sagittarius A . The model consists of a 7 10 5 M black hole accreting at _ M = 1:2 10 5 M yr 1 , where is the usual viscosity parameter. The model spectrum ts the observations from radio to -rays quite well and explains the unusually low luminosity of the source. Since the model explicitly makes use of a horizon at the inner edge to swallow the advected energy, the success of the model strongly suggests that the central object in Sgr A is a black hole. We further show that, if is not much smaller than unity, then advection-dominated models can be applied even to higher luminosity black holes. The existence of Low and High States in black hole X-ray binaries, and the abrupt transition between the two states, nd a natural explanation. The models also explain the close similarity in the hard X-ray/ -ray spectra of black hole X-ray binaries and active galactic nuclei.