ADVECTION-DOMINATED ACCRETION MODEL OF SAGITTARIUS A-ASTERISK AND OTHER ACCRETING BLACK-HOLES

Viscous rotating accretion hows around black holes become advection-do minated when the accretion rate M over dot is sufficiently low. Most o f the accretion energy in such flows is stored within the gas and adve cted radially inward. The temperature is therefore very high, and much of the radiation comes out in hard X-rays and gamma-rays. We have con structed an advection-dominated accretion flow model for the Galactic Center source Sagittarius A. The model consists of a 7 10(5) M(.) bla ck hole accreting at M over dot = 1.2 10(-5) alpha M(.)yr(-1), where a lpha is the usual viscosity parameter. The model spectrum fits the obs ervations from radio to gamma-rays quite well and explains the unusual ly low luminosity of the source. Since the model explicitly makes use of a horizon at the inner edge to swallow the advected energy, the suc cess of the model strongly suggests that the central object in Sgr A is a black hole. We further show that, if alpha is not much smaller th an unity, then advection-dominated models can be applied even to highe r luminosity black holes. The existence of Low and High States in blac k hole X-ray binaries, and the abrupt transition between the two state s, find a natural explanation. The models also explain the close simil arity in the hard X-ray/gamma-ray spectra of black hole X-ray binaries and active galactic nuclei.