ADVECTION-DOMINATED ACCRETION MODEL OF SAGITTARIUS A-ASTERISK - EVIDENCE FOR A BLACK-HOLE AT THE GALACTIC-CENTER

Sagittarius A, which is located at the Galactic center, is a puzzling source. It has a mass of M = (2.5) +/- 0.4) x 10(6) M., which makes i t an excellent black hole candidate. Observations of stellar winds and other gas hows in its vicinity suggest a mass accretion rate of M ove r dot greater than or similar to few x 10(-6) M. yr(-1). However, such an accretion rate would imply a luminosity greater than 10(40) ergs(- 1) if the radiative efficiency is the usual 10%, whereas observations indicate a bolometric luminosity less than 10(37) ergs(-1). The spectr um of Sgr A is unusual, with emission extending over many decades of wavelength. We present a model of Sgr A that is based on a two-temper ature optically thin advection-dominated accretion flow. The model is consistent with the estimated M and M over dot and fits the observed f luxes in the centimeter/millimeter and X-ray bands, as well as upper l imits in the submillimeter and infrared bands; the fit is less good in the radio spectrum below 86 GHz and in gamma-rays above 100 MeV. The very low luminosity of Sgr A is explained naturally in the model by m eans of advection. Most of the viscously dissipated energy is advected into the central mass by the accreting gas, and therefore the radiati ve efficiency is extremely low, similar to 5 x 10(-6). A critical elem ent of the model is the presence of an event horizon at the center tha t swallows the advected energy. The success of the model could thus be viewed as confirmation that Sgr A is a black hole.