Advection-dominated accretion model of Sagittarius A*: 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 it an excellent black hole candidate. Observations of stellar winds and other gas hows in its vicinity suggest a mass accretion rate of M over 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 spectrum 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-temperature optically thin advection-dominated accretion flow. The model is consistent with the estimated M and M over dot and fits the observed fluxes in the centimeter/millimeter and X-ray bands, as well as upper limits 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 means of advection. Most of the viscously dissipated energy is advected into the central mass by the accreting gas, and therefore the radiative efficiency is extremely low, similar to 5 x 10(-6). A critical element of the model is the presence of an event horizon at the center that swallows the advected energy. The success of the model could thus be viewed as confirmation that Sgr A* is a black hole.