Advection‐dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center

Sgr A* at the Galactic Center is a puzzling source. It has a massM=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black holecandidate. Observations of stellar winds and other gas flows in its vicinitysuggest a mass accretion rate approximately few x 10^{-6} solar masses peryear. However, such an accretion rate would imply a luminosity > 10^{40} erg/sif the radiative efficiency is the usual 10 percent, whereas observationsindicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* isunusual, with emission extending over many decades of wavelength. We present amodel of Sgr A* which is based on a two-temperature optically-thinadvection-dominated accretion flow. The model is consistent with the estimatedmass and accretion rate, and fits the observed fluxes in the cm/mm and X-raybands as well as upper limits in the sub-mm and infrared bands; the fit is lessgood in the radio below 86 GHz and in gamma-rays above 100 MeV. The very lowluminosity 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 bythe accreting gas, and therefore the radiative efficiency is extremely low,approximately 5 x 10^{-6}. A critical element of the model is the presence ofan event horizon at the center which swallows the advected energy. The successof the model could thus be viewed as confirmation that Sgr A* is a black hole.Comment: 41 pages (Latex) including 6 Figures and 2 Tables. Final Revised Version changes to text, tables and figures. ApJ, 492, in pres