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 mass M= (2. 5 +/- 0. 4) x 10⁶ solar masses which makes it an excellent black hole candidate. Observations of stellar winds and other gas flows in its vicinity suggest a mass accretion rate approximately few x10^-6 solar masses per year. However, such an accretion rate would imply a luminosity > 10^40 erg/s if the radiative efficiency is the usual 10 percent, whereas observations indicate a bolometric luminosity < 10^37 erg/s. The spectrum of Sgr A* is unusual, with emission extending over many decades of wavelength. We present a model of Sgr A* which is based on a two-temperature optically-thin advection-dominated accretion flow. The model is consistent with the estimated mass, M, and accretion rate, and it fits the observed fluxes in the radio and X-ray bands as well as measured upper limits in the sub-millimeter and infrared bands. The model explains the very low luminosity of Sgr A* by invoking advection. Most of the viscously dissipated energy is carried into the central star by the accreting gas, and therefore the radiative efficiency is extremely low, approximately 5 x 10^-6. A critical element of the model is the presence of an event horizon at the center. The success of the model may thus be viewed as confirmation that Sgr A* is a black hole.