Advection-Dominated Accretion Model for Sagittarius A$^*$: Evidence for a $10^6\msun$ Black Hole at the Galactic Center

The enigmatic radio source Sagittarius A* at the centre of our Galaxy appears to be a low-luminosity version of active galactic nuclei in other galaxies. By analogy with active galactic nuclei models, it has been proposed that Sgr A* may be a massive accreting black holes. The black hole hypothesis is, however, problematic because no model of Sgr A* has been able to explain the observed spectrum in any self-consistent way, and there is no consensus on either the mass of the black hole or the mass accretion rate. Sgr A* has been observed in the radio, sub-mm, infrared, and X-ray bands, and the various various detections and flux upper limits cover more than ten decades of frequency, from $\nu\sles\ 10^9$ Hz (meter wavelength radio bands) up to $\nu\ \sgreat\ 10^{19}$ Hz (150 keV X-rays). We present a robust model of Sgr A* in which a million solar mass black hole accretes at the rate of a few $\times10^{-6}$ solar masses per year. This model fits the entire spectrum self-consistently. The unique feature of the model is that the flow is advection-dominated, i.e. most of the energy viscously dissipated in the differentially rotating flow is carried along with the gas and lost through the horizon. The apparent success of this model in explaining the data may be considered ``proof'' that horizons are real and that a massive black hole does exist at the Galactic center.