On the Nature of the Compact Dark Mass at the Galactic Center

We consider a model in which Sgr A*, the 3.5x10^6 M_sun supermassive blackhole candidate at the Galactic Center, is a compact object with a surface.Given the very low quiescent luminosity of Sgr A* in the near infrared, theexistence of a hard surface, even in the limit in which the radius approachesthe horizon, places severe constraints upon the steady mass accretion rate inthe source, requiring dM/dt < 10^-12 M_sun/yr. This limit is well below theminimum accretion rate needed to power the observed submillimeter luminosity ofSgr A*. We thus argue that Sgr A* does not have a surface, i.e., it must havean event horizon. The argument could be made more restrictive by an order ofmagnitude with microarcsecond resolution imaging, e.g., with submillimeterVLBI.Comment: 6 pages, 2 figures, submitted to ApJ Letter