The missing x-rays in Sgr A*: evidence for - not against - a supermassive black hole

Abstract

We present a simple argument that the missing x-ray flux from the Galactic Center source Sgr~A* ist not evidence {\it against} -- as claimed by Goldwurm et al. 1994 -- but rather indirect evidence {\it for} the presence of a supermassive black hole. The radio spectrum provides a strict {\it lower} limit for the size of Sgr~A* ($R>3\cdot 10^{11}$cm). A more compact source would be completely synchrotron self-absorbed. This size is $10^6$ times larger than a stellar-mass black hole, yet the bolometric radio luminosity is comparable to or even larger than the x-ray luminosity where matter accreting onto a {\it stellar-mass} black hole would inevitably radiate the {\it bulk} of its luminosity. Hence, either the bulk of the accretion power is radiated in the UV (where the limits are higher), or the accretion has to stop at the radio-scale to avoid producing x-rays brighter than the radio emission. Both would be a natural consequence of a supermasive black hole with $\sim10^6M_{\sun}$.