The Missing X-rays in Sgr A*: Evidence for a Supermassive Black Hole in the Galactic Center

We present a simple argument that the missing x-ray flux from the Galactic Center source Sgr A* ist not evidence against – as claimed by Goldwurm et al. 1994 – but rather indirect evidence for the presence of a supermassive black hole. The radio spectrum provides a strict lower limit for the size of Sgr A* (R > 3 · 10cm). A more compact source would be completely synchrotron self-absorbed. This size is 10 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 stellar-mass black hole would inevitably radiate the 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