X-ray Evidence for Supermassive Black Holes in Non-Active Galaxies. Subtitle: Detection of X-ray Flare Events, Interpreted as Tidal Disruptions of Stars by SMBHs

It has long been suggested that supermassive black holes in non-active galaxies might be tracked down by occasional tidal disruptions of stars on nearly radial orbits. A tidal disruption event would reveal itself by a luminous flare of electromagnetic radiation. Theorists argued that the convincing detection of such a tidal disruption event would be the observation of an event which fulfills the following three criteria: (1) the event should be of finite duration (a `flare'), (2) it should be very luminous (up to L_{max} ~ 10^{45} erg/s in maximum), and (3) it should reside in a galaxy that is otherwise perfectly {\em non-active} (to be sure to exclude an upward fluctuation in gaseous accretion rate of an {\em active} galaxy). During the last few years, several X-ray flare events were detected which match exactly the above criteria. We therefore consider these events to be excellent candidates for the occurrences of the theoretically predicted tidal disruption flares. In this contribution, we review the previous observations of giant X-ray flares from normal galaxies, present new results on these objects, critically discuss alternatives to the favored outburst scenario, and report results from our ongoing search for further tidal disruption flares based on the ROSAT all-sky survey database.