Large-Amplitude X-ray Outbursts from Galactic Nuclei: A Systematic   Survey Using ROSAT Archival Data
  In recent years, luminous X-ray outbursts with variability amplitudes as high as ~ 400 have been serendipitously detected from a small number of active and inactive galaxies. These outbursts may result from the tidal disruptions of stars by supermassive black holes as well as accretion-disk instabilities. In order to place the first reliable constraints on the rate of such outbursts in the Universe and test the stellar tidal disruption hypothesis, we have performed a systematic and complete survey for them by cross-correlating ROSAT All-Sky Survey (RASS) and pointed PSPC data. We have detected five galaxies that were in outburst during the RASS, three of which show no signs of nuclear activity; these objects had been reported on individually in previous studies. After making reasonable corrections for the complicated selection effects, we conclude that the rate of large-amplitude X-ray outbursts from inactive galaxies in the local Universe is ~ 9.1 x 10^{-6} galaxy^{-1} yr^{-1}. This rate is consistent with the predicted rate of stellar tidal disruption events in such galaxies. When only the two active galaxies are considered, we find a rate for active galaxies of ~ 8.5 x 10^{-4} galaxy^{-1} yr^{-1}. In order to place tighter constraints on these rates, additional outbursts must be detected. 
J.L. Donley, W.N. Brandt, M.C. Eracleous (Penn State), Th. Boller   (MPE)