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J. Donley, W.N. Brandt, Michael Eracleous (Penn State), Th. Boller (MPE-Garching)
We present the results from a study designed to detect extragalactic large-amplitude X-ray outbursts using ROSAT archival data. Such outbursts are known to decay over a period of months to years, to have maximum X-ray luminosities of order 1043 to 1044 erg/s, and to often have very soft spectra. As such they are excellent ROSAT targets. By comparing different ROSAT observations, we have identified all galactic nuclei that (1) were in outburst during the six-month ROSAT All-Sky Survey and that (2) then faded by a minimum factor of 20 to low or undetectable count rates in later pointed ROSAT observations. The systematic nature of this study has allowed us to place the first reliable constraints on the number of such outbursts that occur in the Universe. We discuss these constraints and their implications for future missions such as MAXI and Lobster-eye X-ray telescopes. We also discuss possible mechanisms for the outbursts, the most likely of which is the tidal disruption of a star by a supermassive black hole.