In recent years , luminous X-ray outbursts with variability amplitudes as high as \approx 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 \approx 9.1 \times 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 \approx 8.5 \times 10 ^ { -4 } galaxy ^ { -1 } yr ^ { -1 } .
In order to place tighter constraints on these rates , additional outbursts must be detected .