Accepted for publication in The Astronomical Journal 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 to 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 Position Sensitive Proportional Counter 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 × 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 × 10-4 galaxy-1 yr-1. In order to place tighter constraints on these rates, additional outbursts must be detected.

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