Ray Flares from Inactive Galaxies

Given its broad astrophysical relevance and the intense theoretical attention (e.g. [2,4,10,20,24] and Sect. 3 of [11] and references therein]), a key question is: do these flares occur in nature, how frequent is tidal disruption of stars, and what are its properties?

All-sky X-ray surveys, similar to the one performed with the ROSAT satellite [23], are the ideal experiment to detect such flares, since hundredths of thousands of galaxies are sampled in the soft X-ray band.

With ROSAT, several giant-amplitude, nonrecurrent X-ray outbursts from a handful of "normal" galaxies - NGC 5905 [12], RX J1242-1119 [13], RX J1624 + 7554 [8], RX J1420 + 5334 [6] - were detected (see [11] for a review). All of them share similar properties:

- Huge soft X-ray peak luminosities (up to Lsx ~ 1044 erg s-1)

- Large amplitudes of variability (up to a factor ^200)

- Ultra-soft X-ray spectra (kTbb ~ 0.04-0.1 keV, where Tbb is the black body temperature)

- Complete absence of any signs of ongoing Seyfert activity in ground-based optical spectra (confirmed with the Hubble Space Telescope, except for NGC 5905 that shows a faint high-excitation core in its nucleus [5])

The observed events were interpreted as excellent candidates for the long-sought tidal disruption events. To perform key tests of the favored outburst scenario, follow-up observations with the new generation of X-ray observatories, Chandra and XMM-Newton, were obtained.

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