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S. Gezari, J. Halpern (Columbia University), S. Komossa, D. Grupe (Max-Planck-Institut), K. Leighly (The University of Oklahoma)
Dormant, supermassive black holes, suspected to be present in the centers of normal galaxies, should reveal themselves by a UV/X-ray flare when they tidally disrupt a star and some fraction of the tidal debris is accreted. Such an event is very rare in the nucleus of a galaxy (10-4 /yr); however, the ROSAT All-Sky Survey in 1990-91 was an ideal experiment to detect these flares since it sampled hundreds of thousands of galaxies in the soft X-ray band. Several flares were detected by ROSAT in galaxies with no evidence for AGN activity in their ground-based optical spectra. These large amplitude X-ray flares had the properties predicted for a tidal disruption event: a soft X-ray spectrum, time-scale of months, and a large X-ray luminosity (1042 to 1044 erg/s). In order to evaluate the alternative hypothesis that the flares could have been some form of extreme AGN variability, we obtained follow-up optical spectroscopy of three of these flaring galaxies a decade later with the HST STIS and a narrow slit to search for or place stringent limits on the presence of any permanent Seyfert-like emission in the their nuclei. Two of the galaxies, RXJ1624.9+7554 and RXJ1242.6-1119, show no evidence for emission lines or non-stellar continuum in their HST nuclear spectra, consistent with their ground-based classification as inactive galaxies. NGC 5905, previously known as a starburst HII galaxy due to it strong emission lines, has in its inner 0.1 arcseconds a nucleus with narrow emission-line ratios consistent with a Seyfert 2 galaxy in the diagnostic diagrams of Veilleux and Osterbrock. The weak Seyfert nucleus in NGC 5905, which was masked by the many surrounding H II regions in ground-based spectra, raises questions about the nature of its X-ray flare.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.