Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aas...201.1117g&link_type=abstract
American Astronomical Society, 201st AAS Meeting, #11.17; Bulletin of the American Astronomical Society, Vol. 34, p.1111
Astronomy and Astrophysics
Astronomy
Scientific paper
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.
Gezari Suvi
Grupe Dirk
Halpern Joseph
Komossa Stefanie
Leighly Karen
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