The giant-amplitude X-ray outburst in NGC 5905 - a tidal disruption   event ?
  NGC 5905 is one of the very few galaxies that underwent a giant X-ray outburst, with a change in photon countrate of a factor 100. The outburst spectrum is both, very soft and luminous (Bade, Komossa & Dahlem 1996). Remarkably, the optical pre-outburst emission line spectrum of NGC 5905 is that of an HII-type galaxy, i.e. does not show any signs of Seyfert activity.   One exciting explanation of the X-ray observation is that we may have witnessed the tidal disruption of a star by a supermassive black hole (SMBH) residing in the nucleus of this galaxy. The expected flare of electromagnetic radiation being produced when the stellar debris is swallowed by the black hole was proposed by Rees (1988) as a means of tracing SMBHs in nearby non-active galaxies.   In the present work, we discuss this and other possible outburst scenarios through an analysis of all ROSAT PSPC and HRI X-ray observations of NGC 5905. We also present simultaneous and long-term optical photometry of NGC 5905 as well as follow-up optical spectroscopy. 
Stefanie Komossa (MPE, Garching), Norbert Bade (Hamburger Sternwarte)