The giant X-ray outbursts in NGC 5905 and IC 3599: Follow-up observations and outburst scenarios

Huge amplitude X-ray outbursts in a few galaxies were reported in the last few years. As one of the exciting possibilities to explain these observatio ns, tidal disruption of a star by a central supermassive black hole has bee n proposed. In the present paper, we perform a detailed discussion of this and other possible scenarios for the X-ray outburst in NGC 5905, and a comp arison of NGC5905 and IC3599 in outburst as well as in quiescence. To this end we present (i) a thorough analysis of all ROSAT PSPC X-ray observations of NGC 5905 and new HRI data, (ii) optical photometry of NGC5905 quasi-sim ultaneous to the X-ray outburst and on longer terms, (iii) the first post-o utburst optical spectra of NGC 5905 and high-resolution post-outburst spect ra of IC3599, and (iv) photoionization models for the high-excitation emiss ion lines that were discovered in the optical outburst spectrum of IC3599. The investigated outburst models include, besides the tidal disruption even t, a supernova in dense medium, an accretion-disk instability, an event of extreme gravitational lensing, the X-ray afterglow of a GRB, and the possib ility of a warm-absorbed hidden Seyfert nucleus in the center of the galaxy . The successful models, all involving the presence of a central supermassi ve black hole, are selected and implications are discussed. The optical spe ctra of both galaxies in quiescence are carefully examined for signs of per manent low-level Seyfert-activity. Whereas IC3599 shows several signs of ac tivity, none is revealed for NGC 5905. At present, and among the X-ray burs ts, this makes NGC 5905 the only safe candidate for a tidal disruption even t in an otherwise non-active galaxy. The physical conditions in the HII emi ssion-line gas are also investigated. We briefly comment on a search for fu rther highly variable objects on the basis of ROSAT observations. Several w ith factors 10-20 are found; all of them are known to harbour warm absorber s.