The giant X-ray outburst in NGC 5905 - a tidal disruption event ?

Abstract

NGC 5905 is one of the very few galaxies that underwent a giant X-ray outburst, with a change in ROSAT photon countrate of a factor ~100. The outburst spectrum is both, very soft and luminous. Our high-resolution follow-up optical spectroscopy of NGC 5905 does not reveal any signs of Seyfert activity. At present, this makes NGC 5905 the only non-active galaxy among the X-ray outbursting ones. We discuss several scenarios to account for the exceptional properties of NGC 5905, including a supernova in dense medium, an accretion-disk instability, an event of extreme gravitational lensing, and the X-ray afterglow of a GRB to account for the X-ray outburst. We find that the most likely model to explain the observations seems to be tidal disruption of a star by a central SMBH, a scenario proposed by Rees (1988) as a tracer of SMBHs in nearby non-active galaxies. The X-ray outburst in this HII galaxy then lends further support to the scenario that all galaxies passed through an active phase, leaving dormant SMBHs in their centers.