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X-ray outbursts from nearby `normal' and active galaxies. A review, new radio observations, and an X-ray search for further tidal disruption flares

In the last few years, giant-amplitude, non-recurrent X-ray flares have been observed from several non-active galaxies (NGC 5905, RXJ1242-11, RXJ1624+75, RXJ1420+53, RXJ1331-32). All of them share similar properties, namely: extreme X-ray softness in outburst, huge peak luminosity (up to ~10^{44} erg/s), and the absence of optical signs of Seyfert activity. Tidal disruption of a star by a supermassive black hole is the favored explanation of these unusual events. We present a review of the previous results, a search for radio emission from all outbursters, based on the NVSS database, and dedicated radio observations of NGC 5905 carried out with the VLA. These provide important constraints on the presence of an (obscured) active nucleus (AGN) at the center of each flaring galaxy. We rigorously explore AGN scenarios to account for the unusual X-ray outbursts from the optically `normal' galaxies and find AGN-related models highly unlikely. We conclude that the previously favored scenario - tidal disruption of a star by a supermassive black hole at the center of each of the outbursters - provides the best explanation for the X-ray observations. Finally, we present results from our on-going search for further X-ray flares from a sample of ~140 nearby active and non-active galaxies, using the ROSAT data base. While we do not find another X-ray flaring normal galaxy among this sample - entirely consistent with the predictions of the tidal disruption scenario - several highly variable active galaxies are detected. Their variability is not linked to tidal disruption, but best explained in terms of absorption or accretion-disk-related models.