A luminous recurrent supersoft X-ray source in NGC 300

We report the results of XMM-Newton observations for an especially luminous supersoft X-ray source (SSS) with a bolometric luminosity of 10(39) ergs s(-1) in the spiral galaxy NGC 300. The source was detected as an SSS in 1992 and disappeared in subsequent X-ray observations. The source was active again during recent XMM-Newton observations. It appeared to be very soft (kT similar to 60 eV) and very luminous (similar to10(38)-10(39) ergs s(-1)). The two XMM-Newton observations also reveal that the source went from a high state to a low state in 6 days. We also found a 5.4 hr periodicity during the low state. We consider white dwarf, black hole, and neutron star models in order to explain the nature of the source.