DETECTION OF AN EXTREMELY SOFT-X-RAY OUTBURST IN THE HII-LIKE NUCLEUSOF NGC-5905

NGC 5905 was observed with ROSAT five times, three times during the RO SAT all-sky survey (RASS) and twice in the pointed observing mode. The soft X-ray properties of the nucleus of NGC 5905 derived from the RAS S observations, namely high-amplitude flux variability on timescales o f days, the extremely soft spectrum (photon index r = 4), and high pea k luminosity of L(X) = 7 . 10(42) ergs s(-1), are all inconsistent wit h its optical classification as HII-type, i.e. starburst galaxy. Durin g the pointed observations taken two years later, the Xray luminosity was down by a factor of > 80. Possible mechanisms, which can produce t he high state, are discussed. Tidal disruption of a star near a centra l black hole is in good agreement with the observations.