A Supernova Candidate atz=0.092 in XMM-Newton Archival Data

During a search for X-ray transients in the XMM-Newton archive within the EXTraS project, we discovered a new X-ray source that is detected only during an similar to 5 min interval of an similar to 21 hr-long observation performed on 2011 June 21 (EXMM 023135.0-603743, probability of a random Poissonian fluctuation: similar to 1.4 x 10(-27)). With dedicated follow-up observations, we found that its position is consistent with a star-forming galaxy (SFR = 1-2Myr(-1)) at redshiftz = 0.092 0.003 (d = 435 15 Mpc). At this redshift, the energy released during the transient event was 2.8 x 10(46)erg in the 0.3-10 keV energy band (in the source rest frame). The luminosity of the transient, together with its spectral and timing properties, make EXMM 023135.0-603743 a gripping analog to the X-ray transient associated to SN 2008D, which was discovered during a Swift/XRT observation of the nearby (d= 27 Mpc) supernova-rich galaxy NGC 2770. We interpret the XMM-Newton event as a supernova shock break-out or an early cocoon, and show that our serendipitous discovery is broadly compatible with the rate of core-collapse supernovae derived from optical observations and much higher than that of tidal disruption events.