2003ApJ...596..323B

When a massive star produces a powerful stellar wind prior to its supernova event, theory predicts that the collision of the exploded stellar ejecta with the wind leads to a reverse shock, creating soft X-ray emission. To understand the frequency at which luminous young X-ray supernovae occur, we used ROSAT to observe a complete sample of nearby supernovae (v_helio<1700 km/s) that occurred in the period 1985.5 through 1994.3, which included eight Type Ia supernovae and 19 Type Ib and Type II events. Three supernovae are detected in this time frame, SN 1987A (LMC), SN 1993J (NGC 3031), and a previously unreported source, SN 1992ad, a Type II supernova in NGC 4411b. No supernova had 0.5-2 keV luminosities exceeding 2x10³⁹ ergs/s, so at the 95% confidence level, the probability of an individual supernova exceeding this luminosity limit is less than 12%. Two of these supernovae had luminosities brighter than 6x10³⁸ ergs/s and at the 95% confidence level, the probability of a supernova being detected above this luminosity is in the range 8.7%-51%. It is unlikely for young supernovae to be a large component of the Intermediate Luminosity X-Ray Object (IXO or ULX) class, where the luminosity exceeds 2x10³⁹ ergs/s. The rate of successful detections appears to increase for sensitivities in the 10³⁷ ergs/s range, especially when obtained close to the time of the event.