DETECTION OF SOFT X-RAYS FROM SUPERNOVA 1993J 6 DAYS AFTER OUTBURST

ON 28 March 1993, a new supernova was discovered(1) in the nearby galaxy M81. The proximity of the event (the distance(2) to M81 is only 3.6 Mpc), and the fact that the supernova was detected at an early stage of its outburst(3), makes SN1993J an ideal candidate for the detailed study of the evolution of a supernova in all wavelength regimes. We report here the detection of soft X-ray emissions from SN1993J, six days after the initial discovery, and the subsequent evolution of the X-ray light curve over the next 41 days. The spectral characteristics of the emissions can be readily explained if the X-rays originate in strong shock fronts produced by the rapid expansion of the supernova ejecta into the slow, dense wind of a red supergiant progenitor star(4). The low intrinsic absorption of the earliest emissions requires that any circumstellar material is ionized, probably by the intense radiation of the initial outburst. The decay of the X-ray luminosity with time should provide important constraints on the density profiles of both the circumstellar gas and the outermost layers of the supernova ejecta.