Detection of soft X-rays from supernova 1993J six days after outburst

Abstract

ON 28 March 1993, a new supernova was discovered¹ in the nearby galaxy M81. The proximity of the event (the distance² to M81 is only 3.6 Mpc), and the fact that the supernova was detected at an early stage of its outburst³, 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 star⁴. 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.