Discovery of X-ray emission from supernova 1970G with Chandra:: Filling the void between supernovae and supernova remnants

We report the discovery of X-ray emission from SN 1970G in M101, 35 yr after its outburst, using deep X-ray imaging with the Chandra X-Ray Observatory. The Chandra ACIS spectrum shows that the emission is soft (less than or similar to 2 keV) and characteristic of the reverse-shock region. The X-ray luminosity, L0.3-2 = (1.1 +/- 0.2) x 10(37) ergs s(-1), is likely caused by the interaction of the supernova shock with dense circumstellar matter. If the material was deposited by the stellar wind from the progenitor, a mass-loss rate of <(M)over dot> = (2.6 +/- 0.4) x 10(-5) M circle dot yr(-1) (v(w)/10 is inferred. Utilizing the high-resolution Chandra ACIS data of SN 1970G and its environment, we reconstruct the X- ray lightcurve from previous ROSAT HRI, PSPC, and XMM- Newton EPIC observations, and find a best-fit linear rate of decline of L proportional to t(-s) with index s = 2.7 +/- 0.9 over a period of approximate to 20-35 yr after the outburst. As the oldest supernova detected in X- rays, SN 1970G allows, for the first time, direct observation of the transition from a supernova to its supernova remnant phase.