2005ApJ...632L..99I

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 (≲2 keV) and characteristic of the reverse-shock region. The X-ray luminosity, L_0.3–2=(1.1±0.2)x10³⁷ ergs/s, 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{dot}=(2.6±0.4)x10^–5 M_☉/yr(v_w/10 km/s) 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∝t^–s with index s=2.7±0.9 over a period of ~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.