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 (≲2 keV) and characteristic of the reverse-shock region. The X-ray luminosity, L0.3-2 = (1.1 ± 0.2) × 1037 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 = (2.6 ± 0.4) × 10-5 M☉ yr-1 (vw/10 km s-1) 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.