  We report on the discovery of X-ray emission from SN 1970G in M101, 35 years
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 for the reverse shock region. The X-ray luminosity (1 x
10^37 ergs/s) is likely caused by the interaction of the supernova (SN) 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) x 10^-5 M_sun/year 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 \propto t^-s with index s = 1.7+/-0.6 over a period of 12-35 years after the
outburst. As the oldest SN detected in X-rays, SN 1970G allows, for the first
time, direct observation of the transition from a SN to its supernova remnant
(SNR) phase.
