2008ApJ...683..767S

We have studied the X-ray emission from four historical Type II supernovae (the newly recovered 1941C in NGC 4136 and 1959D in NGC 7331; and 1968D and 1980K in NGC 6946), using Chandra ACIS-S imaging. In particular, the first three are the oldest ever found in the X-ray band, and provide constraints on the properties of the stellar wind and circumstellar matter encountered by the expanding shock at more advanced stages in the transition toward the remnant phase. We estimate emitted luminosities ~5x10³⁷ erg/s for SN 1941C, ∼a few x10³⁷ erg/s for SN 1959D, ~2x10³⁸ erg/s for SN 1968D, and ~4x10³⁷ erg/s for SN 1980K, in the 0.3-8 keV band. X-ray spectral fits to SN 1968D suggest the presence of a harder component, possibly a power law with photon index ~2, contributing ~10³⁷ erg/s in the 2-10 keV band. We speculate that it may be evidence of nonthermal emission from a Crab-like young pulsar.