  We describe an observation of the X-ray luminous SN1978K in NGC 1313 using
the ACIS detector on Chandra and an archival Newton EPIC observation. The
models that provided good fits to the ASCA SIS and GIS and the ROSAT PSPC
spectra no longer do so for the ACIS or EPIC spectra. The best-fit model to the
ACIS and EPIC spectra are dual hot plasma model (vmekal); one component is soft
(T = 0.61+0.04-0.05 keV, 90% errors) and the other is harder (T=3.16+0.44-0.40
keV). For the varying abundances permitted within the model, only the Si
abundance of the soft component differs from solar, with a value
n(Si)(SN78K)/n(Si)(solar) = 3.20+1.80-1.90 (90% errors). From a ratio of the
low- and high-T model fits to the Chandra and XMM-Newton spectra, we infer an
exponent n of the ejecta density distribution, rho_ejecta ~ r^(-n), of ~5.2,
adopting a circumstellar matter distribution exponent of s = 2 (rho_cs ~
r^(-s)). The 0.5-2 keV light curve shows essentially no decline; the 2-10 keV
light curve, constructed only of the ASCA, XMM-Newton, and Chandra
observations, shows a drop of 1.5 from the ASCA epoch. The hard band decline,
together with the apparently enhanced Si emission, signal the start of the
X-ray decline of SN1978K.
