2004ApJ...603..644S

We describe an observation of the X-ray-luminous SN 1978K in NGC 1313 using the ACIS detector on board Chandra and an archival XMM-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 and EPIC spectra. The best-fit models to the ACIS and EPIC spectra are dual hot plasma models (VMEKAL); one component is soft (T=0.61^+0.04_–0.05keV, 90% errors), and the other is harder (T=3.16^+0.44_–0.40keV). 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, ρ_ejecta∝r^–n, of ∼5.2, adopting a circumstellar matter distribution exponent of s=2(ρ_cs∝r^–s). The 0.5-2 keV light curve shows essentially no decline; the 2-10 keV light curve, comprised of only 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 SN 1978K.