2017MNRAS.468.1551Z

The Type IIn supernova (SN) 1978K in the nearby galaxy NGC 1313 has remained bright in multiwavelengths for more than ∼25 yr. The archival data of SN 1978K collected with ROSAT, ASCA, XMM-Newton and Chandra from 1990 to 2006 show no significant variation of the soft X-ray emission but a hint of flux decrease in hard X-rays. In this work, we perform a detailed analysis using more than 15 yr of XMM-Newton observations. Both the 0.5-2 and 2-10 keV light curves decline as t^–1 from 2000 to 2015. The transition of light-curve profiles can be explained in a way that the reverse shock was radiative at an early phase and then became adiabatic at late times. Such a scenario is also supported by the spectral analysis results. We also found a decrease in the absorption column density, which indicates the presence of a cool shell during the radiative phase.