The X-ray Decay of the ultraluminous supernova SN 1978K in NGC 1313

TL;DR

This study analyzes over 15 years of X-ray observations of supernova SN 1978K, revealing a decline in X-ray flux consistent with shock evolution from radiative to adiabatic phases and indicating a cooling shell formation.

Contribution

It provides a detailed long-term X-ray analysis of SN 1978K, demonstrating the transition of shock phases and changes in absorption, which enhances understanding of supernova remnant evolution.

Findings

X-ray flux declines as t^{-1} over 15 years

Transition from radiative to adiabatic shock phase

Decrease in absorption column density indicating cooling shell

Abstract

The Type IIn supernova (SN) 1978K in the nearby galaxy NGC 1313 has remained bright in multiwavelengths for more than $\sim 25$ 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 keV 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.