First detection of X-ray line emission from Type IIn supernova 1978K with XMM-Newton's RGS

We report on robust measurements of elemental abundances of the Type IIn supernova SN 1978K, based on the high-resolution X-ray spectrum obtained with the Reflection Grating Spectrometer (RGS) onboard XMM-Newton. The RGS clearly resolves a number of emission lines, including N Ly $\alpha$ , O Ly $\alpha$ , O Ly $\beta$ , Fe xvii, Fe xviii, Ne He $\alpha$ , and Ne Ly $\alpha$ for the first time from SN 1978K. The X-ray spectrum can be represented by an absorbed, two-temperature thermal emission model, with temperatures of $kT \sim 0.6$ keV and 2.7 keV. The elemental abundances are obtained to be N $=$ $2.36_{{-0.80}}^{{+0.88}}$ , O $=$ $0.20 \pm {0.05}$ , Ne $=$ $0.47 \pm {0.12}$ , Fe $=$ $0.15_{{-0.02}}^{{+0.01}}$ times the solar values. The low metal abundances except for N show that the X-ray emitting plasma originates from the circumstellar medium blown by the progenitor star. The abundances of N and O are far from the CNO-equilibrium abundances expected for the surface composition of a luminous blue variable, and resemble the H-rich envelope of less massive stars with masses of 10-25 $\, M_{\odot }$ . Together with other peculiar properties of SN 1978K, i.e., a low expansion velocity of 500-1000 km s $^{-1}$ and SN IIn-like optical spectra, we propose that SN 1978K is a result of either an electron-capture SN from a super asymptotic giant branch star, or a weak Fe core-collapse explosion of a relatively low-mass ( $\sim \! \! 10\, M_{\odot }$ ) or high-mass ( $\sim$ 20-25 $\, M_{\odot }$ ) red supergiant star. However, these scenarios cannot naturally explain the high mass-loss rate of the order of $\dot{M} \sim 10^{-3}\, M_{\odot }\:{\rm yr^{-1}}$ over $\gtrsim$ 1000 yr before the explosion, which is inferred by this work as well as many other earlier studies. Further theoretical studies are required to explain the high mass-loss rates at the final evolutionary stages of massive stars.