SN 1978K is one of the most remarkable X-ray supernovae. It is located in the nearby late-type SB spiral galaxy NGC 1313 (at a distance of 4.1 Mpc). A 1980, Einstein IPC image showed two bright sources in NGC 1313 with LX > 1039 ergs-1, one near the nucleus and one 6arcmin to the south. A 1992 ROSAT PSPC image revealed a third source to the east-southeast of the nucleus, nearly as bright as the other two, and nearly forming an equilateral triangle with them. An optical counterpart had been discovered 2 yrs earlier and reported as a nova. Also, it had been detected as a bright radio source in 1982. Coupled with the X-ray luminosity, these observations led to the conclusion that this object is a supernova [135]. SN 1978K is thus the first supernova identified as such from X-ray observations. Studies of archival plates indicated an explosion date around June 1, 1978, approximately 19 months before the IPC observation.

SN 1978K has been monitored extensively, initially using ROSAT and ASCA, and subsequently XMM-Newton and Chandra. Its 0.5-2.0 keV flux since discovery has remained remarkably constant, while its 2-10 keV light curve shows a slight decrease starting after 2000 (22 years post-explosion). The 0.5-10 keV ASCA spectra taken in 1993 were reasonably well fit by a single thermal spectrum with kT < 3.0 keV, with low metal abundances [116]. Recent ACIS and EPIC spectra require two thermal components, with temperatures of <0.6keV and <3.0keV [139]. These spectra show a hint of enhanced Si. Schlegel et al. interpret the warmer and cooler components as emanating from the forward and reverse shocks [139]. The decline of the flux from the hotter component is interpreted as the first hint of increasing transparency of the matter surrounding the supernova. The flat light curve is consistent with an unusually flat CSM density profile (p « r-1), indicating an unusual mass loss history [72].

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