THE X-RAY LIGHT-CURVE OF THE VERY LUMINOUS SUPERNOVA SN-1978K IN NGC-1313

We present the 0.5-2.0 keV light curve of the X-ray luminous supernova SN 1978K in NGC 1313, based on six ROSAT observations spanning 1990 J uly to 1994 July. SN 1978K is one of a few supernovae or supernova rem nants that are very luminous (similar to 10(39)-10(40) ergs s(-1)) in the X-ray, optical, and radio bands, and the first, at a supernova age of 10-20 yr, for which sufficient data exist to create an X-ray light curve. The X-ray flux is approximately constant over the 4 yr sampled by our observations, which were obtained 12-16 yr after the initial e xplosion. Three models exist to explain the large X-ray luminosity: pu lsar input, a reverse shock running back into the expanding debris of the supernova, and the outgoing shock crushing of cloudlets in the deb ris field. Based upon calculations of Chevalier & Fransson, a pulsar c annot provide sufficient energy to produce the soft X-ray luminosity. Based upon the models and the light curve to date, it is not possible to discern the evolutionary phase of the supernova.