I. A. Smith, S. D. Ryder, M. Böttcher,
S. J. Tingay, A. Stacy, M. Pakull, and E. P. Liang
Astrophysical Journal, 669, 1130 (2007)
We discuss our radio (Australia Telescope Compact Array and Australian Long Baseline Array) and X-ray (XMM-Newton) monitoring observations of the unusual ultraluminous supernova SN 1978K in NGC 1313 at ~ 25 years after the explosion. SN 1978K is a rate example of a Type IIn supernova that has remained bright enough to have long-term X-ray and radio observations. The observations probe the dense medium that was ejected by the progenitor star prior to its explosion; the star might have been a Luminous Blue Variable. The radio imaging shows that the source remains compact, but may be marginally resolved. The radio monitoring shows deviations from a smooth decay suggesting that inhomogeneities are present in the radio emitting region.. It appears that a major change occurred in the mass-loss rate of the progenitor star shortly before the supernova event. The X-ray emission between 2000 and 2005 is consistent with the radiation coming from two shocks. All the X-ray data can be fit using the same model (with no systematic evolution or short-term variability), but this has a surprising requirement: The X-ray emitting regions have a very large abundance of helium. This would be consistent with the X-ray emitting shocks being located in a helium-rich layer that was ejected in the supernova explosion. The unusual properties of the supernova motivated a search for an associated gamma-ray burst (GRB). We show that SN K was inside the ~ 4 sigma error bos of GRB 771029. If this association is correct, the GRB was exceptionally underluminous. However, the quality of the gamma-ray burst locations at that time was poor, and this is likely just a chance alignment.