Multiwavelength Monitoring of the Unusual Ultraluminous Supernova Sn 1978k in Ngc 1313 and the Search for an Associated Gamma-ray Burst

We discuss our radio (Australia Telescope Compact Array and Australian Long Baseline Array) and X-ray (XMMNewton) monitoring observations of the unusual ultraluminous supernova SN 1978K inNGC 1313 at 25 yr after the explosion. SN 1978K is a rare 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 it 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 2006 is consistent with the radiation coming from two shocks. All the X-ray data can be fit using the samemodel (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 by the progenitor star, or helium-rich material 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 1978K was inside the 4 error box 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. Subject headinggs: galaxies: individual (NGC 1313) — gamma rays: bursts — supernova remnants — supernovae: individual (SN 1978K) Online material: color figures