Multiwavelength Monitoring of the Unusual Ultraluminous Supernova
SN 1978K in NGC 1313 and the Search for an Associated Gamma-Ray Burst
Smith, I. A., Ryder, S. D., Böttcher, M., Tingay, S. J.,
Stacy, A., Pakull, M., & Liang, E. P.
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 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 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 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 sigma 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.
Status:
2007, ApJ, 669, 1130.
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