X-ray and radio emission from the luminous supernova 2005kd

SN 2005kd is among the most luminous supernovae (SNe) to be discovered at X-ray wavelengths. We have re-analysed all good angular resolution (better than 20 arcsec full width at half-maximum point spread function) archival X-ray data for SN 2005kd. The data reveal an X-ray light curve that decreases as t^-1.62±0.06. Our modelling of the data suggests that the early evolution is dominated by emission from the forward shock in a high-density medium. Emission from the radiative reverse shock is absorbed by the cold dense shell formed behind the reverse shock. Our results suggest a progenitor with a mass-loss rate towards the end of its evolution of ≥4.3 × 10^{-4} {M_{{⊙}}} yr^{-1}, for a wind velocity of 10 km s^-1, at 4.0 × 10¹⁶ cm. This mass-loss rate is too high for most known stars, except perhaps hypergiant stars. A higher wind velocity would lead to a correspondingly higher mass-loss rate. A luminous blue variable star undergoing a giant eruption could potentially fulfill this requirement, but would need a high mass-loss rate lasting for several hundred years, and need to explain the plateau observed in the optical light curve. The latter could perhaps be due to the ejecta expanding in the dense circum-stellar material at relatively small radii. These observations are consistent with the fact that Type IIn SNe appear to expand into high-density and high mass-loss rate environments, and also suggest rapid variability in the wind mass-loss parameters within at least the last 5000 yr of stellar evolution prior to core-collapse.