SN 1988Z

This event was discovered independently, both by C. Pollas at the Côte d'Azur Observatory on a photographic plate taken December 12, 1988, and by G. Candeo at the Asiago Astrophysical Observatory from a plate taken December 14.^[5] The supernova was already past maximum when it was discovered. A spectrum taken December 17 showed this was most likely a Type II supernova.^[6]

This supernova displayed a number of unusual characteristics. It was unusually bright at maximum and showed very slow fading. There were strong, narrow emission lines caused by thick circumstellar material. Unlike a typical Type II supernova, no P Cygni profiles or absorption lines were observed.^[8] Emission lines of neutral helium were also visible. The spectral lines displayed a complex structure that evolved over time. Decline in the Hydrogen-alpha line strength was unusually slow and lacked an explanation in terms of radioactive decay. The overall picture suggested interaction between the supernova ejecta and a dense circumstellar medium.^[7]

A year after the event, radio emission from the supernova was detected using the Very Large Array. The host galaxy shows a redshift of z equal to 0.022, making this the most distant radio supernova detected at that time. It was also one of the most luminous radio supernova discovered. The radio properties indicated a very massive progenitor star in the range of 20–30 M_☉. In the late evolutionary stages of the star, it underwent a high rate of mass loss on the order of 10⁻⁴ M_☉·yr⁻¹, which created a dense circumstellar cocoon.^[9] In 1996, X-ray emission from the supernova was detected by ROSAT, making it the most distant supernova to be detected in this band. The estimated X-ray luminosity was 10⁴¹ erg·s⁻¹, which is consistent with a supernova event within dense circumstellar material.^[10]

Most studies now favor a model of a very massive progenitor that ejected up to 10 M_☉ at a rate of around 10⁻³ M_☉·yr⁻¹ for a period of about 10,000 years prior to the explosion. The mass loss rate ramped up during the final millennium prior to core collapse.^[3]