SN 1988Z: The Most Distant Radio Supernova

We present observations of the early radio emission from the unusual supernova SN 1988Z in MCG +03-28-022 made with the Very Large Array at 20, 6, 3.6, and 2 cm from 1989 December, 1 year after optical discovery, through 1992 December. At the redshift z = 0.022 of the parent galaxy, SN 1988Z is the most distant radio supernova ever discovered. With a 6 cm maximum flux density of 1.90 mJy, SN 1988Z is ~1.2 times more luminous than the unusually powerful radio supernova SN 1986J in NGC 891, making SN 1988Z also one of the most luminous radio supernovae ever discovered. Our analysis and model fitting of these initial light curves indicates that the overall radio properties of SN 1988Z are quite similar to those of SN 1986J and can be described by a modified Chevalier model involving the supernova shock interacting with a high-density circumstellar cocoon, with an additional component of internal thermal absorbers and nonthermal emitters. The radio properties of SN 1988Z indicate that the cocoon resulted from a very high mass-loss rate (M~10^-4^ M_sun_ yr^-1^) in the late stages of the evolution of a very massive (20-30 M_sun_) presupernova star.