At its peak, SN 1993J was one of the brightest supernovae in this century, and it is being studied more thoroughly than any supernova except SN 1987A. It is proving to be similar to the transition object SN 1987K, which metamorphosed from being a hydrogen-rich Type II near peak to having a hydrogen-deficient nebular phase. SN 1993J has been observed throughout the electromagnetic spectrum and with optical spectropolarimetry. It is interacting with a dense circumstellar nebula and is generating radio and X-ray flux, but it has probably not been detected in gamma rays. The photometric and spectral evolution are consistent with a star of original mass ∼ 15 M⊙ that lost appreciable mass to a binary companion leaving an extended, helium-rich hydrogen envelope of ≲ 0.5 M⊙ and a helium core of ∼ 4 M⊙. The spectral evolution will put strong constraints on the mixing of 56Ni and other species.

Introduction

SN 1993J was discovered on March 28.9 by F. Garcia (Ripero 1993) in the Sab galaxy NGC 3031 = M81. It was the brightest supernova observable from mid-northern latitudes since SN 1972E and has been the subject of intense observation by a large number of major and minor optical observatories, the VLA and other radio telescopes, IUE, the Compton Gamma Ray Observatory, ROSAT, and the newly launched ASCA satellite, as well as by a host of amateur astronomers. In addition, SN 1993J has proven exceptional on a number of grounds and has prompted considerable theoretical modeling.