Shell revealed in SN 1979C

TL;DR

VLBI observations at 5 GHz have revealed a nearly circular shell structure in supernova 1979C, with detailed measurements of its position, brightness distribution, and spectral evolution, providing insights into its morphology and radio emission.

Contribution

This study provides the first detailed VLBI imaging of supernova 1979C's shell structure, including precise position and brightness distribution measurements.

Findings

Shell is approximately circular with minor deviations.

No central component detected above 0.15 mJy.

Radio spectrum has become flatter over time.

Abstract

VLBI observations at 5 GHz have revealed that supernova 1979C, in the galaxy M100 in the Virgo cluster, has shell structure. The shell is approximately circular with the 50% contour deviating from a circle by an rms of no more than 8% of the radius. The brightness distribution along the ridge may vary. The position of the center of the shell is at R.A.=12h 22m 58.66758s and decl.=15^o 47' 51.9695" (J2000), with a standard error of 0.8 mas in each of the coordinates. No isolated central component is visible above a flux density limit of ~0.15 mJy which corresponds to an upper limit of ~15 times the corresponding spectral luminosity of the Crab Nebula. The radio lightcurve is clearly falling again and the radio spectrum is now flatter than at earlier times. SN 1979C is only the fourth, and oldest (optically identified) supernova of which a detailed image could be obtained.