SN 1993J VLBI. III. The Evolution of the Radio Shell

A sequence of images of supernova 1993J at 31 epochs, from 50 days to ~9 yr after shock breakout, shows the evolution of the expanding radio shell of an exploded star in detail. The images were obtained from 24 observing sessions at 8.4 GHz and 19 at 5.0 GHz and from our last session at 1.7 GHz. The images are all phase-referenced to the stable reference point of the core of the host galaxy M81. This allows us to display them relative to the supernova explosion center. The earliest image shows an almost unresolved source with a radius of 520 AU. The shell structure becomes discernible 175 days after shock breakout. The brightness of the ridge of the projected shell is not uniform, but rather varies by a factor of 2, having a distinct peak or maximum to the southeast and a gap or minimum to the west. Over the next ~350 days, this pattern rotates counterclockwise, with the gap rotating from west to north-northeast. After 2 years, the structure becomes more complex with hot spots developing in the east, south, and west. The pattern of modulation continues to change, and after 5 years the hot spots are located to the north-northwest, south, and south-southeast. After 9 years, the radio shell has expanded to a radius of 19,000 AU. The brightness in the center of the images is lower than expected for an optically thin, spherical shell. Absorption in the center is favored over a thinner shell in the back and/or front. Allowing for absorption, we find that the thickness of the shell is 25% ± 3% of its outer radius. We place a 3 σ upper limit of 4.4% on the mean polarization of the bright part of the shell, consistent with internal Faraday depolarization. We find no compact source in the central region above a brightness limit of 0.05 mJy beam-1 at 8.4 GHz, corresponding to 30% of the current spectral luminosity of the Crab Nebula. We conclude either that any pulsar nebula in the center of SN 1993J is much fainter than the Crab or that there is still significant internal radio absorption.