How is really decelerating the expansion of SN1993J?

TL;DR

SN1993J's radio shell expansion has been extensively monitored, revealing complex deceleration behavior, structural features, and spectral evolution, with some discrepancies between optical and radio observations.

Contribution

This study provides a detailed observational analysis of SN1993J's expansion, highlighting potential multiple deceleration slopes and structural features, advancing understanding of supernova radio evolution.

Findings

Shell expansion generally follows shock models with near circular symmetry.

Spectral index flattened from -1 to -0.67 over time.

Hints of complex deceleration and structural features in the expansion curve.

Abstract

SN1993J is to date the radio supernova whose evolution has been monitored in greatest detail and the one which holds best promise for a comprehensive theoretical-observational analysis. The shell-like radio structure of SN1993J has expanded in general accord with models of shock excited emission, showing almost circular symmetry for over 8 years, except for a bright feature at the south-eastern region of the shell that has been observed at every epoch. The spectrum of SN1993J has flattened from alpha =-1 to alpha =-0.67 (S_(\nu) propto nu**(alpha)). The decelerated expansion can be modeled well with a single slope but apparently better with two slopes. There are also intriguing hints of structure in the expansion curve. The results by the two VLBI groups carrying out this research show general agreement, but also some differences. A comparison of the optical and VLBI results about the details of the deceleration show some discrepancies.