The radial distribution of radio emission from SN 1993J: Magnetic field amplification due to the Rayleigh-Taylor instability

Context. Observations of radio emission from young core-collapse supernovae (CCSNe) allow one to study the history of the pre-supernova stellar wind, trace the density structure of the ejected material, and probe the magnetohydrodynamics that describe the interaction between the two, as the forward shock expands into the circumstellar medium. The radio shell of supernova SN 1993J has been observed with very long baseline interferometry (VLBI) for ∼20 years, giving one of the most complete pictures of the evolution of a CCSN shock. However, different results about the expansion curve and properties of the radio-emitting structure have been reported by different authors, likely due to systematics in the data calibration and/or model assumptions made by each team. Aims. We aim to perform an analysis of the complete set of VLBI observations of SN 1993J that accounts for different instrumental and source-intrinsic effects, in order to retrieve robust conclusions about the shock expansion and physics in SN 1993J. Methods. We have explored the posterior probability distribution of a complete data model, using a technique based on Markov chains. Our model accounts for antenna calibration effects, as well as different kinds of radio-emission structures for the supernova. Results. The posterior parameter distributions strongly favor a spherical shell-like radio structure with a nonuniform radial intensity profile, with a broad brightness distribution that peaks close to or just above the region where the contact discontinuity is expected to be located. Regarding the shell expansion, the full dataset can be well described using one single deceleration parameter, β ∼ 0.80, being the shell outer radius R ∝ t β . There is clear evidence of a relative widening of the shell width beyond day 2600−3300 after the explosion, which is due to an increased deceleration of the inner shell boundary. This is similar to findings previously reported by other authors. Conclusions. The radial intensity profile and the late evolution of the shell suggest a scenario in which the magnetic field is amplified mainly by the Rayleigh-Taylor instability, which emanates from the contact discontinuity. Furthermore, the increased deceleration of the inner boundary indicates that the reverse shock enters a region of the ejecta at around 3000 days, where the density distribution is substantially flatter. Such a weakening of the reverse shock can also explain the achromatic break in the radio light curves, which occurs at the same time. The deduced radial intensity distribution for SN 1993J is quite similar to that observed in the spatially well-resolved supernova remnant Cassiopeia A.