Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data

We have performed a complete re-calibration and re-analysis of all the available Very-Long-Baseline-Interferometry (VLBI) observations of supernova SN1993J, following an homogeneous and well-defined methodology. VLBI observations of SN1993J at 69 epochs, spanning 13 years, were performed by two teams, which used different strategies and analysis tools. The results obtained by each group are similar, but their conclusions on the supernova expansion and the shape and evolution of the emitting region differ significantly. From our analysis of the combined set of observations, we have obtained an expansion curve with unprecedented time resolution and coverage. We find that the data from both teams are compatible when analyzed with the same methodology. One expansion index (m₃ = 0.87 {\plusmn} 0.02) is enough to model the expansion observed at 1.7 GHz, while two expansion indices (m₁ = 0.933 {\plusmn} 0.010 and m₂ = 0.796 {\plusmn} 0.005), separated by a break time, t$_{br}$ = 390 {\plusmn} 30 days, are needed to model the data, at frequencies higher than 1.7 GHz, up to day \~{}4000 after explosion. We thus confirm the wavelength dependence of the size of the emitting region reported by one of the groups. We also find that all sizes measured at epochs later than day \~{}4000 after explosion are systematically smaller than our model predictions (i.e., an additional expansion index might be needed to properly model these data). We also estimate the fractional shell width (0.31 {\plusmn} 0.02, average of all epochs and frequencies) and the level of opacity to the radio emission by the ejecta. We find evidence of a spectral-index radial gradient in the supernova shell, which is indicative of a frequency-dependent ejecta opacity. Finally, we study the distribution and evolution of the azimuthal anisotropies (hot spots) found around the radio shell during the expansion. These anisotropies have intensities of \~{}20% of the mean flux density of the shell, and appear to systematically evolve during the expansion.