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 VLBI observations of supernova SN1993J, following an homogeneous and
well-defined methodology. 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 ($m_3 = 0.87 \pm
0.02$) is enough to model the expansion observed at 1.7\,GHz, while two
expansion indices ($m_1 = 0.933\pm0.010$ and $m_2 = 0.796\pm0.005$), separated
by a break time, $t_{br} = 390\pm30$ 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.
We estimate the fractional shell width ($0.31 \pm 0.02$, 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 $\sim 20$% of the mean flux density of the shell, and appear to
systematically evolve during the expansion.
