Radio Imaging of SN 1993J: The Story Continues
  We present the most recent VLBI images of SN 1993J, taken at 1.7 GHz on 2010
March 5-6, along with a discussion of its evolution with time. The new image is
the latest in a sequence covering almost the entire lifetime of the supernova.
For these latest observations we used an "in beam calibrator" technique, and
obtained a background rms brightness of 3.7 micro-Jy/beam. The supernova shell
remains quite circular in outline. Modulations in brightness are seen around
the rim which evolve relatively slowly, having remained generally similar over
the last several years of observation. We determine the outer radius of the
supernova using visibility-plane model-fitting. The supernova has slowed down
to around 30% of its original expansion velocity, and continues to expand with
radius approximately proportional to t^0.8, however, deviations from a strict
power-law evolution are seen. We do not find any clear-cut evidence for
systematically frequency-dependent evolution, suggesting that the radii as
determined from visibility-plane model-fitting continue to provide reasonable
estimates of the physical outer shock-front radius.
