A Decade of SN1993J: Discovery of Wavelength Effects in the Expansion
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  We have studied the growth of the shell-like radio structure of supernova
SN1993J in M81 from September 1993 through October 2003 with very-long-baseline
interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18cm. For
this purpose, we have developed a method to accurately determine the outer
radius (R) of any circularly symmetric compact radio structure like SN1993J.
  The source structure of SN1993J remains circularly symmetric (with deviations
from circularity under 2%) over almost 4000 days. We characterize the
decelerated expansion of SN 1993J through approximately day 1500 after
explosion with an expansion parameter $m= 0.845\pm0.005$ ($R \propto t^{m}$).
However, from that day onwards the expansion is different when observed at 6
and 18cm. Indeed, at 18cm, the expansion can be well characterized by the same
$m$ as before day 1500, while at 6cm the expansion appears more decelerated,
and is characterized by another expansion parameter, $m_{6}= 0.788\pm0.015$.
Therefore, since about day 1500 on, the radio source size has been
progressively smaller at 6cm than at 18cm. These findings are in stark contrast
to previous reports by other authors on the details of the expansion. In our
interpretation the supernova expands with a single expansion parameter, $m=
0.845\pm0.005$, and the 6cm results beyond day 1500 are due to physical
effects, perhaps also coupled to instrumental limitations. Two physical effects
may be involved: (a) a changing opacity of the ejecta to the 6cm radiation, and
(b) a radial decrease of the magnetic field in the emitting region.
  (Long abstract cut. Please, read full abstract in manuscript).
