The TeV blazar Markarian 421 at the highest spatial resolution
  We report the results obtained for the AGN Markarian 421 by model-fitting the
data in the visibility plane, studing the proper motion of jet components, the
light curve, and the spectral index of the jet features. We compare the radio
data with optical light curves obtained at the Steward Observatory, considering
also the optical polarization information. Mrk 421 has a bright nucleus and a
one-sided jet extending towards the north-west for a few parsecs. The
model-fits show that brightness distribution is well described using 6-7
circular Gaussian components, four of which are reliably identified at all
epochs; all components are effectively stationary except for component D, at
~0.4 mas from the core, whose motion is however subluminal. Analysis of the
light curve shows two different states, with the source being brighter and more
variable in the first half of 2011 than in the second half. The highest flux
density is reached in February. A comparison with the optical data reveals an
increase of the V magnitude and of the fractional polarization simultaneous
with the enhancement of the radio activity.
