VLBA monitoring of Mrk 421 at 15 and 24 GHz during 2011
  We present a preliminary analysis of new high resolution radio observations
of the nearby TeV blazar Markarian 421 (z=0.031). This study is part of an
ambitious multifrequency campaign, with observations in sub-mm (SMA),
optical/IR (GASP), UV/X-ray (Swift, RXTE, MAXI), and gamma rays (Fermi-LAT,
MAGIC, VERITAS). In this manuscript we consider only data obtained with the
Very Long Baseline Array (VLBA) at seven epochs (one observation per month from
January to July 2011) at 15 and 23.8 GHz. We investigate the inner jet
structure on parsec scales through the study of model-fit components for each
epoch. We identified 5-6 components which are consistent with being stationary
during the 6-month period reported here. The aim is to try to shed light on
questions such as the nature of radiating particles, the connection between
radio and gamma-ray emission, the location of emitting regions and the origin
of the flux variability.
