Unexpected High Brightness Temperature 140 PC from the Core in the Jet
  of 3C 120
  We present 1.7, 5, 15, 22 and 43 GHz polarimetric multi-epoch VLBA
observations of the radio galaxy 3C 120. The higher frequency observations
reveal a new component, not visible before April 2007, located 80 mas from the
core (which corresponds to a deprojected distance of 140 pc), with a brightness
temperature about 600 times higher than expected at such distances. This
component (hereafter C80) is observed to remain stationary and to undergo small
changes in its brightness temperature during more than two years of
observations. A helical shocked jet model - and perhaps some flow acceleration
- may explain the unusually high Tb of C80, but it seems unlikely that this
corresponds to the usual shock that emerges from the core and travels
downstream to the location of C80. It appears that some other intrinsic process
in the jet, capable of providing a local burst in particle and/or magnetic
field energy, may be responsible for the enhanced brightness temperature
observed in C80, its sudden appearance in April 2007, and apparent
stationarity.
