2010ApJ...712L.160R

We present 1.7, 5, 15, 22 and 43 GHz polarimetric multi-epoch Very Long Baseline Array observations of the radio galaxy 3C 120. The higher frequency observations reveal a new component, not visible before 2007 April, 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 T_b 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 2007 April, and apparent stationarity.