Unexpected High Brightness Temperature 140 PC from the Core in the Jet of 3C 120

TL;DR

This study reports a stationary, highly bright component in the jet of 3C 120 at 140 parsecs from the core, exhibiting unexpectedly high brightness temperature likely due to intrinsic jet processes rather than shock propagation.

Contribution

It introduces the discovery of a stationary, high-brightness component in 3C 120's jet at 140 pc, suggesting intrinsic jet activity as the cause.

Findings

Discovery of a stationary component with high brightness temperature at 140 pc.

The component's brightness temperature is about 600 times higher than expected.

Possible explanation involves intrinsic jet processes, not standard shock models.

Abstract

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.