The connection between the radio jet and the gamma-ray emission in the radio galaxy 3C 120

TL;DR

This study investigates the link between radio jet activity and gamma-ray emission in galaxy 3C 120, revealing that gamma-ray flares are associated with specific jet ejections and orientations, suggesting synchrotron self-Compton as the likely emission mechanism.

Contribution

It demonstrates a direct connection between gamma-ray activity and radio jet ejections, emphasizing the importance of jet orientation and local photon fields in gamma-ray production in 3C 120.

Findings

Gamma-ray activity coincides with radio core flares and superluminal ejections.

Gamma-ray detections depend on jet component orientation relative to the observer.

Gamma-ray production region is within 0.13 pc of the mm-VLBI core.

Abstract

We present the analysis of the radio jet evolution of the radio galaxy 3C 120 during a period of prolonged gamma-ray activity detected by the Fermi satellite between December 2012 and October 2014. We find a clear connection between the gamma-ray and radio emission, such that every period of gamma-ray activity is accompanied by the flaring of the mm-VLBI core and subsequent ejection of a new superluminal component. However, not all ejections of components are associated with gamma-ray events detectable by Fermi. Clear gamma-ray detections are obtained only when components are moving in a direction closer to our line of sight.This suggests that the observed gamma-ray emission depends not only on the interaction of moving components with the mm-VLBI core, but also on their orientation with respect to the observer. Timing of the gamma-ray detections and ejection of superluminal components locate the gamma-ray production to within almost 0.13 pc from the mm-VLBI core, which was previously estimated to lie about 0.24 pc from the central black hole. This corresponds to about twice the estimated extension of the broad line region, limiting the external photon field and therefore suggesting synchrotron self Compton as the most probable mechanism for the production of the gamma-ray emission. Alternatively, the interaction of components with the jet sheath can provide the necessary photon field to produced the observed gamma-rays by Compton scattering.