Six Years of Fermi-LAT and Multi-wavelength Monitoring of the Broad-Line Radio Galaxy 3C 120: Jet Dissipation at Sub-parsec Scales from the Central Engine

TL;DR

This six-year multi-wavelength study of 3C 120 reveals that gamma-ray emission originates from sub-parsec scales near the black hole, with jet dissipation linked to core brightening and superluminal knot ejection, favoring synchrotron self-Compton processes.

Contribution

The paper provides the first detailed multi-wavelength temporal correlation analysis linking gamma-ray emission to jet activity at sub-parsec scales in 3C 120.

Findings

Gamma-ray emission is produced inside the unresolved 43 GHz core.

Jet dissipation occurs at sub-parsec distances from the black hole.

Synchrotron self-Compton process is favored for gamma-ray production.

Abstract

We present multi-wavelength monitoring results for the broad-line radio galaxy 3C 120 in the MeV/GeV, sub-millimeter, and 43 GHz bands over six years. Over the past two years, Fermi-LAT sporadically detected 3C 120 with high significance and the 230 GHz data also suggest an enhanced activity of the source. After the MeV/GeV detection from 3C 120 in MJD 56240-56300, 43 GHz VLBA monitoring revealed a brightening of the radio core, followed by the ejection of a superluminal knot. Since we observed the gamma-ray and VLBA phenomena in temporal proximity to each other, it is naturally assumed that they are physically connected. This assumption was further supported by the subsequent observation that the 43 GHz core brightened again after a gamma-ray flare occurred around MJD 56560. We can then infer that the MeV/GeV emission took place inside an unresolved 43 GHz core of 3C 120 and that the jet dissipation occurred at sub-parsec distances from the central black hole, if we take the distance of the 43 GHz core from the central black hole as ~ 0.5 pc, as previously estimated from the time lag between X-ray dips and knot ejections (Marscher et al. 2002; Chatterjee et al. 2009). Based on our constraints on the relative locations of the emission regions and energetic arguments, we conclude that the gamma rays are more favorably produced via the synchrotron self-Compton process, rather than inverse Compton scattering of external photons coming from the broad line region or hot dusty torus. We also derived the electron distribution and magnetic field by modeling the simultaneous broadband spectrum.