Relativistic plasma as the dominant source of the optical continuum emission in the broad-line radio galaxy 3C 120

TL;DR

This study links optical flares in 3C 120 to superluminal jet components, suggesting a non-thermal origin of the optical continuum from relativistic plasma in the jet, impacting our understanding of AGN emission regions.

Contribution

It demonstrates that optical continuum emission in 3C 120 is primarily non-thermal and jet-related, providing new insights into AGN emission mechanisms and broad-line region ionization.

Findings

Optical flares coincide with superluminal jet component emergence.

Optical maxima occur when jet components pass stationary features.

Significant non-thermal optical emission influences broad-line region ionization.

Abstract

We report a relation between radio emission in the inner jet of the Seyfert galaxy 3C 120 and optical continuum emission in this galaxy. Combining the optical variability data with multi-epoch high-resolution very long baseline interferometry observations reveals that an optical flare rises when a superluminal component emerges into the jet and its maxima is related to the passage of such component through the location a stationary feature at a distance of ~1.3 parsecs from the jet origin. This indicates that a significant fraction of the optical continuum produced in 3C 120 is non-thermal and it can ionize material in a sub-relativistic wind or outflow. We discuss implications of this finding for the ionization and structure of the broad emission line region, as well as for the use of broad emission lines for determining black hole masses in radio-loud AGN.