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