Observational evidence for the link between the variable optical continuum and the subparsec-scale jet of the radio galaxy 3C 390.3

The mechanism and the region of generation of variable continuum emission are poorly understood for radio-loud active galactic nuclei because of the complexity of the nuclear region. High-resolution radio very long baseline interferometry (VLBI) observations have allowed us to zoom into a subparsec-scale region of the jet in the radio-loud galaxy 3C 390.3. We have combined the radio VLBI and the optical data covering the time period of 14 yr in order to look for a link between the optical flares and the parsec-scale jet. We have identified two stationary and nine moving radio features in the innermost subparsec-scale region of the jet. All nine ejections are associated with optical flares. We have found a significant correlation (at a confidence level of > 99.99 per cent) between the ejected jet components and the optical continuum flares. The epochs at which the moving knots pass through the location of a stationary radio feature and at which the optical light curve reaches maximum are correlated. The radio events follow the maxima of optical flares with a mean time delay of 0.10 +/- 0.04 yr. This correlation can be understood if the variable optical continuum emission is generated in the innermost part of the jet. A possible mechanism of the energy release is the ejection of knots of high-energy electrons that are accelerated in the jet flow and generate flares of synchrotron continuum emission in the wide range of frequencies from radio to X-ray bands. In this scenario, the beamed optical continuum emission from the jet and counterjet ionizes a gas in a subrelativistic outflow surrounding the jet. This results in the formation of two outflowing conical regions with broad emission lines (in addition to the conventional broad-line region around the central nucleus).