Observational consequences of optical band milliarcsec-scale structure in active galactic nuclei discovered by Gaia

Abstract

We interpret the recent discovery of a preferred very long baseline interferometry (VLBI)/Gaia offset direction for radio-loud active galactic nuclei (AGNs) along pc-scale radio jets as a manifestation of their optical structure on scales of 1-100 milliarcsec (mas). The extended jet structure affects the Gaia position more strongly than the VLBI position, due to the difference in observing techniques. Gaia detects total power, while VLBI measures a correlated quantity, visibility, and is therefore sensitive to compact structures. The synergy of VLBI, which is sensitive to the position of the most compact source component, usually associated with the opaque radio core, and Gaia, which is sensitive to the centroid of optical emission, opens a window of opportunity to study optical jets at milliarcsec resolution, two orders of magnitude finer than the resolution of most existing optical instruments. We demonstrate that strong variability of optical jets is able to cause a jitter comparable to the VLBI/Gaia offsets in a quiet state, I.e. several mas. We show that the VLBI/Gaia position jitter correlation with the AGN optical light curve may help to locate the region where a flare has occurred and estimate its distance from the supermassive black hole and the ratio of the flux density in the flaring region to the total flux density.