Observational consequences of optical band milliarcsecond-scale
  structure in active galactic nuclei discovered by Gaia
  We interpret the recent discovery of a preferable VLBI/Gaia offset direction
for radio-loud active galactic nuclei (AGNs) along the parsec-scale radio jets
as a manifestation of their optical structure on scales of 1 to 100
milliarcseconds. The extended jet structure affects the Gaia position stronger
than the VLBI position due to the difference in observing techniques. Gaia
detects total power while VLBI measures the correlated quantity, visibility,
and therefore, sensitive to compact structures. The synergy of VLBI that is
sensitive to the position of the most compact source component, usually
associated with the opaque radio core, and Gaia that is sensitive to the
centroid of optical emission, opens a window of opportunity to study optical
jets at milliarcsecond 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 at a quiet state, i.e. several milliarcseconds. We show that
the VLBI/Gaia position jitter correlation with the AGN optical light curve may
help to locate the region where the flare occurred, estimate its distance from
the super-massive black hole and the ratio of the flux density in the flaring
region to the total flux density.
