Gaia GraL II - Gaia DR2 Gravitational Lens Systems: The Known Multiply
  Imaged Quasars
  Thanks to its spatial resolution the ESA/Gaia space mission offers a unique
opportunity to discover new multiply-imaged quasars and to study the already
known lensed systems at sub-milliarcsecond astrometric precisions. In this
paper, we address the detection of the known multiply-imaged quasars from the
Gaia Data Release 2 and determine the astrometric and photometric properties of
the individually detected images found in the Gaia DR2 catalogue. We have
compiled an exhaustive list of quasar gravitational lenses from the literature
to search for counterparts in the Gaia Data Release 2. We then analyze the
astrometric and photometric properties of these Gaia's detections. To highlight
the tremendous potential of Gaia at the sub-milliarcsecond level we finally
perform a simple Bayesian modeling of the well-known gravitational lens system
HE0435-1223, using Gaia Data Release 2 and HST astrometry. From 478 known
multiply imaged quasars, 200 have at least one image found in the Gaia Data
Release 2. Among the 41 known quadruply-imaged quasars of the list, 26 have at
least one image in the Gaia Data Release 2, 12 of which are fully detected, 6
have three counterparts, 7 have two and 1 has only one. As expected, the
modeling of HE0435-1223 shows that the model parameters are significantly
better constrained when using Gaia astrometry compared to HST astrometry, in
particular the relative positions of the background quasar source and the
centroid of the deflector. The Gaia sub-milliarcsecond astrometry also
significantly reduces the parameter correlations. Besides providing an
up-to-date list of multiply imaged quasars and their detection in the Gaia DR2,
this paper shows that more complex modeling scenarios will certainly benefit
from Gaia sub-milliarcsecond astrometry.
