Gravitationally Lensed Quasars in Gaia: I. Resolving Small-Separation
  Lenses
  Gaia's exceptional resolution (FWHM $\sim$ 0.1$^{\prime\prime}$) allows
identification and cataloguing of the multiple images of gravitationally lensed
quasars. We investigate a sample of 49 known lensed quasars in the SDSS
footprint, with image separations less than 2$^{\prime\prime}$, and find that 8
are detected with multiple components in the first Gaia data release. In the
case of the 41 single Gaia detections, we generally are able to distinguish
these lensed quasars from single quasars when comparing Gaia flux and position
measurements to those of Pan-STARRS and SDSS. This is because the multiple
images of these lensed quasars are typically blended in ground-based imaging
and therefore the total flux and a flux-weighted centroid are measured, which
can differ significantly from the fluxes and centroids of the individual
components detected by Gaia. We compare the fluxes through an empirical fit of
Pan-STARRS griz photometry to the wide optical Gaia bandpass values using a
sample of isolated quasars. The positional offsets are calculated from a
recalibrated astrometric SDSS catalogue. Applying flux and centroid difference
criteria to spectroscopically confirmed quasars, we discover 4 new
sub-arcsecond-separation lensed quasar candidates which have two distinct
components of similar colour in archival CFHT or HSC data. Our method based on
single Gaia detections can be used to identify the $\sim$ 1400 lensed quasars
with image separation above 0.5$^{\prime\prime}$, expected to have only one
image bright enough to be detected by Gaia.
