Discovery of Strongly Lensed Quasars in the Ultraviolet Near Infrared
  Optical Northern Survey (UNIONS)
  We report the discovery of five new doubly-imaged lensed quasars from the
first 2500 square degrees of the ongoing Canada-France Imaging Survey (CFIS),
which is a component of the Ultraviolet Near Infrared Optical Northern Survey
(UNIONS), selected from initial catalogues of either Gaia pairs or MILLIQUAS
quasars. We take advantage of the deep, 0.6'' median-seeing $r$-band imaging of
CFIS to confirm the presence of multiple point sources with similar colour of
$u-r$, via convolution of the Laplacian of the point spread function. Requiring
similar-colour point sources with flux ratios less than 2.5 mag in $r$-band,
reduces the number of candidates from 256314 to 7815. After visual inspection
we obtain 30 high-grade candidates, and prioritise spectroscopic follow-up for
those showing signs of a lensing galaxy upon subtraction of the point sources.
We obtain long-slit spectra for 18 candidates with ALFOSC on the 2.56-m Nordic
Optical Telescope (NOT), confirming five new doubly lensed quasars with
$1.21<z<3.36$ and angular separations from 0.8'' to 2.5''. One additional
system is a probable lensed quasar based on the CFIS imaging and existing SDSS
spectrum. We further classify six objects as nearly identical quasars -- still
possible lenses but without the detection of a lensing galaxy. Given our
recovery rate ($83\%$) of existing optically bright lenses within the CFIS
footprint, we expect that a similar strategy, coupled with $u-r$
colour-selection from CFIS alone, will provide an efficient and complete
discovery of small-separation lensed quasars of source redshifts below $z=2.7$
within the CFIS $r$-band magnitude limit of 24.1 mag.
