Kavli Affiliate: R. G. McMahon
| First 5 Authors: Frédéric Dux, Martin Millon, Aymeric Galan, Eric Paic, Cameron Lemon
| Summary:
We present new time delays, the main ingredient of time delay cosmography,
for 22 lensed quasars resulting from high-cadence r-band monitoring on the 2.6
m ESO VLT Survey Telescope and Max-Planck-Gesellschaft 2.2 m telescope. Each
lensed quasar was typically monitored for one to four seasons, often shared
between the two telescopes to mitigate the interruptions forced by the COVID-19
pandemic. The sample of targets consists of 19 quadruply and 3 doubly imaged
quasars, which received a total of 1 918 hours of on-sky time split into 21 581
wide-field frames, each 320 seconds long. In a given field, the 5-{sigma}
depth of the combined exposures typically reaches the 27th magnitude, while
that of single visits is 24.5 mag – similar to the expected depth of the
upcoming Vera-Rubin LSST. The fluxes of the different lensed images of the
targets were reliably de-blended, providing not only light curves with
photometric precision down to the photon noise limit, but also high-resolution
models of the targets whose features and astrometry were systematically
confirmed in Hubble Space Telescope imaging. This was made possible thanks to a
new photometric pipeline, lightcurver, and the forward modelling method
STARRED. Finally, the time delays between pairs of curves and their
uncertainties were estimated, taking into account the degeneracy due to
microlensing, and for the first time the full covariance matrices of the delay
pairs are provided. Of note, this survey, with 13 square degrees, has
applications beyond that of time delays, such as the study of the structure
function of the multiple high-redshift quasars present in the footprint at a
new high in terms of both depth and frequency. The reduced images will be
available through the European Southern Observatory Science Portal.
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