Gaia GraL: Gaia DR2 Gravitational Lens Systems. IV. Keck/LRIS
  spectroscopic confirmation of GRAL113100-441959 and model prediction of
  time-delays
  We report the spectroscopic confirmation and modeling of the quadruply imaged
quasar GRAL113100-441959, the first gravitational lens (GL) to be discovered
mainly from astrometric considerations. Follow-up spectra obtained with
Keck/LRIS reveal the lensing nature of this quadruply-imaged quasar with
redshift $z_s = 1.090 \pm 0.002$, but show no evidence of the central lens
galaxy. Using the image positions and $G$-band flux ratios provided by Gaia
Data Release 2 as constraints, we model the system with a singular power-law
elliptical mass distribution (SPEMD) plus external shear, to different levels
of complexity. We show that relaxing the isothermal constraint of the SPEMD is
not statistically significant, and thus we simplify the SPEMD to a singular
isothermal ellipsoid to estimate the Einstein radius of the main lens galaxy
$\theta_{\text{E}} = 0.\!\!^{\prime\prime}851$, the intensity and position
angle of the external shear $(\gamma,\theta_{\gamma}) =
(0.044,11.\!\!^{\circ}5)$, and we predict the lensing galaxy position to be
$(x_{\text{gal}},y_{\text{gal}}) =
(-0.\!\!^{\prime\prime}424,-0.\!\!^{\prime\prime}744)$ with respect to image A.
We provide time delay predictions for pairs of images, assuming a plausible
range of lens redshift values $z_l$ between $0.5$ and $0.9$. We finally examine
the impact on time delays of the so-called Source Position Transformation, a
family of degeneracies existing between different lens density profiles that
reproduce most of the lensing observables equally well. We show that this
effect contributes significantly to the time delay error budget and cannot be
ignored during the modeling. This has implications for robust cosmography
applications of lensed systems. GRAL113100-441959 is the first in a series of
seven new spectroscopically confirmed GLs discovered from Gaia Data Release 2.
