Spectroscopic characterization of a remarkable temporally varying, triple-lensed quasar at z=2.67
Gravitationally lensed quasars are viable cosmic tools for constraining a diversity of fundamental astrophysical phenomena; They enable identification of faint, low-mass supermassive black holes, provide unique constraints on the intervening intergalactic or interstellar medium in their sightlines, and can be used to determine key cosmological quantities such as the Hubble constant, $H_0$. However, they are rare phenomena, and it has proven difficult to define efficient, unbiased selection methods.} In this study, we report the independent spectroscopic identification of a remarkable triple-lensed quasar at $z=2.67$, identified based on astrometric measurements from the {\em Gaia} mission, previously identified in Pan-STARRS. Furthermore, a larger spectroscopic follow-up survey of {\em Gaia}-detected candidate lensed quasars. We characterize in detail the three mirror images of the quasar and their spatial and temporal spectroscopic coverage, with focus on the emission-line properties which shows variation across sigthlines and temporal evolution over the $\sim 11$months spectroscopic campaign. We construct a lens model of the foreground source from a combination of the multiple spectra and deep optical imaging, providing a robust halo mass of $M_{\rm h} = (2.78 \pm 0.05)\times 10^{10}M_\odot$. Based on the lens model, the time delay between each sightline is translated into an intrinsic quasar time, allowing us to construct a quasar timeseries over $\sim18$months with monthly cadence. Over months timescales the broad emission lines vary in both velocity offset and equivalent width (EW) as well as an overall increase in ionization. This exemplary triple-lensed quasars demonstrates the viability of identifying such rare lens configurations based purely on the astrometric measurements from the {\em Gaia} mission, which we here provide optimized selection criteria for, for future studies.