Time-delay and lens galaxy redshift in the doubly imaged quasar PS J2305+3714

ABSTRACT We present results from a seven-season (2018–2024) monitoring campaign of the gravitationally lensed quasar system PS J2305+3714 using the 1.5-m Maidanak Telescope in the optical R band. We measure a time delay of $t_{\mathrm{ AB}} = 52.2 \pm 2.5$ d, with the brighter image A leading. No significant microlensing variability is detected, with an upper limit on the amplitude of $\lesssim$10 mmag over a 7-yr time-scale. Long-slit WHT/ISIS spectroscopy refines the quasar redshift to $z_s$ = 1.791 and provides the first measurement of the lens redshift, $z_d$ = 0.473. The flux ratios of the quasar images in the Mg ii $\lambda$2800 emission line and in the adjacent continuum are nearly identical, indicating minimal microlensing effects in the spectral domain, which is consistent with the very weak microlensing signal in the time domain. Using precise astrometry from recent Hubble Space Telescope (HST) imaging and the Mg ii flux ratio, we also build two simple mass models for the lens system. The close agreement between the measured delay and the values predicted by the lens mass models, the measured redshifts, and a concordance $\Lambda$ cold dark matter cosmology, supports the robustness of our results and highlights PS J2305+3714 as a promising system for future time-delay cosmography.