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. From these data, the amplitude of possible microlensing variability does not exceed 10 mmag on a 7-yr timescale. Additionally, we measure a time delay of $t_{AB}$ = 52.2$\pm$2.5 days, with the brighter image A leading. Long-slit \WHT 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 MgII $\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 HST imaging and the MgII flux ratio, we also built two simple mass models for the lens system. The close agreement between the measured delay and those predicted by the mass models, measured redshifts, and a concordance $\Lambda$CDM cosmology, confirms the robustness of our results and highlights PS J2305+3714 as a promising system for future time-delay cosmography.