Time delays for the quadruply lensed QSO GraL J1651-0417 based on ZTF light curves

Aims We aim to test the possibility of measuring significant and accurate time delays between images of lensed quasars using public photometric surveys. We show the case for the large-separation, quadruply lensed quasi-stellar object (QSO) GraL J1651-0417. Methods We retrieved publicly available light curves in the g and r bands from the ZTF forced photometry service centred on the four images of the quasar. We estimated the time delays of the flux variations in the different images using the discrete correlation function and bootstrapping methods separately for each band. The delays were independently predicted by fitting two mass lens model (SIS+γ and SPL+γ). Results The ZTF light curves reveal significant correlations and time delays. In the r band, we measure delays of 579 ± 40 days for A-B, 366 ± 26 days for A-C, and 698 ± 29 days for A-D, which are fully consistent with the predictions from the lens model.