14.5yr r-band phot. of ClG J1004+4142 QSO images

The light curves span 14.5yr and allow the measurement of the time delay between the trailing bright quasar image D and the leading image C. When we fit all four light curves simultaneously and combine the models using the Bayesian information criterion, we find a time delay of {Delta}t_DC_=2458.47+/-1.02d (6.73yr), the longest ever measured for a gravitational lens.

For the other two independent time delays we obtain {Delta}t_BC_=782.20+/-0.43d (2.14yr) and {Delta}t_AC_=825.23+/-0.46d (2.26yr), in agreement with previous results. The information criterion is needed to weight the results for light curve models with different polynomial orders for the intrinsic variability and the effects of differential microlensing. The results using the Akaike information criterion are slightly different, but, in practice, the absolute delay errors are all dominated by the ~4% cosmic variance in the delays rather than the statistical or systematic measurement uncertainties.