The light-curve reconstruction method for measuring the time delay of gravitational lens systems

We propose a new technique to measure the time delay of radio-loudgravitational lens systems, which does not rely on the excessive use ofinterferometric observations. Instead, the method is based on single-dish fluxdensity monitoring of the (unresolved) lens system's total lightcurve, combinedwith additional interferometric measurements of the flux density ratio at a fewepochs during that monitoring period. The basic idea of the method is toreconstruct the individual image lightcurves from the observed total lightcurveby assuming a range of potential values for the time delay and themagnification ratio of the images. It is then possible to single out thecorrect reconstruction, and therefore determine the time delay, by checking theconsistency of the reconstructed individual lightcurves with the additionalinterferometric observations. We performed extensive numerical simulations ofsynthetic lightcurves to investigate the dependence of the performance of thismethod on various parameters which are involved in the problem. Probably themost promising candidates for applying the method (and also for determining theHubble constant) are lens systems consisting of multiply imaged compact sourcesand an Einstein ring, such as B0218+357 from which some of the parameters usedfor our simulations were adopted.