The lightcurve reconstruction method for measuring the time delay of   gravitational lens systems
  We propose a new technique to measure the time delay of radio-loud gravitational lens systems, which does not rely on the excessive use of interferometric observations. Instead, the method is based on single-dish flux density monitoring of the (unresolved) lens system's total lightcurve, combined with additional interferometric measurements of the flux density ratio at a few epochs during that monitoring period. The basic idea of the method is to reconstruct the individual image lightcurves from the observed total lightcurve by assuming a range of potential values for the time delay and the magnification ratio of the images. It is then possible to single out the correct reconstruction, and therefore determine the time delay, by checking the consistency of the reconstructed individual lightcurves with the additional interferometric observations. We performed extensive numerical simulations of synthetic lightcurves to investigate the dependence of the performance of this method on various parameters which are involved in the problem. Probably the most promising candidates for applying the method (and also for determining the Hubble constant) are lens systems consisting of multiply imaged compact sources and an Einstein ring, such as B0218+357 from which some of the parameters used for our simulations were adopted. 
Bernhard Geiger and Peter Schneider (Max-Planck-Institut fuer   Astrophysik)