A microlensing model for QSO 2237 + 0305

We present models of microlensing for the four images of QSO 2237+0305, with the surface mass density and shear for each image based on the macrolensing model developed by Schneider et al. Our computations were done with a new code that makes use of the hierarchical tree method. The high efficiency of this approach allowed us to include up to 60,000 lensing stars with different masses. We present examples of amplification maps, light curves, correlation functions, and amplification probabilities. All these are very strongly affected by the shear due to macrolensing. The observed changes in the luminosity of the four images of QSO 2237+0305 are most likely induced by microlensing.