Gravitational microlensing - The effect of a single lens and the impact of sparse sampling

We try to clarify some topics in gravitational microlensing. One issue is that even in cases in which the surface mass density is quite low (κ ~0.2), microlensing events cannot be considered as caused by individual stars. Whenever a high-magnification event occurs, the quasar has crossed a caustic. These caustics, however, are complex structures even for low surface mass densities. We illustrate this by removing a single low-mass star from an ensemble of microlenses. We compare the magnification patterns and light curves with and without this star. The differences are striking and much more complex than expected. Another issue we investigate is the effect of sparse sampling on the perception of microlensed light curves. We show the influence of poor time coverage on simulated light curves. We demonstrate how important it is to set up a regular monitoring program for microlensing candidates (especially 2237 + 0305) with a dedicated telescope, so that one can make observations in different wavebands each night.