VARIABILITY OF MICROLENSING LIGHT CURVES .2. MAGNIFICATION FLUCTUATIONS, AUTOCORRELATION FUNCTIONS, AND APPLICATIONS TO QSO-2237+0305

Using the method of correlated deflection probabilities of microlensed light rays, we show how to determine the magnification auto-correlati on function and the magnification fluctuations in various microlensing situations. We compare our results with those obtained with other met hods, and investigate the practical limitations inherent in the ray-sh ooting method: finite resolution, and the problem of fair sampling of the source plane. We apply our method to the observed light curve of t he microlensed image A of Q2237+0305 and determine correlation times a nd magnification fluctuations. We conclude that it is possible to expl ain this light curve with low-mass lenses.