VARIABILITY OF MICROLENSING LIGHT CURVES .1. AUTOCORRELATION METHOD AND THE CALCULATION OF THE CORRELATED DEFLECTION PROBABILITY

The recently observed microlensing variations in the gravitational len s system 2237+0305 has renewed the interest in gravitational microlens ing. Most theoretical investigations of microlensing are based on a nu merical scheme called ray-shooting. We present here a new method for c alculating the autocorrelation function of microlensing light curves a nd the magnification fluctuations, following in part a method presente d by Deguchi & Watson (1987). In the present paper, we outline the met hod and investigate in detail its major ingredient, the probability di stribution for the difference of the deflection angles of two light ra ys with fixed separation in the lens plane, the so-called correlated d eflection probability. The application to the calculation of autocorre lation functions and magnification fluctuations, in particular the app lication to the system 2237+0305, is presented in a forthcoming paper.