CAUSTIC-INDUCED FEATURES IN MICROLENSING MAGNIFICATION PROBABILITY-DISTRIBUTIONS

Our numerical simulations have uncovered a previously unrecognized "bump" in the macroimage magnification probabilities produced by a planar distribution of point masses. The result could be relevant to cases of microlensing by star fields in single galaxies, for which this lensing geometry is an excellent approximation. The bump is produced by bright pairs of microimages formed by sources lying near the caustics of the lens. The numerically calculated probabilities for the magnifications in the range 3 < A < 30 are significantly higher than those given by the asymptotic relation derived by Schneider, which itself is significantly above the low optical depth formula: p(A) = 2-sigma-A-3. The numerical probabilities are higher than the latter formula by factors as large as 1.7, 2.5, and 4, for sigma = 0.1, 0.2, and 0.3, respectively. The bump present in the two-dimensional lenses appears not to exist in the magnification probability distribution produced by a fully three-dimensional lens. The reason for this difference is not well understood by us.