Observed Bimodality of the Einstein Crossing Times of Galactic Microlensing Events

The OGLE data$^1$ for Einstein ring crossing times, $t_E$, for microlensing events toward the galactic bulge are analyzed. The analysis shows that the crossing times are bimodal, indicating that two populations of lenses could be responsible for observed microlensing events. Given the possibility that microlensing in this direction can be due to both main-sequence stars and white dwarfs, we analyze and show that the observed bimodality of $t_E$ can be derived from the accepted density distributions of both populations. Our Kolmogorov-Smirnov (KS) one sample test shows that that a white dwarf population of about $25% $ of all stars in the galaxy agrees well with the observed bimodality with a KS significance level greater than 97%.