Blending in Future Space-based Microlensing Surveys

We investigate the effect of blending in future gravitational microlensing surveys by carrying out simulations of Galactic bulge microlensing events to be detected by a proposed space-based lensing survey. From this simulation, we find that the contribution of the flux from background stars to the total blended flux would be equivalent to that from the lens itself despite the greatly improved resolution of space-based observations, implying that characterizing lenses from the analysis of the blended flux would not be easy. As a method to isolate events for which most of the blended flux can be attributed to the lens, we propose using astrometric information of source star image centroid motion. For the sample of events obtained by imposing a criterion that the centroid shift should be less than 3 times the astrometric uncertainty among the events for which blending is noticed with blended light fractions f_B>0.2, we estimate that the contamination of the blended flux by background stars would be less than 20% for most (~90%) of the sample events. The expected rate of these events is >~700 events yr^-1, which is large enough for the statistical analysis of the lens populations.