Rates for Color Shifted Microlensing Events

If the objects responsible for gravitational microlensing of Galactic-bulge stars are faint dwarfs, then blended light from the lens will distort the shape of the microlensing light curve and shift the color of the observed star during the microlensing event. In most cases, the resolution in current microlensing surveys is not accurate enough to observe this color-shift effect. However, such signatures could conceivably be detected with frequent followup observations of microlensing events in progress, providing the photometric errors are small enough. We calculate the expected rates for microlensing events where the shape distortions will be observable by such followup observations, assuming that the lenses are ordinary low-mass main-sequence stars in a rapidly rotating bar and in the disk. We adopt Galactic models consistent with observed microlensing timescale distributions, and consider separately the cases of self-lensing of the bulge, lensing of the bulge by the disk, and self-lensing of the disk, further differentiating between events where the source is a giant or a main-sequence star. We study the dependence of the rates for color-shifted microlensing events on the frequency of followup observations and on the precision of the photometry for a variety of waveband pairings. We find that for hourly observations in B and K with typical photometric errors of 0.01 mag, 28% of the events where a main-sequence bulge star is lensed, and 7% of the events where the source is a bulge giant, will give rise to a measurable color shift at the 95% confidence level. For observations in V and I, the fractions become 18% and 5%, respectively, [email protected] [email protected] [email protected]