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Session 47 - Microlensing.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

[47.03] Rates for Color Shifted Microlensing Events

A. Buchalter, M. Kamionkowski, R. M. Rich (Columbia U.)

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 with good photometry of microlensing events in progress. 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 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 events where a main-sequence bulge star is lensed, and 7% of 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, but may be increased to 40% and 13% by improved photometric accuracy and increased sampling frequency. Unlike standard achromatic lensing events, color shifted events provide information on the lens mass, distance and velocity. We outline how these parameters can be obtained, and give examples of typical errors which may arise in the calculation. We briefly discuss other applications of such light-curve measurements, and address how color-shifted events may be distinguished from events where the source is blended with a binary companion.

This work was supported in part by the U. S. Department of Energy under contract DE-FG02-92ER40699, and by NASA LTSARP grant NAGW-2479.

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