AAS 207th Meeting, 8-12 January 2006
Session 142 Gravitational Lensing
Poster, Wednesday, 9:20am-6:30pm, January 11, 2006, Exhibit Hall

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[142.10] Binary Microlenses: The Search for Missing Events

M E Schwamb (University of Pennsylvania), C Night, R Di Stefano (Harvard-Smithsonian Center for Astrophysics)

Gravitational microlensing has been used to probe the Galactic Halo for evidence of dark matter in the form of MAssive Compact Halo Objects, or MACHOs. The majority of the events detected by the observing teams appear to have been produced by point-lenses.A small fraction of the observed events have, however, clearly been produced by lenses which are binaries. Most of the associated light curves exhibit caustic crossings. Yet, among binary-lens light curves, caustic crossings are unusual. Other perturbations should be more common.

To better quantify theoretical predictions, we conducted simulations in which the lenses were binaries. We sampled light curves produced by various source-lens configurations for varying binary mass ratios and orbital separations. For each light curve with a peak magnification above a well-defined threshold we computed a chi-squared fit to a point-lens model, and kept track of this information and other light curve characteristics as well. We were therefore able to compute upper limits for the relative fraction of caustic crossings.

The high rate of detected caustic crossings is inconsistent with our simulations.The number of non-caustic-crossing events showing clear evidence of binarity should be larger than observed. For example, more double peaked and triple peaked events should have been detected than have been found so far in the current datasets. We explore several ways in which this mystery may be resolved. This research was supported by the NSF REU Program at SAO.

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