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Session 83 - Gravitational Lensing.
Display session, Friday, January 09
Exhibit Hall,
In this poster, we present the prospect that astrometric shifts can be used to identify blended microlensing events in crowded fields. Moreover, by measuring an astrometric shift, one can determine the position of the true lensed star with respect to the local field with very high precision. We first perform several simulations of microlensing searches in crowded fields and find that if we assume a dark lens, and that the lensed star obeys a power law luminosity function, n(L)\propto L^-\beta, over half the simulated events show a measurable astrometric shift. For simulations of 20000 stars on a 256\times 256 Nyquist sampled CCD frame, we found that with \beta=2, 58% of the events were significantly blended (F_\ast/F_tot\leq 0.9), and of those, 73% had a large astrometric shift (\geq 0.5 pixels). For \beta=3, we found that 85% were significantly blended, and that 85% of those had a significant shift. Since we expect most blended events to show a significant shift, we look in the OGLE I database (Wo\'zniak amp; Szyma\'nski 1997), and find measurable and systematic shifts in over half the candidate microlensing events, including OGLE # 5, which was considered to be blended from photometric data.
