Ch. Han et al., Observational evidence for the effect of amplification bias in gravitational microlensing experiments, ASTROPHYS J, 507(1), 1998, pp. 102-107
Recently Alard, Mao, & Guibert and Alard proposed to detect the shift of a
star's image centroid, delta x, as a method to identify the lensed source a
mong blended stars. Goldberg & Wozniak actually applied this method to the
OGLE-1 database and found that seven of 15 events showed significant centro
id shifts of delta x greater than or similar to 0." 2. The amount of centro
id shift has been estimated theoretically by Goldberg; however, he treated
the problem in general and did not apply it to a particular survey or field
and therefore based his estimate on simple toy model luminosity functions
(i.e., power laws). In this paper, we construct the expected distribution o
f delta x for Galactic bulge events based on the precise stellar luminosity
function observed by Holtzman et al. using the Hubble Space Telescope. The
ir luminosity function is complete up to M-I similar to 9.0 (M-V similar to
12), which corresponds to faint M-type stars. In our analysis we find that
regular blending cannot produce a large fraction of events with measurable
centroid shifts. By contrast, a significant fraction of events would have
measurable centroid shifts if they are affected by amplification-bias blend
ing. Therefore, the measurements of large centroid shifts for an important
fraction of microlensing events of Goldberg & Wozniak confirm the predictio
n of Han & Alard that a large fraction of Galactic bulge events are affecte
d by amplification-bias blending.