Microlensing under shear

When a microlensing event occurs in highly dense star fields such as in the Galactic bulge or in a globular cluster, it is necessarily affected by the shear from the global distribution of mass near the lens star. We investigate the distortions due to this shear in the microlensing light curves and in the astrometrie microlensing centroid shift trajectories. As expected, the light curve deviation increases as the shear increases and the impact parameter decreases. Although the light curve in the presence of a small shear is similar to the simple Paczyński curve with a slightly smaller impact parameter, the detailed difference between the light curve with and without shear reflects the direction and the magnitude of the shear. The centroid shift trajectory also deviates from a simple ellipse in the presence of shear. The distortion of the centroid shift trajectory increases as the impact parameter decreases, and the shape of the trajectory becomes complicated when the impact parameter becomes small enough. It is possible to determine the magnitude of the shear and its direction if both the time and the magnitude of the maximum astrometrie distortion are measured. The magnitude of the shear produced by the Galactic bulge or a globular cluster falls in the range 10^-6 to 10 ^-4 in normalized units. Although the actual determination of the shear from the Galactic substructures will not be easy due to complications such as binary companion, future large-scale microlensing experiments may enable us to determine the shear in some high-amplification events, leading eventually to mapping the Galactic mass distribution.