Microlensing under Shear

TL;DR

This paper studies how shear affects microlensing light curves and centroid shifts, showing that shear causes measurable distortions that can help map Galactic mass distribution.

Contribution

It provides a detailed analysis of shear-induced distortions in microlensing signals and proposes methods to determine shear magnitude and direction from observations.

Findings

Shear increases light curve deviations and centroid shift distortions.

Maximum distortion timing depends mainly on impact parameter.

Galactic shear values are estimated to be between 10^{-6} and 10^{-4}.

Abstract

We investigate the distortions due to this shear in the microlensing light curves and in the astrometric 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 Paczynski 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. The magnitude of the maximum distortion depends on the magnitude and the direction of the shear. For a source trajectory in a given direction, the time of the maximum distortion depends mostly on the impact parameter and hardly on the shear. It is possible to determine the magnitude of the shear and its direction if both the time and the magnitude of the maximum astrometric distortion are measured. The magnitude of the shear produced by the Galactic bulge or a globular cluster falls in the range 10^{-6}--10^{-4} in normalized units. Although the actual determination of the shear from the Galactic sub-structures 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.