Extended-Source Effect and Chromaticity in Two-Point-Mass Microlensing

TL;DR

This paper investigates how the extended nature of source stars affects two-point-mass gravitational microlensing, revealing expanded sensitive regions near cusps, deriving analytical formulas, and assessing implications for stellar limb darkening measurements.

Contribution

It introduces new analytical expressions for extended-source effects and chromaticity in binary microlensing, and analyzes the impact of source size and lens configuration on microlensing sensitivity.

Findings

Sensitive regions near cusps are significantly larger than previously thought.

Probability of extended-source effects exceeds caustic crossing likelihood by 40-60%.

Linear-fold caustic approximation can have errors up to a few percent.

Abstract

We explore the sensitivity of two-point-mass gravitational microlensing to the extended nature of the source star, as well as the related sensitivity to its limb darkening. We demonstrate that the sensitive region, usually considered to be limited to a source-diameter-wide band along the caustic, is strongly expanded near cusps, most prominently along their outer axis. In the case of multi-component caustics, facing cusps may form a region with a non-negligible extended-source effect spanning the gap between them. We demonstrate that for smaller sources the size of the sensitive region extending from a cusp measured in units of source radii increases, scaling as the inverse cube root of the radius. We study the extent of different sensitivity contours and show that for a microlensed Galactic bulge giant the probability of encountering at least a 1% extended-source effect is higher than the probability of caustic crossing by 40-60% when averaged over a typical range of lens-component separations, with the actual value depending on the mass ratio of the components. We derive analytical expressions for the extended-source effect and chromaticity for a source positioned off the caustic. These formulae are more generally applicable to any gravitational lens with a sufficiently small source. Using exactly computed amplifications we test the often used linear-fold caustic approximation and show that it may lead to errors on the level of a few percent even in near-ideal caustic-crossing events. Finally, we discuss several interesting cases of observed binary and planetary microlensing events and point out the importance of our results for the measurement of stellar limb darkening from microlensing light curves.