# Polarization in caustic-crossing binary microlensing events

## TL;DR

This paper analyzes polarization signals in caustic-crossing binary microlensing events, revealing how source size and caustic structure influence polarization behavior and peak signals, with implications for observational strategies.

## Contribution

It introduces detailed polarization maps as functions of source position and explores polarization behavior across different caustic configurations, enhancing understanding of microlensing polarization signals.

## Key findings

- Maximum polarization occurs at source edges crossing caustic folds.
- Polarization peaks form around the caustic structures, varying with source size.
- Polarization signals can reach up to 0.2% for early-type stars.

## Abstract

Here, we revisit the polarization signal in caustic-crossing binary microlensing and introduce the maps of the polarization signals of the source star as a function of its position projected on the lens plane. The behavior of these maps depends on the source size in comparison with the caustic. If the source size is so smaller than the caustic curve, the maximum polarization signal occurs while the source edge is crossing the nearest folds to the position of the primary. When the source size is larger than the caustic, the polarization signal maximizes over a circular ring around the primary whose radius normalized to the source radius is ~0.96. In cusp caustic crossings, the polarization curves have three peaks, the largest and widest one happens when the source edge is on the corner of the cusp and the source center is inside the caustic curve. When the source is entirely inside the caustic, the polarization signal significantly decreases. Despite the low magnification factor between two parts of planetary caustics, its polarization signal is considerable and can reach to 0.2\% for early-type stars. While crossing the connection line between different parts of caustic, the polarimetry curve have three close peaks that the middle one appears when the source center is upon the connection line.