2019MNRAS.487..908S

Here, we revisit the polarization signal in caustic-crossing binary microlensing and introduce maps of the polarization signals of the source star as a function of its position projected on the lens plane. The behaviour of these maps depends on the source size in comparison with the caustic. If the source size is 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 centre 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 0.2 per cent for early-type stars. While crossing the connection line between different parts of the caustic, the polarimetry curve has three close peaks, the middle one appearing when the source centre is on the connection line.