Detecting stellar spots through polarimetry observations of microlensing events in caustic-crossing

TL;DR

This paper explores how gravitational microlensing can enhance the polarization signals of stellar spots, enabling their detection and characterization, especially magnetic fields, through polarimetric observations during caustic-crossing events.

Contribution

It demonstrates that polarimetry during microlensing caustic-crossing can reveal stellar spot properties, including magnetic fields, which are less accessible through photometry alone.

Findings

Polarimetric deviations are maximized when the spot is near the source edge during caustic entry.

Photometric deviations are largest when the spot is at the source center.

Polarimetry can determine the magnetic field of stellar spots.

Abstract

In this work, we investigate if gravitational microlensing can magnify the polarization signal of a stellar spot and make it be observable. A stellar spot on a source star of microlensing makes polarization signal through two channels of Zeeman effect and breaking circular symmetry of the source surface brightness due to its temperature contrast. We first explore the characteristics of perturbations in polarimetric microlensing during caustic-crossing of a binary lensing as follows: (a) The cooler spots over the Galactic bulge sources have the smaller contributions in the total flux, although they have stronger magnetic fields. (b) The maximum deviation in the polarimetry curve due to the spot happens when the spot is located near the source edge and the source spot is first entering the caustic whereas the maximum photometric deviation occurs for the spots located at the source center. (c) There is a (partial) degeneracy for indicating spot's size, its temperature contrast and its magnetic induction from the deviations in light or polarimetric curves. (d) If the time when the photometric deviation due to spot becomes zero (between positive and negative deviations) is inferred from microlensing light curves, we can indicate the magnification factor of the spot, characterizing the spot properties except its temperature contrast. The stellar spots alter the polarization degree as well as strongly change its orientation which gives some information about the spot position. Although, the photometry observations are more efficient in detecting stellar spots than the polarimetry ones, but polarimetry observations can specify the magnetic field of the source spots.