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