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

Abstract

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