Detecting stellar spots through polarimetry 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 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.

Accepted for publication in MNRAS, 10 pages, 7 figures, 1 table

References in corpus (5)

• OGLE-III Microlensing Events and the Structure of the Galactic Bulge
• EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars
• Adaptive Contouring -- an efficient way to calculate microlensing light curves of extended sources
• Measuring Polarization in microlensing events
• Binary astrometric microlensing with Gaia

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• Polarimetric microlensing of circumstellar disks
• Stellar rotation effects in polarimetric microlensing
• Non-radially pulsating stars as microlensing sources
• Polarimetry microlensing of close-in planetary systems
• Timing analysis in microlensing
• Identifying low-amplitude pulsating stars through microlensing observations
• Measuring Limb Darkening of Stars in high magnification Microlensing Events by the Finite Element Method
• Polarization in caustic-crossing binary microlensing events
• Variation of the stellar color in high-magnification and caustic-crossing microlensing events