Detecting Stellar Spots by Gravitational Microlensing

During microlensing events with a small impact parameter the amplification of the source flux is sensitive to the surface brightness distribution of the source star. Such events provide a means for studying the surface structure of target stars in the ongoing microlensing surveys, most efficiently for giants in the Galactic Bulge. In this work we demonstrate the sensitivity of point-mass microlensing to small spots with radii $r_s\lesssim0.2$ source radii. We compute the amplification deviation from the light curve of a spotless source and explore its dependence on lensing and spot parameters. During source-transit events spots can cause deviations larger than 2\%, and thus be in principle detectable. Maximum relative deviation usually occurs when the lens directly crosses the spot. Its numerical value for a dark spot with sufficient contrast is found to be roughly equal to the fractional radius of the spot, i.e. up to 20\% in this study. Spots can also be efficiently detected by the changes in sensitive spectral lines during the event. Notably, the presence of a spot can mimic the effect of a low-mass companion of the lens in some events.