Photometric, astrometric and polarimetric observations of gravitational microlensing events

Gravitational microlensing can be used as a unique astrophysical tool to study the atmospheres of stars thousands of parsec away from us. This capability results from the bending of light rays in the gravitational field of a lens that can magnify the light of a background source star during the lensing. Moreover, one of the consequences of this light bending is that the circular symmetry of the source is broken because distorted images are produced at either side of the lens position. This property makes it possible to observe polarization, and also the light centroid shift of images. Assigning vectors for these two parameters, they are perpendicular to each other in simple and binary microlensing events, except in fold singularities. In this work, we investigate the advantages of polarimetric and astrometric observations during microlensing events (i) for studying the surface of the source star and spots on it and (ii) for obtaining extra information to determine the trajectory of source stars with respect to the lens. Finally, we analyse the largest sample of microlensing events from the Optical Gravitational Lensing Experiment (OGLE) catalogue and show that, for almost similar to 4.3 per cent of events in the direction of the Galactic bulge, the polarization signals would be observable with large telescopes.