Chromatic and spectroscopic signatures of microlensing events as a tool for the gravitational imaging of stars

The detection of microlensing events from stars in the Large Magellanic Cloud and in the Galactic bulge raise important constraints on the distribution of dark matter and on galactic structure, although some events may be due to a new type of intrinsic variability. When lenses are relatively close to the sources, we predict that chromatic and spectroscopic effects are likely to appear for a significant fraction of the microlensing events. These effects are due to the differential amplification of the limb and the centre of the stellar disc, and present a systematic dependence with wavelength and time that provide an unambiguous signature of a microlensing event (as opposed to a new type of intrinsic stellar variability). In addition, their measure would provide a direct constraint on stellar atmospheres, allowing a 3-dimensional reconstruction or imaging of its structure, a unique tool to test the current models of stellar atmospheres.