Variation of the stellar color in high-magnification and caustic-crossing microlensing events

Abstract: To a first approximation, the microlensing phenomenon is achromatic, and great advancement has been achieved in the interpretation of the achromatic signals, which among other achievements has led to the discovery and characterization of well above $100$ new exoplanets. At higher order accuracy in the observations, microlensing has a chromatic component (a color term) which has so far been much less explored. Here, we analyze the chromatic microlensing effect of $4$ different physical phenomena, which have the potential to add important new knowledge about the stellar properties not easily reachable with other methods of observations. Our simulation is limited to the case of main-sequence source stars. Microlensing is particularly sensitive to giant and sub-giant stars near the Galactic center. While this population can be studied in short snapshots by use of the largest telescopes in the world, a general monitoring and characterization of the population can be achieved by use of more accessible medium-sized telescopes with specialized equipments through dual-color monitoring from observatories at sites with excellent seeing. We quantify our results to what will be achievable from the Danish $1.54$m telescope at La Silla observatory by use of the existing dual-color lucky imaging camera. Such potential monitoring programs of the bulge population from medium-sized telescopes include the characterization of starspots, limb-darkening, the frequency of close-in giant planet companions, and gravity darkening for blended source stars. We conclude our simulations with quantifying the likelihood of detecting these different phenomena per object where they are present to be $\sim 60\%$ and $\sim 30\%$ for the mentioned phenomena, when monitored during high magnification and caustic crossings, respectively.