Possible evidence of axion stars in HSC and OGLE microlensing events

Abstract

Dark matter in the form of axions is expected to form axion stars. Such axion stars could be discovered by microlensing events. In particular, some candidate events reported by Subaru HSC and OGLE can be explained simultaneously if the axion stars with masses of the order of the Earth mass make up about ∼27−13+7 percent of dark matter. For QCD axions, this corresponds to the axion mass in the range 10−9−10−6 eV, which is consistent with the experimental constraints, as well as the cosmological anthropic window of parameters.