On the Prospects for Microlensing WIMPy Halos

Abstract. The kinetic decoupling of thermal dark matter candidates sets a model dependent mass scale below which structure formation is heavily suppressed. Probing the population of dark sub-halos therefore provides invaluable information about the particle nature of dark matter. Due to their elusive nature, however, the abun- dance of small-scale dark halos remains unknown to this day. We investigate the prospects for their detection using gravitational microlensing, a technique utilizing the temporary, apparent increase in the brightness of a distant star arising from the passage of a massive intermediate object. It is found that while one can put strong bounds on highly concentrated NFW-profiled halos, there is a minimal con- centration needed for NFW-halos to produce observable microlensing effects. This minimal concentration is related to an exponential shrinkage of the Einstein ring in lensing geometries only sensitive to the inner 1/r profile of the halo. We conclude that WIMPy halos nearby are likely never to be constrained by microlensing. At cosmological scales, microlensing of WIMPy halos could produce observable effects. Strong constraints from cosmological microlensing of WIMPy halos is doubted, but remains a possibility and should be investigated further.