Microlensing by a Cluster of Primordial Black Holes

Abstract

Numerous programs to search for microlensing events have constrained the possibility of the existence of dark matter (DM) in the form of compact objects in the halo of our Galaxy. These constraints on the DM fraction have been obtained under the assumption of single, widely spaced objects. We investigate the microlensing by primordial black holes (PBHs) gathered into clusters. In this case, during the microlensing by PBHs the influence of both neighboring PBHs and the combined gravitational potential from the entire cluster should be taken into account, making the shape of the light curve for a microlensing event much more complicated. Events with such complex light curves are not detected in the observational MACHO, EROS, OGLE, POINT-AGAPE, and HSC experiments. It has been shown that the fraction of DM in the form of PBHs (up to \(93\%\) for the models under study) is not recorded in these observational experiments. However, a significant fraction of PBHs that may be considered as single lenses remains for all models; therefore, PBH clustering does not allow the constraint from microlensing on the fraction of PBHs in DM to be completely lifted.