Microlensing by Cluster of Primordial Black Holes

Abstract

Numerous microlensing survey programs have constrained the possibility of dark matter existing in the form of compact objects within the Galactic halo. These constraints on the dark matter fraction were derived under the assumption of isolated, widely separated objects. This work investigates microlensing by primordial black holes (PBHs) organized into clusters. In this scenario, it is necessary to account for both the influence of neighboring PBHs and the collective gravitational potential of the entire cluster, which significantly complicates the microlensing light curve. Events exhibiting such complex light curves elude detection in observational experiments such as MACHO, EROS, OGLE, POINT-AGAPE, and HSC. It is demonstrated that a significant fraction of PBH dark matter (up to 93\% for the models studied) remains undetected in these observational data. However, for all considered cluster models, a substantial population of PBHs still behaves as isolated lenses. Consequently, the clustering of PBHs does not completely eliminate the microlensing constraints on the PBH contribution to dark matter.

Figures (3)

• Figure 1: Parameters of the PBH cluster models.
• Figure 2: Illustration of the magnification map at $r = 0.014$ pc from the cluster center (for model M1) with four sample trajectories of background sources (left). Light curves of the background sources along the corresponding trajectories featuring microlensing events (right). The microlensing maxima (peaks) are marked with green diamonds. The results of fitting the light curve peaks with the PLPS profile are shown by red dashed lines.
• Figure 3: The radial dependence of the $\chi^2_{d.o.f}$ goodness-of-fit criterion for the PBH cluster models. The black dashed line at $\chi_{crit}^2\xspace = 3.2$ separates PLPS events (to the left) from no-PLPS events (to the right). As the distance from the cluster center increases, the fraction of no-PLPS events decreases