Lensing Constraints on PBHs: Substellar to Intermediate Masses

Abstract

Gravitational microlensing is a robust tool to detect and directly measure the abundance and mass of any kind of compact objects, either in our galaxy or in the extragalatic domain. On basis to generic, broadly applicable arguments, it is concluded that the observed microlensing magnifications are too small and the microlensing events less frequent than the expectations for a significant population of compact objects (other than normal stars). The detection of chromatic effects of microlensing, neither supports the presence of BHs. Detailed statistical studies of the observed microlensing magnifications and events frequency impose strict upper limits to the fraction of total mass of BHs ($\ltsim$ 1\%) from $10^{-7}M_\odot$ to indefinitely large masses. These results hold even when the BHs are distributed according to a mass spectrum or are forming clusters.