Gravitational microlensing constraints on primordial black holes by Euclid

Primordial black holes (PBHs) may form in the early stages of the Universe via the collapse of large density perturbations. Depending on the formation mechanism, PBHs may exist and populate today the galactic halos and have masses in a wide range, from about 10−14M⊙\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$10^{-14} \, M_{\odot }$\end{document} up to thousands, or more, of solar masses. Gravitational microlensing is the most robust and powerful method to constrain primordial black holes (PBHs), since it does not require that the lensing objects be directly visible. We calculate the optical depth and the rate of microlensing events caused by PBHs eventually distributed in the Milky Way halo, towards some selected directions of observation. Then we discuss the capability of Euclid, a space-based telescope which might perform microlensing observations at the end of its nominal mission, to probe the PBH populations in the Galactic halo.