Constraints on the Abundance of PBHs from X-ray Quasar Microlensing Observations: Substellar to Planetary Mass Range

TL;DR

This study uses X-ray quasar microlensing data to constrain the abundance of low-mass black holes, setting new upper limits on their contribution to dark matter across substellar to planetary mass ranges.

Contribution

It provides the first constraints on planetary-mass black holes using X-ray microlensing and compares these with optical data to refine dark matter composition limits.

Findings

Negligible BH contribution in substellar mass range.

Planetary-mass BHs could evade detection in X-ray microlensing.

New upper limits on BHs as dark matter candidates.

Abstract

We use X-ray observations of quasar microlensing (sensitive to smaller compact objects than in the optical) to study the possible presence of a population of low mass black holes (from $\sim$ $10^{-3}M_{\odot}$ to $10^{-1}M_{\odot}$) in lens galaxies. We compare these observations with microlensing magnification simulations of a mixed population of stars and black holes (BHs) plus a smooth matter component. We estimate the individual mass fractions of both, stars and BHs, for three different BH masses in the range of substellar to planetary masses. Our Bayesian analysis indicates that the contribution of BHs is negligible in the substellar mass range but that a population of BHs of planetary mass (M $\lesssim$ $10^{-3}M_{\odot}$) could pass unnoticed to X-ray microlensing. We provide new upper limits to the contribution of BHs to the fraction of dark matter based on both, the quasar microlensing data in the X-ray band, and our previous estimates in the optical of intermediate-mass BHs with an additional upper limit at $M=3M_{\odot}$.