Constraints on primordial black holes from the observed number of Icarus-like ultrahigh magnification events

TL;DR

This paper uses the observed high-magnification event Icarus to constrain the abundance and mass of primordial black holes, finding that PBHs with around one solar mass cannot account for the observed events if they constitute a significant fraction of dark matter.

Contribution

It introduces a method to constrain primordial black hole properties using ultrahigh magnification events near macro-critical curves.

Findings

PBHs with ~1 solar mass and fraction >0.2 are excluded as primary microlenses.

Stars from intracluster light are consistent with observed events at 95% confidence.

The approach can be applied to future observations for tighter constraints.

Abstract

Icarus is an individual star observed near the macro-critical curve of the MACS J1149 cluster, with the magnification factor estimated to be an order of thousands. Since microlenses near the macro-critical curve influence the number of such high-magnification events, the observed occurrence of Icarus-like events is expected to provide a useful constraint on the properties of microlenses. We first study the mass and mass fraction of microlenses consistent with the observed number of events assuming a single microlens component with a monochromatic mass function, finding that stars that contribute to the intracluster light (ICL) are consistent at the 95% confidence level. We then consider the contribution of primordial black holes (PBHs), which are one of the alternatives to the standard cold dark matter, as microlenses in addition to ICL stars. The derived parameter space indicates that a large abundance of PBHs with a mass around $1\ M_{\odot}$ and a fraction of PBHs to the total dark matter of $f_{\rm PBH} \gtrsim 0.2$ cannot explain the observed number of Icarus-like events and therefore is excluded. The methodology developed in this paper can be used to place tighter constraints on the fraction of PBHs from ongoing and future observations of ultrahigh magnification events.