Optical Microlensing by Primordial Black Holes with IACTs

TL;DR

This paper explores the potential of using Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect microlensing events caused by primordial black holes in a low mass range, which could extend current dark matter constraints.

Contribution

It proposes a novel application of IACTs for low-mass PBH microlensing detection and evaluates their potential effectiveness with current and future telescopes.

Findings

IACTs can detect microlensing events caused by PBHs with masses below 10^{-10} M_sun.

Fast sampling detectors are crucial for detecting low-mass PBHs with short event durations.

Expected event rates suggest IACTs could significantly extend PBH dark matter constraints.

Abstract

Primordial black holes (PBHs), hypothesized to be the result of density fluctuations during the early universe, are candidates for dark matter. When microlensing background stars, they cause a transient apparent enhancement of the flux. Measuring these signals with optical telescopes is a powerful method to constrain the PBH abundance in the range of $10^{-10}\,M_{\odot}$ to $10^{1}\,M_{\odot}$. Especially for galactic stars, the finiteness of the sources needs to be taken into account. For low PBH masses (in this work $\lesssim 10^{-8}\,M_{\odot}$) the average duration of the detectable event decreases with the mass $\langle t_e\rangle \propto M_{\mathrm{PBH}}$. For $M_{\mathrm{PBH}}\approx 10^{-11}\,M_{\odot}$ we find $\langle t_e\rangle \lesssim\,1 \mathrm{s}$. For this reason, fast sampling detectors may be required as they could enable the detection of low mass PBHs. Current limits are set with sampling speeds of 2 minutes to 24 hours in the optical regime. Ground-based Imaging Atmospheric Cherenkov telescopes (IACTs) are optimized to detect the $\sim$ns long optical Cherenkov signals induced by atmospheric air showers. As shown recently, the very-large mirror area of these instruments provides very high signal to noise ratio for fast optical transients ($\ll 1\,$s) such as asteroid occultations. We investigate whether optical observations by IACTs can contribute to extending microlensing limits to the unconstrained mass range $M_{\mathrm{PBH}}<10^{-10}M_\odot$. We discuss the limiting factors to perform these searches for each telescope type. We calculate the rate of expected detectable microlensing events in the relevant mass range for the current and next-generation IACTs considering realistic source parameters.