Optical Microlensing by Primordial Black Holes with IACTs

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 to 101 M . Especially for galactic stars, the finiteness of the sources needs to be taken into account. For low PBH masses (in this work . 10−8 M ) the average duration of the detectable event decreases with the mass 〈te〉 ∝ MPBH. For MPBH ≈ 10−11 M we find 〈te〉 . 1s. 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. Groundbased Imaging Atmospheric Cherenkov telescopes (IACTs) are optimized to detect the ∼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 ( 1 s) such as asteroid occultations. We investigate whether optical observations by IACTs can contribute to extending microlensing limits to the unconstrained mass range MPBH < 10−10M . 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.