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Primordial black holes (PBHs), hypothesized to be the result of densityfluctuations during the early universe, are candidates for dark matter. Whenmicrolensing background stars, they cause a transient apparent enhancement ofthe flux. Measuring these signals with optical telescopes is a powerful methodto constrain the PBH abundance in the range of $10^{-10}\,M_{\odot}$ to$10^{1}\,M_{\odot}$. Especially for galactic stars, the finiteness of thesources needs to be taken into account. For low PBH masses (in this work$\lesssim 10^{-8}\,M_{\odot}$) the average duration of the detectable eventdecreases 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 berequired as they could enable the detection of low mass PBHs. Current limitsare set with sampling speeds of 2 minutes to 24 hours in the optical regime.Ground-based Imaging Atmospheric Cherenkov telescopes (IACTs) are optimized todetect the $\sim$ns long optical Cherenkov signals induced by atmospheric airshowers. As shown recently, the very-large mirror area of these instrumentsprovides very high signal to noise ratio for fast optical transients ($\ll1\,$s) such as asteroid occultations. We investigate whether opticalobservations by IACTs can contribute to extending microlensing limits to theunconstrained mass range $M_{\mathrm{PBH}}<10^{-10}M_\odot$. We discuss thelimiting factors to perform these searches for each telescope type. Wecalculate the rate of expected detectable microlensing events in the relevantmass range for the current and next-generation IACTs considering realisticsource parameters.

Optical Microlensing by Primordial Black Holes with IACTs