Picolensing as a Probe of Primordial Black Hole Dark Matter

The gravitational-lensing parallax of gamma-ray bursts (GRBs) is an intriguing probe of primordial black hole (PBH) dark matter in the asteroid-mass window, $2\times 10^{-16}M_{\odot} \lesssim M_{\text{PBH}} \lesssim 5 \times 10^{-12}M_{\odot}$. Recent work in the literature has shown exciting potential reach for this "picolensing" signal if a future space mission were to fly two x-/$\gamma$-ray detectors in the Swift/BAT class, with inter-spacecraft separation baselines on the order of the Earth-Moon distance. We revisit these projections with a view to understanding their robustness to various uncertainties related to GRBs. Most importantly, we investigate the impact of uncertainties in observed GRB angular sizes on reach projections for a future mission. Overall, we confirm that picolensing shows great promise to explore the asteroid-mass window; however, we find that previous studies may have been too optimistic with regard to the baselines required. Detector baselines on the order of at least the Earth-L2 distance would make such a mission more robust to GRB size uncertainties; baselines on the order of an astronomical unit (AU) would additionally enable reach that equals or exceeds existing microlensing constraints up to $M_{\text{PBH}} \sim 2 \times 10^{-8} M_{\odot}$.