Lensing of Fast Radio Bursts as a Probe of Compact Dark Matter

The possibility that part of the dark matter is made of massive compact halo objects (MACHOs) remains poorly constrained over a wide range of masses, and especially in the 20-100 M window. We show that strong gravitational lensing of extragalactic fast radio bursts (FRBs) by MACHOs of masses larger than ∼20 M would result in repeated FRBs with an observable time delay. Strong lensing of a FRB by a lens of mass ML induces two images, separated by a typical time delay ∼few×(ML/30 M) msec. Considering the expected FRB detection rate by upcoming experiments, such as canadian hydrogen intensity mapping experiment (CHIME), of 104 FRBs per year, we should observe from tens to hundreds of repeated bursts yearly, if MACHOs in this window make up all the dark matter. A null search for echoes with just 104 FRBs would constrain the fraction fDM of dark matter in MACHOs to fDM0.08 for ML 20 M.