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 $20-100\, M_\odot$ window. We show that strong gravitational
lensing of extragalactic fast radio bursts (FRBs) by MACHOs of masses larger
than $\sim20\,M_\odot$ would result in repeated FRBs with an observable time
delay. Strong lensing of an FRB by a lens of mass $M_L$ induces two images,
separated by a typical time delay $\sim$ few $\times(M_L/30\, M_\odot)$
milliseconds. Considering the expected FRB detection rate by upcoming
experiments, such as CHIME, of $10^4$ 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 $10^4$ FRBs, would
constrain the fraction $f_{\rm DM}$ of dark matter in MACHOs to $f_{\rm
DM}\lesssim 0.08$ for $M_L\gtrsim 20\,M_\odot$.
