First Constraints on Compact Dark Matter from Fast Radio Burst
  Microstructure
  Despite existing constraints, it remains possible that up to $35\%$ of all
dark matter is comprised of compact objects, such as the black holes in the
10-100\,M$_\odot$ range whose existence has been confirmed by LIGO. The strong
gravitational lensing of transients such as FRBs and GRBs has been suggested as
a more sensitive probe for compact dark matter than intensity fluctuations
observed in microlensing experiments. Recently ASKAP has reported burst
substructure down to $15\mu$s timescales in FRBs in the redshift range
$0.3-0.5$. We investigate here the implications of this for the detectability
of compact dark matter by FRBs. We find that a sample size of $\sim130$ FRBs
would be required to constrain compact dark matter to less than the existing
35$\%$ limit with 95$\%$ confidence, if it were distributed along $\gtrsim
1\,$Gpc-long FRB sightlines through the cosmic web. Conversely, existing
constraints on the fraction of compact dark matter permit as many as 1 in
$\approx 40$ of all $z \lesssim 0.4$ FRBs to exhibit micro-lensed burst
structure. Approximately $170$ FRBs intercepting halos within $\sim 50\,$kpc
would be required to exclude the fraction of compact dark matter in each
intercepted halo to a similar level. Furthermore, we consider the cumulative
effects of lensing of the FRB signal by a macroscopic dark matter distribution.
We conclude that lensing from a uniform distribution of compact objects is
likely not observable, but suggest that FRBs may set meaningful limits on
power-law distributions of dark matter.
