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.