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 10-100Mrange whose existence has been confirmed by LIGO. The strong gravitational lensing of transients such as fast radio bursts (FRBs) and gamma-ray bursts has been suggested as a more sensitive probe for compact dark matter than intensity fluctuations observed in microlensing experiments. Recently the Australian Square Kilometre Array Pathfinder 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 similar to 130 FRBs would be required to constrain compact dark matter to less than the existing 35% limit with 95% confidence, if it were distributed along greater than or similar to 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 40 of allz less than or similar to 0.4 FRBs to exhibit microlensed burst structure. Approximately 170 FRBs intercepting halos within similar to 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.