First Constraints on Compact Dark Matter from Fast Radio Burst Microstructure

Abstract

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 ⊙ range 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 μs 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 ∼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 ≳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 all z ≲ 0.4 FRBs to exhibit microlensed burst structure. Approximately 170 FRBs intercepting halos within ∼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.

Fast Radio Burst Morphology in the First CHIME/FRB Catalog

“…FRBs can potentially be observably gravitationally lensed and probe dark matter distributions in the Universe (e.g., Muñoz et al 2016;Sammons et al 2020). Whether wave effects are directly visible in the burst morphology of an FRB depends on the Fresnel scale of the lensing scenario, which in turn depends on the physical parameters of the lens and the observing frequency.…”

Probing the Universe with Fast Radio Bursts

“…FRBs have temporal profiles that range from tens of microseconds to several milliseconds, which is shorter than the anticipated delay (∼1 ms) caused by multi-path propagation due to the gravitational lensing of compact objects (e.g., 10-100 M blackholes) [194]. Extremely high time resolution studies of FRBs and detection of few-microsecond temporal structure such as for FRB 181112 [195] could potentially detect multiple burst copies generated as a consequence of lensing [194]. Methods to distinguish copies of a burst through a lensing event have been developed and applied to FRB data by Farah et al [71] and Cho et al [195], based on the expectation of spatial coherence of the emission, which will be manifested as correlation in the voltages between pulses.…”

“…FRBs have temporal profiles that range from tens of microseconds to several milliseconds, which is shorter than the anticipated delay (∼1 ms) caused by multi-path propagation due to the gravitational lensing of compact objects (e.g., 10-100 M blackholes) [194]. Extremely high time resolution studies of FRBs and detection of few-microsecond temporal structure such as for FRB 181112 [195] could potentially detect multiple burst copies generated as a consequence of lensing [194].…”

“…The current small sample of ASKAP localised FRBs have started to provide an observational evidence of the Macquart-relation [13] and unique extragalatic sightlines to study intervening halos [113] and the cosmic web [130]. The high-time resolution analysis of ASKAP FRBs reporting burst substructure down to ∼15 µs timescales in the redshift range 0.3-0.5 has also provided first constraints on compact dark matter [194]. Thus, the potential of FRBs as probes of cosmology has begun to be realised.…”

The physics of fast radio bursts

“…They concluded that f DM in MACHO is constrained to < 0.001 when there were no search of multiplepeaked FRBs for time intervals larger than 1 ms and flux ratio less than 10 3 . These methods of using FRBs to find dark matter in MACHO is widely studied later [261][262][263][264][265].…”