Lensing of Fast Radio Bursts as a Probe of Compact Dark Matter

Abstract

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 window. We show that strong gravitational lensing of extragalactic fast radio bursts (FRBs) by MACHOs of masses larger than ∼ 20 M would result in repeated FRBs with an observable time delay. Strong lensing of a FRB by a lens of mass ML induces two images, separated by a typical time delay ∼ few ×(ML/30 M ) milliseconds. Considering the expected FRB detection rate by upcoming experiments, such as CHIME, of 104 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 fDM of dark matter in MACHOs to fDM 0.08 for ML 20 M .Although observations indicate that dark matter accounts for a significant share of the energy density of our Universe [1], we do not know its composition. Longtime candidates to make up the dark matter are massive compact halo objects (MACHOs) [2]. They were originally proposed to be as light as 10 −7 M and as heavy as the first stars (∼ 10 3 M ) [3]. Over the years, different experiments have progressively constrained the fraction f DM of dark matter that can reside in MACHOs with a given mass, placing tight upper bounds over most of the vast range above. High-mass ( 100 M ) MACHOs, for example, are constrained by the fact that they would perturb wide stellar binaries in our Galaxy [4]. Meanwhile, lower-mass ( 20M ) MACHOs are effectively ruled out as the sole component of Galactic dark matter, as they would create artificial variability in stars, due to gravitational microlensing [5][6][7].However, there remains a window of masses between 20 and 100 M , where the constraints are weaker, and in which arguably all the cosmological dark matter could be in the form of MACHOs [6][7][8][9][10]. This is a particularly interesting window, as it has been recently argued in Ref.[11] that if primordial black holes (PBHs) [12,13] in the ∼ 30 M mass range are the constituents of dark matter, they form binaries in halos, coalesce, and emit observable gravitational waves, with an event rate consistent with the published LIGO detection [14].In this Letter we propose to use the strong lensing of fast radio bursts (FRBs) to probe MACHOs of masses 20 M , including PBHs, and either confirm that they make up the dark matter or close this window. FRBs are strong radio bursts with a very short duration, which makes them ideal as microlensing targets. Their temporal width is increased by the dispersion measure (DM), which measures the time delay of photons with different radio frequencies due to scattering by free electrons on their way to Earth. All detected FRBs to date possess high DMs, which yield burst widths of ∼ 1 − 10 ms [15][16][17][18][19][20][21][22][23][24]. These values of the DM are several times larger than the expected contribution from free electrons within the Milky Way ...

Cosmic microwave background limits on accreting primordial black holes

“…In the mean time, there are a number of other interesting astrophysical probes in that mass range. These include future measurements of the stochastic gravitational-wave background [56][57][58][59][60] and of the mass spectrum [61], redshift distribution [62], and orbital eccentricies [63] for future binary-black-hole mergers; lensing of fast radio bursts by PBHs [64]; pulsar timing [65,66]; radio/x-ray sources [67] or the cosmic infrared background [68]; the dynamics of compact stellar systems [54]; strong-lensing systems [69]; and perhaps clustering of GW events [70][71][72][73]. The conclusions of our work suggest that it will be important to pursue vigorously these alternative avenues.…”

Black hole mass function from gravitational wave measurements

“…[39]. While other methods have been proposed to constrain PBH dark matter in the stellar-mass range [81][82][83][84], some even based on GW measurements [75,85], we conclude that a simple examination of the mass spectrum is an efficient way of testing this scenario. We leave a more detailed investigation of this to future work [86].…”

Microlensing and dynamical constraints on primordial black hole dark matter with an extended mass function

“…[11] argues that there are significant uncertainties associated with modelling the complex physical processes involved. The strong gravitational lensing of extragalactic fast radio bursts will place tight limits on MACHOs in this mass range in the future [23].…”