Abstract
ABSTRACT We explore a novel search strategy for dark matter in the form of massive compact halo objects (MACHOs) such as primordial black holes or dense mini-haloes in the mass range from $10^{-4}\, \mathrm{M}_{\odot }$ to $0.1\, \mathrm{M}_{\odot }$. These objects can gravitationally lens the signal of fast radio bursts (FRBs), producing a characteristic interference pattern in the frequency spectrum, similar to the previously studied femtolensing signal in gamma-ray burst spectra. Unlike traditional searches using microlensing, FRB lensing will probe the abundance of MACHOs at cosmological distance scales (∼Gpc) rather than just their distribution in the neighbourhood of the Milky Way. The method is thus particularly relevant for dark mini-haloes, which may be inaccessible to microlensing due to their finite spatial extent or tidal disruption in galaxies. We find that the main complication in FRB lensing will be interstellar scintillation in the FRB’s host galaxy and in the Milky Way. Scintillation is difficult to quantify because it heavily depends on turbulence in the interstellar medium, which is poorly understood. We show that, nevertheless, for realistic scintillation parameters, FRB lensing can set competitive limits on compact dark matter object, and we back our findings with explicit simulations.
Highlights
Massive compact halo objects (MACHOs) have been widely discussed in the literature as possible dark matter candidates
We review the relevant properties of the interstellar medium (ISM) and intergalactic medium (IGM), and we analytically estimate the impact of scintillation on diffractive lensing of fast radio bursts (FRBs)
To anticipate the sensitivity of FRB lensing to compact dark matter objects, we investigate the significance of the lensing signal as a function of the model parameters, in particular the lens mass M, its position DL along the line of sight and angular distance β from the line of sight, as well as the diffractive scintillation scale rdiff in the Milky Way
Summary
Massive compact halo objects (MACHOs) have been widely discussed in the literature as possible dark matter candidates. On the lower end of this range, the PBH abundance is strongly constrained by non-observation of the γ -ray flux that would be produced by PBHs due to their Hawking evaporation (Carr et al 2010; Ballesteros, Coronado-Blazquez & Gaggero 2019; Arbey, Auffinger & Silk 2020), and by measurements of the cosmic microwave background (CMB) anisotropies that would be affected by Hawking radiation in the early Universe (Poulter et al 2019) (see Boudaud & Cirelli 2019; DeRocco & Graham 2019; Dasgupta, Laha & Ray 2019, for constraints from injection of positrons and neutrinos). A massive effort has been put into exploring the available parameter space of PBH dark matter, in particular via microlensing searches like MACHO (Allsman et al 2001), EROS (Tisserand et al 2007), OGLE (Wyrzykowski et al 2011), and most recently SUBARU-HSC (Niikura et al 2019). Big portions of MACHO parameter space remain unconstrained, while others are only loosely constrained and can still accommodate a sizable fraction of dark matter in the form of MACHOs
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