Cosmic radio background from primordial black holes at cosmic dawn

The presence of an extra radio background besides the cosmic microwave background has important implications for the observation of the 21-cm signal during the cosmic dark ages, cosmic dawn, and the epoch of reionization. The strong absorption trough found in the 21-cm global spectrum measured by the EDGES experiment, which has a much greater depth than the standard model prediction, has drawn great interest to this scenario, but more generally it is still of great interest to consider such a cosmic radio background in the early Universe. To be effective in affecting the 21-cm signal at early time, such a radio background must be produced by sources that can emit strong radio signals but a modest amount of x rays so that the gas is not heated up too early. We investigate the scenario where such a radio background is produced by the primordial black holes (PBHs). For PBHs with a single mass, we find that if the PBHs' abundance log(fPBH) (ratio of total PBH mass density to total matter density) and mass satisfy the relation log(fPBH)∼-1.8log(M•/M⊙)-3.5 for 1M⊙≲M• ≲300M⊙, and have jet emission, they can generate a cosmic radio background required for reproducing the 21-cm absorption signal seen by the EDGES. The accretion rate can be boosted if the PBHs are surrounded by dark matter halos, which permits a lower fPBH value to satisfy the EDGES observation. In the latter scenario, since the accretion rate can evolve rapidly during the cosmic dawn, the frequency (redshift), and depth of the absorption trough can determine the mass and abundance of the PBHs simultaneously. For absorption trough redshift ∼17 and depth ∼-500 mK, it corresponds to M•∼1.05M⊙ and fPBH∼1.5×10-4.