Stimulated Axion Decay around Primordial Black Holes and the Diffuse Radio Background
We study stimulated axion decay in dense axion halos around primordial black holes (PBHs), assuming that both PBHs and axions contribute to dark matter. We construct the axion density profile by matching a hydrogenic soliton-like core dominated by the PBH potential to the $r^{-9/4}$ spike produced by secondary infall. We also impose an inner cutoff at the innermost stable circular orbit. We derive local Boltzmann equations for the number densities of photons and axions, including spontaneous and stimulated decay, inverse decay, photon escape, and axion filling. We solve these equations numerically and with a semi-analytic approximation. Stimulated emission occurs within an inner region where photon amplification overcomes escape. We calculate the photon emission from an individual PBH halo and integrate it over the population of PBHs throughout cosmic history. By requiring the resulting diffuse radio spectrum not to exceed the radio background measured by ARCADE~2, we derive upper bounds on the fraction of dark matter in PBHs. The bounds are strongest at intermediate PBH masses and apply over a wider mass range as the coupling between axions and photons increases.