Superradiant axion clouds around asteroid-mass primordial black holes

TL;DR

This paper explores how axion clouds formed around primordial black holes could produce observable X-ray and gamma-ray signals, potentially constraining dark matter models involving these objects and particles.

Contribution

It analyzes the formation and observational signatures of superradiant axion clouds around primordial black holes, including effects of axion self-interactions and potential detection prospects.

Findings

Axion clouds can produce non-relativistic axions in the 0.1 eV to 1 MeV range.

Current observations exclude certain parameter spaces for axion-photon coupling.

Future X-ray telescopes like Athena can further test these models.

Abstract

We analyze the dynamics and observational signatures of axion clouds formed via the superradiant instability around primordial black holes, focusing on the mass range $10^{14}-10^{18}$ kg where the latter may account for all the dark matter. We take into account the leading effects of axion self-interactions, showing that, even though these limit the number of axions produced within each cloud, a large number of superradiant axions become free of the black hole's gravitational potential and accumulate in the intergalactic medium or even in the host galaxy, depending on their escape velocity. This means that primordial black hole dark matter may lead to a sizeable astrophysical population of non-relativistic axions, with masses ranging from 0.1 eV to 1 MeV, depending on the primordial black hole mass and spin. We then show that if such axions couple to photons their contribution to the galactic and extragalactic background flux, mainly in the X-ray and gamma-ray band of the spectrum, is already beyond current observational limits for a large range of parameters that are, therefore, excluded. We finish by showing the prospects of the Athena X-ray telescope to further probe this co-existence of primordial black holes and axions.