Determine whether the axion is sourced for QCD axion models near the standard mass

Determine whether the axion is sourced inside neutron stars for the largest values of the light-QCD-axion parameter ε, particularly ε≈1, where the dense-matter calculation becomes non-perturbative or otherwise loses calculability.

Background

The authors analyze a parameter ε that controls the light-QCD-axion mass. For sufficiently small ε, the axion is sourced throughout the star in their free-Fermi-gas treatment, while for larger ε the sourcing behavior depends sensitively on nuclear interactions and the equation of state.

At ε≈1, corresponding to the largest values considered and close to the standard QCD axion mass relation, the authors state that it is speculative whether sourcing occurs at all because the theoretical description ceases to be calculable at densities below those required for sourcing. This uncertainty is distinct from, but related to, the N=1 sourcing question discussed later.

References

For the largest values, $\epsilon \simeq 1$, it remains a point of speculation if the axion gets sourced inside NSs (see e.g. ), as calculability breaks down at densities before the sourcing happens.

— Axion Hair and Pulsar Electrodynamics: modelling, discharge dynamics, and particle-in-cell simulations  (2609.08840 - Witte et al., 8 Sep 2026) in Section 2, “The sourcing of axion hair by neutron stars”