Solar axion flux from the axion-electron coupling

TL;DR

The paper develops a framework to compute the solar axion flux from the axion–electron coupling by relating axion emission to photon opacity via detailed balance, allowing the use of monochromatic opacity data from OP, LEDCOP, and OPAS. It shows that the ABC flux is ∼$30\%$ larger than previous estimates due to including atomic recombination (FB) and deexcitation (BB) contributions, with the flux peaking at $ω\sim$ keV and robust cross-code agreement across opacity datasets. The approach generalizes to axion-like particles and other WISPs, enabling refined predictions for helioscope experiments such as IAXO and impacting current CAST limits. Overall, the work provides a practical, data-driven method to quantify stellar axion production channels tied to electron couplings and improves the reliability of solar-axion flux calculations.

Abstract

In non-hadronic axion models, where axions couple to electrons at tree level, the solar axion flux is completely dominated by the ABC reactions (Atomic recombination and deexcitation, Bremsstrahlung and Compton). In this paper the ABC flux is computed from available libraries of monochromatic photon radiative opacities (OP, LEDCOP and OPAS) by exploiting the relations between axion and photon emission cross sections. These results turn to be ~ 30% larger than previous estimates due to atomic recombination (free-bound electron transitions) and deexcitation (bound-bound), which where not previously taken into account.

Figures (8)

• Figure 1: ABC reactions responsible for the solar axion flux in non-hadronic axion models.
• Figure 2: Flux of solar axions due to ABC reactions driven by the axion-electron coupling (for $g_{ae}=10^{-13}$). The different contributions are shown as red lines: Atomic recombination and deexcitation (FB+BB, solid), Bremsstrahlung (FF, dot-dashed) and Compton (dashed). The Primakoff flux from the axion-photon coupling is shown for comparison using $g_{a\gamma}=10^{-12}$, a typical value for meV axions having $g_{ae}=10^{-13}$. Note that has been scaled up by a factor 50 to make it visible.
• Figure 3: Solar model (black line) and points of temperature and electron density $n_e$ where monochromatic opacities from the OP are available.
• Figure 4: Solar axion flux ($g_{ae}=10^{-13}$) from reactions that involve all cosmologically abundant metals ($Z\geq 6$, upper black) and three groups that account for most of the flux: CNO, NeMgSiS and Fe. The sum of the latter adds up to the think gray line which tracks to a good approximation the full result.
• Figure 5: Solar axion flux ($g_{ae}=10^{-13}$) from reactions that involve CNO (left) and NeMgSiS (right)