A Continuous Galactic Line Source of Axions: The Remarkable Case of 23Na

Abstract

We argue that $^{23}$Na is a potentially significant source of galactic axions. For temperatures $\gtrsim 7 \times 10^8$K -- characteristic of carbon burning in the massive progenitors of supernovae and ONeMg white dwarfs -- the 440 keV first excited state of $^{23}$Na is thermally populated, with its repeated decays pumping stellar energy into escaping axions. Odd-A nuclear abundances are typically very low in high-temperature stellar environments (or absent entirely due to burn-up). $^{23}$Na is an exception: $\approx 0.1 M_\odot$ of the isotope is synthesized during carbon burning then maintained at $\approx 10^9$K for times ranging up to $6 \times 10^4$y. Using MESA simulations, a galactic model, and sampling over progenitor masses, locations, and evolutionary stages, we find a continuous flux at earth of $\langle φ_a \rangle \approx 22$/cm$^2$s for $g^\mathrm{eff}_{aNN} = 10^{-9}$. Some fraction of these axions convert to photons as they propagate through the galactic magnetic field, producing a distinctive 440 keV line $γ$ ray detectable by all-sky detectors like the Compton Spectrometer and Imager (COSI). Assuming a 1$μ$G galactic magnetic field and a sufficiently light axion mass, we find that COSI will be able to probe $| g_{aNN}^\mathrm{eff} g_{a γγ} | \gtrsim1.8 \times 10^{-22}$ GeV$^{-1}$ at $3σ$ after two years of surveying.

Figures (4)

• Figure 1: Mass fractions of C, O, Ne, and $^{23}$Na ($\times10$) at the start of C+C burning (top), at the end of C burning in the innermost zone, and at the end of C burning (bottom) for a 9$M_\odot$ SN progenitor.
• Figure 2: From left to right, the Monte Carlo axion flux probability distributions for SN and WD sources separately, and combined. The solid lines are the analytic fits (see text).
• Figure 3: As in Fig 2, but for the conversion $\gamma$s. The Monte Carlo results are shown with simple Gaussian fits (solid lines).
• Figure 4: Expected $3\sigma$ limit on the ALP coupling $|g_{aNN}^\mathrm{eff} g_{a \gamma \gamma}|$ from a COSI search for $^{23}$Na $a \rightarrow \gamma$ conversion, after two years of surveying (blue shaded). The other constraints shown (gray shaded) were obtained by combining leading limits on $g_{a \gamma \gamma}$, from axion-photon conversion in M82 and axion-induced polarization measurements in magnetic white dwarfs Ning:2024ekyBenabou:2025jcv, with the bound on $g_{aNN}^\mathrm{eff}$ obtained from isolated neutron star cooling Buschmann:2021juv.