We argue that ${}^{23}\mathrm{Na}$ is a potentially significant source of galactic axions. For temperatures $\gtrsim 7\times {10}^8\mathrm{K}$—characteristic of carbon burning in the massive progenitors of supernovae and ONeMg white dwarfs—the 440 keV first excited state of ${}^{23}\mathrm{Na}$ 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). ${}^{23}\mathrm{Na}$ is an exception: $\approx 0.1M_{\odot }$ of the isotope is synthesized during carbon burning then maintained at $\approx {10}^9\mathrm{K}$ for times ranging up to $6\times {10}^4\mathrm{y}r$. Using MESA simulations, a galactic model, and sampling over progenitor masses, locations, and evolutionary stages, we find a continuous flux at Earth of $⟨{\varphi }_a⟩\approx 22/{\mathrm{cm}}^2\mathrm{s}$ for $g_{aNN}^{\mathrm{eff}}={10}^{-9}$. Some fraction of these axions converts to photons as they propagate through the galactic magnetic field, producing a distinctive 440 keV line $\gamma$-ray detectable by all-sky detectors like the Compton Spectrometer and Imager (COSI). Assuming a $1\mathrm{\mu }\mathrm{G}$ galactic magnetic field and a sufficiently light axion mass, we find that COSI will be able to probe $|g_{aNN}^{\mathrm{eff}}{g}_{a\gamma \gamma }|\gtrsim 1.8\times {10}^{-22}{\mathrm{GeV}}^{-1}$ at $3\sigma$ after two years of surveying.

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• Dark matter
• Formation & evolution of stars & galaxies
• Gamma ray astronomy
• Novae & supernovae
• Nucleosynthesis in explosive environments
• Particle dark matter