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

We argue that ^{23}Na is a potentially significant source of galactic axions. For temperatures ≳7×10^{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: ≈0.1M_{⊙} of the isotope is synthesized during carbon burning then maintained at ≈10^{9}  K for times ranging up to 6×10^{4}  yr. Using MESA simulations, a galactic model, and sampling over progenitor masses, locations, and evolutionary stages, we find a continuous flux at Earth of ⟨ϕ_{a}⟩≈22/cm^{2} s for g_{aNN}^{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 γ-ray detectable by all-sky detectors like the Compton Spectrometer and Imager (COSI). Assuming a 1  μG galactic magnetic field and a sufficiently light axion mass, we find that COSI will be able to probe |g_{aNN}^{eff}g_{aγγ}|≳1.8×10^{-22}  GeV^{-1} at 3σ after two years of surveying.