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

We argue that ^23Na is a potentially significant source of galactic axions. For temperatures 710^8 K---characteristic of carbon burning in the massive progenitors of supernovae and ONeMg white dwarfs---the 440 keV first excited state of ^23Na 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). ^23Na is an exception: 0. 1M_ of the isotope is synthesized during carbon burning then maintained at 10^9 K for times ranging up to 610^4 yr. Using MESA simulations, a galactic model, and sampling over progenitor masses, locations, and evolutionary stages, we find a continuous flux at Earth of ⟨₀⟩22/cm^2 s for g₀₍₍^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 -ray detectable by all-sky detectors like the Compton Spectrometer and Imager (COSI). Assuming a 1 galactic magnetic field and a sufficiently light axion mass, we find that COSI will be able to probe |g₀₍₍^effg₀|1. 810^-22 GeV^-1 at 3 after two years of surveying.