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: $\approx 0.1M_{\odot }$ 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 $⟨{\varphi }_a⟩\approx 22$/cm${}^2$s for $g_{aNN}^{\mathrm{eff}}=10^{-9}$. Some fraction of these axions convert 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$\mu$G galactic magnetic field and a sufficiently light axion mass, we find that COSI will be able to probe $|g_{aNN}^{\mathrm{eff}}{g}_{a\gamma \gamma }|\gtrsim 1.8\times 10^{-22}$ GeV${}^{-1}$ at $3\sigma$ after two years of surveying.

6 pages, 4 figures; updated to reflect the published version

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• Axion-photon conversion in transient compact stars: Systematics, constraints, and opportunities