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

N3AS-24-014

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

W. C. Haxton, Xing Liu, Annie McCutcheon, Anupam Ray.
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Abstract

We argue that ^{23}Na is a potentially significant source of galactic axions. For temperatures \gtrsim 7 \times 10^8K — 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 10^9K for times ranging up to 6 \times 10^4y. Using MESA simulations, a galactic model, and sampling over progenitor masses, locations, and evolutionary stages, we find a continuous flux at earth of \langle \phi_a \rangle \approx 22/cm^2s 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 \gamma ray detectable by all-sky detectors like the Compton Spectrometer and Imager (COSI). Assuming a 1\muG 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} | \gtrsim1.8 \times 10^{-22} GeV^{-1} at 3\sigma after two years of surveying.

Associated Fellows