Line Axion Emission from Supernova Progenitors

During the late stages of the evolution of a supernova progenitor, up to 0.1 solar masses of 23Na can be synthesized in the star's neon zone. 23Na has a strong M1 transition to its first excited state at 440 keV. At a temperature of about a billion degrees, the excited state has a significant population, so that its decay become a source of mono-energetic axions. For interestiong axion couplings to nucleons, a single progenitor at the center of the Milky Way would produce an axion flux at earth of about 1 axion/cm2/s. As the axion passes through the stellar and then galactic magnetic fields, it can convert to a monoenergetic photon. We discuss the limits that can be placed on axion couplings using instruments like the Compton Spectrometer and Imager, scheduled for launch in 2027.