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Probing the axion-nucleon coupling with supergiant stars

Francisco R. Candón1,2, Pablo Casaseca2, Maurizio Giannotti2, Mathieu Kaltschmidt2, Jaime Ruz1,2, and Julia K. Vogel1,2

  • 1Fakultät für Physik, TU Dortmund, Otto-Hahn-Straße 4a, Dortmund D-44221, Germany
  • 2CAPA and Departamento de Física Teórica, Universidad de Zaragoza, C. Pedro Cerbuna 12, 50009 Zaragoza, Spain

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Phys. Rev. D 112, 123050 – Published 29 December, 2025

DOI: https://doi.org/10.1103/xv5c-j2rv

Abstract

A finite axion-nucleon coupling enables the production of axions in stellar environments via the thermal excitation and subsequent deexcitation of the Fe57 isotope. Given its low-lying excited state at 14.4 keV, Fe57 can be efficiently excited in the hot cores of supergiant stars, possibly leading to axion emission. The conversion of these axions into photons in the Galactic magnetic field results in a characteristic 14.4 keV line, potentially detectable by hard x-ray telescopes such as NASA’s Nuclear Spectroscopic Telescope Array (nustar). In this work, we present the first constraints on axion-nucleon couplings derived from nustar observations of Betelgeuse and discuss the potential insights that could be gained from detecting this line in other nearby supergiants. Our results establish significantly more stringent bounds than those obtained from solar observations, setting a limit of |gaγgaNeff|<(1.2–2.7)×10−20  GeV−1 for ma≲10−10  eV.

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Phys. Rev. D 112, 123050– Published 29 December, 2025
  • Received 20 May 2025
  • Accepted 5 December 2025
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