Constraints on Axion-Like Particles from a Hard X-Ray Observation of Betelgeuse

Axion-like particles (ALPs) can be produced in stellar plasmas via the Primakoff process due to the photon-ALP coupling. Light ALPs produced in this way can easily escape the star and be converted back into photons in the Galactic magnetic field. Betelgeuse ($\alpha$-Orionis, spectral type M2Iab), a nearby red supergiant star, provides an excellent laboratory for ALP searches, as it (i) has a hot core, and thus is potentially a copious producer of ALPs that, after re-conversion, produces a photon signal peaked in the hard X-ray (E$>$10 keV) range, (ii) is in region of Hertzsprung-Russel diagram where no stable corona is expected, and thus has essentially zero standard astrophysical X-ray background, and (iii) is nearby, at a distance d~200 pc, and thus in a region of the local magnetic field that is relatively easier to constrain with future observations. We use the first observation of Betelgeuse in hard X-rays to perform a novel search for ALPs. With a 50 ks observation by the NuSTAR satellite telescope, we find no significant excess of events above the expected background and set a stringent upper limit on the ALP-photon coupling. In this contribution, I will detail this work while focusing on the data analysis, results and impact, and emphasizing the uncertainty.