Constraints on Axion-like Particles from a Hard $X$-ray Observation of Betelgeuse

Abstract

We use the first observation of Betelgeuse in hard $X$-rays to perform a novel search for axion-like particles (ALPs). Betelgeuse is not expected to be a standard source of $X$-rays, but light ALPs produced in the stellar core could be converted back into photons in the Galactic magnetic field, producing a detectable flux that peaks in the hard $X$-ray band ($E_{\gamma }>10\mathrm{keV}$). Using a 50 ks observation of Betelgeuse by the $NuSTAR$ satellite telescope, we find no significant excess of events above the expected background. Using models of the regular Galactic magnetic field in the direction of Betelgeuse, we set a 95% C.L. upper limit on the ALP-photon coupling of $g_{a\gamma }<(0.5-1.8)\times 10^{-11}$ GeV${}^{-1}$ (depending on magnetic field model) for ALP masses $m_a<(5.5-3.5)\times 10^{-11}$ eV.