Probing the Axion-Electron Coupling with NuSTAR Observations of Galaxies

We search for the existence of ultralight axions coupling to electrons and photons using data from the NuSTAR telescope directed toward the galaxies M82, M87, and M31. We focus on electron bremsstrahlung and Compton scattering for axion production in stars, summing over the stellar populations found in the target galaxies when computing the axion luminosity. We then compute the hard X-ray signal that arises from the conversion of these axions to photons in each galaxy's magnetic fields, inferred from analog galaxies in cosmological magnetohydrodynamic simulations. Analyzing NuSTAR data toward these galaxies between roughly 20 to 70 keV, we find no evidence for axions and set leading constraints on the combined axion-electron and axion-photon coupling at the level of $|g_{aee} \times g_{a \gamma \gamma}| \lesssim 8.3 \times 10^{-27}$ GeV$^{-1}$ for $m_a \lesssim 10^{-10}$ eV at 95% confidence, with M82 providing the most stringent constraints.