Abstract: The dense core of a neutron star may provide an ideal environment for the production of axions. Axions created in the core via nucleon bremsstrahlung processes would escape the neutron star and, with some conversion efficiency, turn into photons in the magnetosphere. The large external magnetic field of a magnetar enhances the conversion probability, making magnetars a particularly strong source of axion-induced photons. I will review the mechanisms of axion production in dense nuclear matter and calculate the energy spectrum of the photons coming from axion-photon conversion. This spectrum is compared to observed hard X-ray spectra from several magnetars in the McGill magnetar catalog to put limits on the product of the axion couplings to nucleons and photons as a function of the magnetar core