Constraining the photon-axion coupling constant with magnetic white dwarfs

The light pseudoscalar particle, dubbed the axion, borne out of the Peccei-Quinn solution to the strong $CP$ problem in QCD remains elusive. One avenue of inferring its existence is through its coupling to electromagnetic radiation. So far, laboratory experiments have dedicated all efforts to detect the axion in the mass range 10^-6<mₐ<10^-3 eV with a photon-axion coupling strength g_aγγ<10^-10 GeV^-1, where the limits are derived from astrophysical considerations. In this study, we present a novel way of constraining g_aγγ by looking at the level of linear polarization in the radiation emerging from magnetic white dwarfs. We find that photon-axion oscillations in white dwarf magnetospheres can enhance the degree of linear polarization. Observing that most magnetic white dwarfs show only 5% linear polarization, we derive upper limits on g_aγγ for different axion masses.