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 $10^{-6} < m_a < 10^{-3}$ eV with a photon-axion coupling strength $g_{a\gamma\gamma} < 10^{-10}GeV^{-1}$, where the limits are derived from astrophysical considerations. In this study, we present a novel way of constraining $g_{a\gamma\gamma}$ by looking at the level of linear polarization in the radiation emerging from magnetic white dwarfs (mWDs). We find that photon-axion oscillations in WD magnetospheres can enhance the degree of linear polarization. Observing that most mWDs show only 5% linear polarization, we derive upper limits on $g_{a\gamma\gamma}$ for different axion masses.