Upper limit on the axion-photon coupling from magnetic white dwarf polarization

Polarization measurements of thermal radiation from magnetic white dwarf (MWD) stars have been proposed as a probe axion-photon mixing. The leaving the surface MWD is unpolarized, but if low-mass axions exist then photons polarized parallel to direction MWD's field may convert into axions, which induces linear polarization dependent on strength coupling $g_{a\gamma\gamma}$. We model this process by using formalism mixing in presence strong-field vacuum birefringence show that all stellar types MWDs are most promising targets for axion-induced searches. consider data multiple MWDs, including SDSS J135141 and Grw+70$^\circ$8247, after rigorously accounting astrophysical uncertainties constrained $|g_{a\gamma\gamma}| \lesssim 5.4 \times 10^{-12}$ GeV$^{-1}$ at 95% confidence axion masses $m_a 3 10^{-7}$ eV. This upper limit puts tension previously-suggested explanation anomalous transparency Universe TeV gamma-rays terms axions. identify future modeling efforts could further improve sensitivity