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

Abstract

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