Resonant axion and dark photon production in magnetic white dwarfs

Using magnetic white dwarfs as a case study, we show that the emission of sub-MeV bosons from stellar plasmas can be substantially modified in the presence of a magnetic field. In particular, the magnetic field-induced anisotropy and cyclotron resonance both significantly affect the in-medium dispersion relations of the Standard Model photon. As a result, resonant level crossing between photons and other light bosons occurs under environmental conditions that differ from the resonance criteria in unmagnetized environments. We find that the magnetic field opens additional regions within magnetic white dwarfs where resonance can occur. These findings motivate revisiting astrophysical constraints on light bosons in systems where the cyclotron frequency is comparable to or larger than the plasma frequency.