Resonant Conversion of Photon Modes Due to Vacuum Polarization in a Magnetized Plasma: Implications for X-ray Emission from Magnetars

It is known that vacuum polarization can modify the photon propagation modes in the atmospheric plasma of a strongly magnetized neutron star. A resonance occurs when the effect of vacuum polarization on the photon modes balances that of the plasma. We show that a photon (with energy E >∼ a few keV) propagating outward in the atmosphere can convert from one polarization mode into another as it traverses the resonant density, ρres ≃ Y −1 e η (B/10 G)(E/1 keV) g cm, where Ye is the electron fraction and η ∼ 1 is a slowly-varying function of the magnetic field B. The physics of this mode conversion is analogous to the Mikheyev-Smirnov-Wolfenstein mechanism for neutrino oscillation. Because the two photon modes have vastly different opacities in the atmosphere, this vacuum-induced mode conversion can significantly affect radiative transport and surface emission from strongly magnetized neutron stars. Subject headings: magnetic fields – radiative transfer – stars: neutron