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 a few E greater than or similar to a few keV) propagating outward in the atmosphere can convert from one polarization mode into another as it traverses the resonant density, rho(res) similar or equal to Y-e(-1) eta(-2)(B/10(14)G)(2)(E/1keV)(2) g cm(-3), where Y-e is the electron fraction and eta similar to 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.