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W.C.G. Ho, D. Lai (Cornell University)
Vacuum polarization modifies the photon propagation modes in the atmospheric plasma of a strongly magnetized neutron star. A resonance feature occurs when the vacuum and plasma effects on the two photon modes balance. We show that a photon (with energy E\ga~a~few keV) propagating outward in the atmosphere can convert from one polarization mode into another as it traverses the resonant density, \rho\rm res\approx Ye-1f(B)-2 (B/1014~{\rm G})2(E/1~{\rm keV})2~g~cm-3, where Ye is the electron fraction and f(B)~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. In general, the emergent thermal radiation from these sources exhibits significant deviation from blackbody, with harder spectra at high energies; however, we show that the inclusion of vacuum polarization can soften this high energy tail.