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W.C.G. Ho (Cornell University & KIPAC)
The spectra of radio pulsars and isolated neutron stars have been observed to possess a thermal component that can be attributed to the neutron star surface at temperatures in the range ~mbox{a few}\times 105-107~K. By investigating the neutron star thermal spectrum, it may be possible to directly measure the neutron star surface magnetic field and composition, obtain a more complete understanding of the evolution of neutron stars, and constrain the properties of matter and physical processes under extreme conditions. To produce spectra that can be compared with the observations, I construct neutron star atmosphere models by solving the full, angle-dependent, coupled radiative transfer equations for the two photon polarization modes in the magnetized electron-ion plasma of the atmosphere. I study the effect of strong magnetic fields on the dielectric property and polarization of the medium, including vacuum polarization. Vacuum polarization gives rise to a resonance feature in the opacity that depends on the plasma density and can also induce resonant conversion of photon modes via a mechanism analogous to the MSW mechanism for neutrino oscillation. I show that for magnetic fields B\ga 1014~G, vacuum polarization softens the high energy tail of the thermal spectrum and strongly suppresses the proton cyclotron line and spectral features due to bound species, making the lines more difficult to observe. For weaker fields (B\la 7\times 1013~G), the surface spectrum is unaffected by vacuum polarization, and the proton cyclotron absorption line can have a large equivalent width; furthermore, variation of magnetic fields over the neutron star surface leads to broadening of the line.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.