| We construct atmosphere models for strongly magnetized neutron stars with surface fields B ∼ 1012–10 15 G and effective temperatures Teff ∼ a few × 105–107 K. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars. For the fully ionized hydrogen or helium atmosphere models, the radiative opacities include free-free absorption and scattering by both electrons and ions computed for the two photon polarization modes in the magnetized electron-ion plasma. We solve the full, angle-dependent, coupled radiative transfer equations for both polarization modes. In general, the emergent thermal radiation exhibits significant deviation from blackbody. The spectra also show a broad feature around the ion cyclotron resonance. We study the effect of vacuum polarization, which modifies the dielectric property of the medium and gives rise to a resonance feature in the opacity; this feature is narrow and occurs at a photon energy that depends on the plasma density. Vacuum polarization can also induce resonant conversion of photon modes via a mechanism analogous to the MSW mechanism for neutrino oscillation. We show that vacuum polarization produces a broad depression in the X-ray flux at high energies as compared to models without vacuum polarization; this arises from the density dependence of the vacuum resonance and the large density gradient present in the atmosphere. Thus the vacuum polarization effect softens the high energy tail of the thermal spectrum. We show that the depression of continuum flux strongly suppresses the equivalent width of the ion cyclotron line and therefore makes the line more difficult to observe. We also construct partially ionized hydrogen atmosphere models using the latest equation of state and opacity results. For the models with B = 1012–1013 G, the spectral features due to neutral atoms lie at extreme UV and very soft X-ray energy bands and therefore are difficult to observe. However, the continuum flux is also different from the fully ionized case. For the superstrong field models (B 1014 G), vacuum polarization not only suppresses the proton cyclotron line but also suppresses spectral features due to bound species. |
