Equation of State and Opacities for Hydrogen Atmospheres of Neutron Stars with Strong Magnetic Fields

We present an equation of state and radiative opacities for a strongly magnetized hydrogen plasma at magnetic fields B, temperatures T, and densities typical for atmospheres of isolated neutron stars. The firstand second-order thermodynamic functions, monochromatic radiative opacities, and Rosseland mean opacities are calculated and tabulated, taking account of partial ionization, for 8 1011 G B 3 1013 G, 2 105 K T 107 K, and a wide range of . We show that bound-bound and bound-free transitions give an important contribution to the opacities at Tdð1 5Þ 106 K in the considered range of B in the outer neutron star atmosphere layers, which may substantially modify the X-ray spectrum of a typical magnetized neutron star. In addition, we reevaluate opacities due to free-free transitions, taking into account the motion of both interacting particles, electron and proton, in a strong magnetic field. Compared to the previous neutron star atmosphere models, the free-free absorption is strongly suppressed at photon frequencies below the proton cyclotron frequency. The latter result holds for any field strength, which prompts a revision of existing theoretical models of X-ray spectra of magnetar atmospheres. Subject headings: equation of state — magnetic fields — plasmas — stars: atmospheres — stars: neutron