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 \rho typical for atmospheres of isolated neutron stars. The first- and second-order thermodynamic functions, monochromatic radiative opacities, and Rosseland mean opacities are calculated and tabulated, taking account of partial ionization, for 8x10^{11} G < B < 3x10^{13} G, 2x10^5 K < T < 10^7 K, and a wide range of \rho. We show that bound-bound and bound-free transitions give an important contribution to the opacities at T < (1-5)x10^6 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 re-evaluate 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. Note added in v4: for free-free photoabsorption (Appendix B), equivalent but much simpler expressions are available in Appendix B of Potekhin & Lai, arXiv:astro-ph/0701285.