Absorption of radiation in hydrogen atmospheres of neutron stars with strong magnetic fields

A.Y. Potekhin
Ioffe Physico-Technical Institute RAS, St.-Petersburg, Russia
G. Chabrier
Ecole Normale Superieure de Lyon, France

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 first- and 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 density. We show that bound-bound and bound-free transitions give an important contribution to the opacities at T < (1-5) 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 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. For the application of these results to the neutron-star atmosphere and spectra modelling, see the talk of 17 April 2003.


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