2007MNRAS.376..793P

We calculate the transition rates between proton Landau levels due to non-radiative and radiative Coulomb collisions in an electron-proton plasma with strong magnetic field B. Both electron-proton collisions and proton-proton collisions are considered. The roles of the first-order cyclotron absorption and second-order free-free absorption and scattering in determining the line strength and shape as well as the continuum are analysed in detail. We solve the statistical balance equation for the populations of proton Landau levels. For temperatures ∼10⁶-10⁷K, the deviations of the proton populations from local thermal equilibrium are appreciable at density ρ ≲ 0.1 B^3.5₁₄g/cm3, where B₁₄= B/(10¹⁴G). We present general formulae for the plasma emissivity and absorption coefficents under a wide range of physical conditions. Our results are useful for studying the possibility and the conditions of proton/ion cyclotron line formation in the near vicinity of highly magnetized neutron stars.