Total cross section for magnetic Compton scattering (purple lines) and the free–free absorption coefficient (cyan lines) for the ordinary (solid) and extraordinary (dashed) polarization mode, taking into account vacuum polarization. Left: coefficients for a tenuous and hot plasma with n_e = 10²⁰ cm⁻³ and kT_e = 35 keV, as expected for the upper, overheated layers of the atmosphere. Right: same but for cold and dense plasma with n_e = 10²⁵ cm⁻³ and kT_e = 2 keV, corresponding to the bottom part of the atmosphere. The cyclotron line energy is E_cyc = 75 keV and the incident photon has a direction of θ = 60° with respect to the magnetic field, for both cases.