Spectral redistribution of gyroresonant photons in magnetized atmospheres of isolated compact stars

Aims. We analyze the spectral redistribution of gyroresonant photons in the course of radiation transfer through magnetized plasma atmospheres of isolated compact stars. Methods. We use analytical estimate and Monte Carlo simulations to prove that this redistribution crucially influences the spectral line formation for atmospheric parameters typical of neutron stars and white dwarfs. Results. We point out the importance of the frequency redistribution of the gyroresonant photons to the process of radiation transfer and analyze its main effects in atmospheres of isolated compact stars with strong magnetic fields, where multiple scattering dominates over the absorption of photons. We estimate analytically and numerically the rate of this redistribution and show that photons' escape from the line center, which in this case is one-dimensional (1D) in origin, is a very pronounced effect despite being strongly inhibited with respect to three-dimensional (3D) photon redistribution, which takes place in the case of atomic or ion spectral lines. The escape of photons from the cyclotron line greatly affects both the line's profile and the characteristic optical depth, from where the outgoing radiation originates. Through this, the spectral redistribution of gyroresonant photons changes the radiation pressure on the atmospheric plasma, what makes it one of the key phenomena need to be included in studies of cyclotron-driven winds.