Cyclotron-line features from near-critical fields. II. On the effect of anisotropic radiation fields

We assess the impact of radiation anisotropy on the line shapes that result from relativistic magnetic Compton scattering in the low-density/high-fiel d regime. A Monte Carlo implementation of radiation transport allows for sp atial diffusion of photons in arbitrary geometries and accounts for relativ istic angular redistribution. The cross section includes natural line width s and photon "spawning" from up to fourth-harmonic photons. In our first pa per we noted that even if the photon injection is isotropic, a strongly ani sotropic radiation field rapidly ensues. We now investigate the angular dis tribution of cyclotron spectra emerging from an internally irradiated magne tized plasma with a prescribed global geometry (either cylindrical or plane parallel) and the effects of anisotropic photon injection on the line shap es. Varying the input angular distribution permits a better understanding o f the line formation process in more realistic scenarios where the radiativ e mechanisms are influenced by the intrinsic anisotropy of the field and by moderate relativistic beaming. In general, the line features are most pron ounced along the directions of the anisotropic continuum injection and tend to be weakened in other directions, relative to the line features resultin g from an isotropic continuum injection. We find that the enhancements at t he line wings of the fundamental, which appear prominently in the case of i sotropic continuum injection, are strongly suppressed along the direction o f anisotropy in the case of beamed continuum injection, regardless of geome try or beaming pattern.