Cyclotron line features from near-critical magnetic fields: The effect of optical depth and plasma geometry

A detailed analytical study of the relativistic profile for magnetic Compto n scattering is performed and use is made of Monte Carlo methods to constru ct a numerical scheme for the production of relativistic theoretical cyclot ron spectra from internally irradiated scattering atmospheres in the low-de nsity/high-field regime. The cross section includes natural line widths, an d the Monte Carlo implementation allows for spatial diffusion of photons in arbitrary geometries while accounting for relativistic angular redistribut ion. Detailed consideration of "Raman-Landau" scattering permits photon spa wning from up to fourth harmonic photons. This scheme is utilized to make s pecific predictions on the effect of geometry and optical depth on the form ation of cyclotron lines in hard incident continua. Even if the injection i s isotropic, a strongly anisotropic radiation field ensues at relatively lo w optical depths. The scattering profiles at large angles and low fields ar e highly asymmetric and peak at the relativistic energy cutoff, not necessa rily at the resonant energy. Higher harmonic features in general bear littl e resemblance to the fundamental even at high fields (B similar to 10 TG), and slabs yield the deepest features, while cylinders present the widest li nes at small angles. Moreover, the fundamental line photons in slab scatter ing regions tend to redistribute to the line wings from all angles, produci ng fairly broad emission line wings at small angles. Higher optical depths at those angles hampers such redistribution in cylindrical geometries.