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 Compton scattering is performed and use is made of Monte Carlo methods to construct a numerical scheme for the production of relativistic theoretical cyclotron spectra from internally irradiated scattering atmospheres in the low-density/high-field regime. The cross section includes natural line widths, and the Monte Carlo implementation allows for spatial diffusion of photons in arbitrary geometries while accounting for relativistic angular redistribution. Detailed consideration of "Raman-Landau" scattering permits photon spawning from up to fourth harmonic photons. This scheme is utilized to make specific predictions on the effect of geometry and optical depth on the formation of cyclotron lines in hard incident continua. Even if the injection is isotropic, a strongly anisotropic radiation field ensues at relatively low optical depths. The scattering profiles at large angles and low fields are highly asymmetric and peak at the relativistic energy cutoff, not necessarily at the resonant energy. Higher harmonic features in general bear little resemblance to the fundamental even at high fields (B similar to 10 TG), and slabs yield the deepest features, while cylinders present the widest lines at small angles. Moreover, the fundamental line photons in slab scattering regions tend to redistribute to the line wings from all angles, producing fairly broad emission line wings at small angles. Higher optical depths at those angles hampers such redistribution in cylindrical geometries.