  (abbreviated version of the abstract) We study the physics of cyclotron line
formation in the high-energy spectra of accreting X-ray pulsars using Monte
Carlo methods, assuming that the line-forming region is a low-density electron
plasma in a sub-critical magnetic field. We investigate the dependence of the
shape of the fundamental line on angle, geometry, optical depth and
temperature. We also discuss variations of the line ratios for non-uniform
magnetic fields. These numerical predictions for the line profiles are linked
to results from observational data analysis using an XSPEC model based on the
Monte Carlo simulations. We apply this model to observational data from RXTE
and INTEGRAL. The predicted strong emission wings of the fundamental cyclotron
feature are not found in observational data, hinting at a bottom illuminated
slab geometry for line formation.
