Effects of the Geometry of the Line-Forming Region on the Properties of Cyclotron Resonant Scattering Lines

We use a Monte Carlo radiative transfer code to examine the dependence of the properties of cyclotron resonant scattering lines on the spatial geometry and the optical depth of the line-forming region. We focus most of our attention on a line-forming region that is a plane-parallel slab. We also consider a cylindrical line-forming region. In both cases, the line-forming region contains an electron-proton plasma at the equilibrium Compton temperature, Tc, and is threaded with a uniform magnetic field with strength ∼ 10 gauss. We consider geometries in which the photon source illuminates the line-forming region from below, and in which the photon source is embedded in the line-forming region. The former may correspond to a line-forming region in the magnetosphere of a neutron star, illuminated from below; the latter to a line-forming region on or near the surface of a neutron star as in an accretion column. In the case of the plane-parallel slab line-forming region, we study the behavior of the resonant Compton temperature and the properties of the cyclotron scattering lines as a function of the column depth of the line-forming region and the orientation of the magnetic field. At small or moderate optical depths (Ne 10 cm [electron scattering optical depth τTo 10]), the resonant Compton temperature ranges from Te/EB ≈ 0.2 when the magnetic field is perpendicular to the slab to Te/EB ≈ 0.4 when the field is parallel to the slab. At large optical depths (Ne ∼> 1.5 × 10 cm [τTo ∼> 10]), the resonant Compton temperature is higher. At small or moderate optical depths (τTo 10), the equivalent widths, WE, of the spectral lines depend on two effects. First, WE increases as the viewing angle with respect to the field decreases, due to Doppler broadening. Second, WE increases as the viewing angle with respect to the slab normal