Sampled electron parallel momenta for one MC run. Each point represents one scattered photon by its energy on the x-axis and the corresponding electron’s momentum before (red) and after (blue) the scattering process on the y-axis. Scattering events involving photons incoming at angles of ~90°, 79°, 60°, 37°, and 1° to the magnetic field (from top to bottom) are shown. The setup of the calculation is the same as in Isenberg et al. (1998) with a magnetic field B = 0.0385 B_crit, electron parallel temperature k_BT = 5 keV, and a bottom-illuminated slab geometry of the line-forming region. Vertical dashed lines show the cyclotron line energy, which is slightly shifted to higher energies for smaller angles to the magnetic field, because of the angular dependence of the resonance energy. This shift is often neglected by observers in favor of using the simplified 12-B-12 rule. Solid vertical lines mark the position of the cut-off energy beyond which no resonant scattering is possible. Dotted horizontal lines enclose the electron momentum range in which 99% of the electrons are located (assuming a relativistic Maxwellian, Eq. (3)). Solid curved lines show the possible solutions for resonant scattering. The vertical dotted lines indicate the Doppler width of the cyclotron lines (see Eq. (9)).