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Context: Accretion-powered X-ray pulsars show highly energy-dependent andcomplex pulse-profile morphologies. Significant deviations from the averagepulse profile can appear, in particular close to the cyclotron line energies.These deviations can be described as energy-dependent phase lags, that is, asenergy-dependent shifts of main features in the pulse profile. Aims: Using anumerical study we explore the effect of cyclotron resonant scattering onobservable, energy-resolved pulse profiles. Methods: We generated theobservable emission as a function of spin phase, using Monte Carlo simulationsfor cyclotron resonant scattering and a numerical ray-tracing routineaccounting for general relativistic light-bending effects on the intrinsicemission from the accretion columns. Results: We find strong changes in thepulse profile coincident with the cyclotron line energies. Features in thepulse profile vary strongly with respect to the average pulse profile with theobserving geometry and shift and smear out in energy additionally when assuminga non-static plasma. Conclusions: We demonstrate how phase lags at thecyclotron energies arise as a consequence of the effects of angularredistribution of X-rays by cyclotron resonance scattering in a strong magneticfield combined with relativistic effects. We also show that phase lags arestrongly dependent on the accretion geometry. These intrinsic effects will inprinciple allow us to constrain a system's accretion geometry.

Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars