2011A&A...532A..76F

High-mass X-ray binaries are among the brightest objects of our Galaxy in the high-energy domain (0.1-100keV). Despite our relatively good knowledge of their basic emission mechanisms, the complex problem of understanding their time- and energy- dependent X-ray emission has not been completely solved. In this paper, we study the energy-dependent pulse profiles of the high-mass X-ray binary pulsar 4U 0115+63 to investigate how they are affected by cyclotron resonant scattering. We analyzed archival BeppoSAX and RXTE observations performed during the giant outburst of the source that occurred in 1999. We exploited a cross correlation technique to compare the pulse profiles in different energy ranges and developed a relativistic ray-tracing model to interpret our findings. We also studied the phase dependency of the cyclotron absorption features by performing phase-resolved spectroscopy. The pulse profiles of 4U 0115+63 displayed clear ``phase-lags'' at energies close to those of the cyclotron absorption features that characterize the X-ray emission of the source. We reproduce this phenomenon qualitatively by assuming an energy-dependent beaming of the emission from the column surface and verify that our model is also compatible with the results of phase-resolved spectral analysis. We show that cyclotron resonant scattering affects the pulse profile formation mechanisms in a complex way, which necessitates both improvements in the modeling and the study of other sources to be better understood.