4U 0115+63: phase lags and cyclotron resonant scattering

Context. High-mass X-ray binaries are among the brightest objects of our Galaxy in the high-energy domain (0.1-100 keV). 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.