4U 0115+63: phase lags and cyclotron resonant scattering
  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 is not completely solved. <P> 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. <P> We analyze archival BeppoSAX and RXTE observations
performed during the giant outburst of the source which occurred in 1999. We
exploit a cross correlation technique to compare the pulse profiles in
different energy ranges and develop a relativistic ray-tracing model to
interpret our findings. We also study the phase dependency of the cyclotron
absorption features by performing phase resolved spectroscopy. <P> 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 qualitatively this phenomenon 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.
<P> We showed 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.
