Study of the accreting pulsar 4U 0115+634 with a bulk and thermal
  Comptonization model
  Highly magnetized pulsars accreting matter in a binary system are bright
sources in the X-ray band (0.1-100 keV). Despite the early comprehension of the
basic emission mechanism, their spectral energy distribution is generally
described by phenomenological or simplified models. We propose a study of the
spectral emission from the high mass X-ray binary pulsar 4U 0115+634 by means
of thermal and bulk Comptonization models based on the physical properties of
such objects. For this purpose, we analyze the BeppoSAX data in the energy
range 0.7-100 keV of the 1999 giant outburst, 12 days after the maximum. We
model the spectral energy distribution of the system using a two-component
continuum. At higher energy, above ~7 keV, the emission is due to thermal and
bulk Comptonization of the seed photons produced by cyclotron cooling of the
accretion column, and at lower energy, the emission is due to thermal
Comptonization of a blackbody source in a diffuse halo close to the stellar
surface. From the best fit parameters, we argue that the cyclotron emission is
produced ~1.7 km above the stellar surface, and escapes from the column near
its base, where the absorption features are generated by the interaction with
the magnetic field in a surrounding halo. We find that in 4U 0115+634, the
observed spectrum is dominated by reprocessed cyclotron radiation, whereas in
other bright sources with stronger magnetic fields such as Her X-1, the
spectrum is dominated by reprocessed bremsstrahlung.
