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

Study of the accreting pulsar 4U 0115+634 with a bulk and thermal Comptonization model