A Bulk and Thermal Comptonization Model for the Accreting Pulsar 4U 0115+63

Abstract

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, usually their spectral energy distribution is described by phenomenological or simplified models.Here, we study the spectral energy distribution of the high mass X‐ray binary pulsar 4U 0115+63, using a thermal and bulk Comptonization model based on the physical properties of the object. We analyze the Beppo‐SAX data of the 1999 giant outburst, taken 12 days after the maximum in the energy range 0.7–100 keV. We focus first on the average emission, and then on the phase resolved spectra. The emission can be modeled as a two component continuum with contributions due to (i) thermal and bulk Comptonization of seed photons produced by cyclotron cooling in the accretion column, and (ii) thermal Comptonization by a lower energy plasma surrounding the base of the column. We find that the magnetic field strength implied by the cyclotron seed photons is lower than the magnetic field responsible for producing the cyclotron absorption lines, supporting the hypothesis of a spatial offset between these two regions. The ongoing phase resolved analysis evidences how most of the emission in the main peak comes from the column, while the low energy component gives a nearly constant contribution throughout the phase.