The 0.1-100 keV spectrum of Centaurus X-3: Pulse phase spectroscopy of thecyclotron line and magnetic field structure

We report spectral and temporal analysis of the X-ray pulsar Centaurus X-3 out of eclipse observed by BeppoSAX. The broadband spectrum (0.12-100 keV) is well described by an absorbed power law modified by a high-energy rollov er at similar to 14 keV (e-folding energy similar to 8 keV) plus an iron em ission line at similar to 6.7 keV. A soft excess below 1 keV is also presen t. Interpreted as a blackbody (kT similar or equal to 0.1 keV), it correspo nds to 58% of the total unabsorbed flux. This component seems to originate from reprocessing of the primary radiation by an opaque shell located at th e magnetosphere. An absorption feature at similar to 30 keV is also present . Interpreted as a cyclotron line, after correction for gravitational redsh ift, this corresponds to a surface magnetic held of similar to 3.5 x 10(12) G. Phase-resolved spectroscopy reveals a variation by about 8 keV of the c yclotron resonance energy along the pulse profile. In particular, the line energy decreases from similar to 36 to similar to 28 keV along the peak, st arting from the ascent. The asymmetric variations of the cyclotron line ene rgy can be explained by assuming an offset of the dipolar magnetic held wit h respect to the neutron star center. The spectral results are discussed in terms of emission from magnetic caps, where Comptonization of soft photons occurs. The soft photons could come from either magnetically resonant doub le Compton scattering or from illumination of the polar cap by the primary radiation reprocessed at the magnetospheric surface, a feedback mechanism s imilar to that proposed for the formation of black hole spectra.