The 0.1-100 keV spectrum of Centaurus X-3: Pulse phase spectroscopy of the cyclotron 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 rollover at similar to 14 keV (e-folding energy similar to 8 keV) plus an iron emission line at similar to 6.7 keV. A soft excess below 1 keV is also present. Interpreted as a blackbody (kT similar or equal to 0.1 keV), it corresponds to 58% of the total unabsorbed flux. This component seems to originate from reprocessing of the primary radiation by an opaque shell located at the magnetosphere. An absorption feature at similar to 30 keV is also present. Interpreted as a cyclotron line, after correction for gravitational redshift, this corresponds to a surface magnetic field of similar to 3.5 x 10(12) G. Phase-resolved spectroscopy reveals a variation by about 8 keV of the cyclotron resonance energy along the pulse profile. In particular, the line energy decreases from similar to 36 to similar to 28 keV along the peak, starting from the ascent. The asymmetric variations of the cyclotron line energy can be explained by assuming an offset of the dipolar magnetic field with 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 double Compton scattering or from illumination of the polar cap by the primary radiation reprocessed at the magnetospheric surface, a feedback mechanism similar to that proposed for the formation of black hole spectra.