SIMBAD references

We present a pulse phase resolved study of the 8-100keV X-ray spectrum of the High Mass X-ray binary Vela X-1, observed by the BeppoSAX satellite. The continuum part of the spectrum is found to be strongly dependent on pulse phase and evidence for saturated and unsaturated Comptonization is found. We confirm the presence of a phase dependent absorption-like feature detected at ∼55keV that we interpret as due to cyclotron resonant scattering. The equivalent width of the feature ranges from ∼10 to ∼40keV whereas its width varies from ∼5 to ∼10keV. The energy of the cyclotron line, whose values range from ∼50 to ∼60keV, is strongly correlated with the trend of the luminosity intensity of the pulsed profile. With regard to the early reported cyclotron absorption at ∼25keV we do not find strong evidence of it in our BeppoSAX observations.