The Vela X{1 pulse-averaged spectrum as observed by BeppoSAX

We report on the 20 ksec observation of Vela X{1 performedbyBeppoSAX on1996July14duringitsScienceVer- ication Phase. We observed the source in two intensity states, characterized by a change in luminosity of a factor 2, and a change in absorption of a factor 10. The single Narrow Field Instrument pulse-averaged spectra are well t by a power law with signicantly different indices. This is in agreement with the observed changes of slope in the wide-band spectrum: a rst change of slope at 10 keV, and a second one at 35 keV.Tomimicthisbehaviourweusedadoublepowerlawmod- ied by an exponential cutoff | the so-called NPEX model | to t the whole 2{100 keV continuum. This functional is able to adequately describe the data, especially the low intensity state. We found an absorption-like feature at 57 keV, very well visible in the ratio performed with the Crab spectrum. We inter- preted this feature as a cyclotron resonance, corresponding to a neutron star surface magnetic strength of 4:9 10 12 Gauss. The BeppoSAX data do not require the presence of a cyclotron resonance at 27 keV as found in earlier works.