THE VELA-X-1 PULSE-AVERAGED SPECTRUM AS OBSERVED BY BEPPOSAX

We report on the 20 ksec observation of Vela X-1 performed by BeppoSAX on 1996 July 14 during its Science Verification Phase. We observed th e source in two intensity states, characterized by a change in luminos ity of a factor similar to 2, and a change in absorption of a factor s imilar to 10. The single Narrow Field Instrument pulse-averaged spectr a are well fit by a power law with significantly different indices. Th is is in agreement with the observed changes of slope in the wide-band spectrum: a first change of slope at similar to 10 keV, and a second one at similar to 35 keV. To mimic this behaviour we used a double pow er law modified by an exponential cutoff - the so-called NPEX model to fit the whole 2-100 keV continuum. This functional is able to adequat ely describe the data, especially the low intensity state. We found an absorption-like feature at similar to 57 keV, very well visible in th e ratio performed with the Crab spectrum. We interpreted this feature as a cyclotron resonance, corresponding to a neutron star surface magn etic strength of 4.9 x 10(12) Gauss. The BeppoSAX data do not require the presence of a cyclotron resonance at similar to 27 keV as found in earlier works.