SIMBAD references

We report on the 20ksec observation of Vela X-1 performed by BeppoSAX on 1996 July 14 during its Science Verification 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 fit by a power law with significantly different indices. This is in agreement with the observed changes of slope in the wide-band spectrum: a first change of slope at ∼10keV, and a second one at ∼35keV. To mimic this behaviour we used a double power law modified by an exponential cutoff – the so-called NPEX model – to fit the whole 2-100keV continuum. This functional is able to adequately describe the data, especially the low intensity state. We found an absorption-like feature at ∼57keV, very well visible in the ratio performed with the Crab spectrum. We interpreted this feature as a cyclotron resonance, corresponding to a neutron star surface magnetic strength of 4.9x10¹² Gauss. The BeppoSAX data do not require the presence of a cyclotron resonance at ∼27keV as found in earlier works.