BeppoSAX observation of the X-ray binary pulsar Vela X-1

We report on the spectral (pulse averaged) and timing analysis of the ∼20 ksec observation of the X-ray binary pulsar Vela X-1 performed during the BeppoSAX Science Verification Phase. The source was observed in two different intensity states: the low state is probably due to an erratic intensity dip and shows a decrease of a factor ∼2 in intensity, and a factor 10 in NH. We have not been able to fit the 2–100 keV continuum spectrum with the standard (for an X-ray pulsar) power law modified by a high energy cutoff because of the flattening of the spectrum in ∼10–30 keV. The timing analysis confirms previous results: the pulse profile changes from a five-peak structure for energies less than 15 keV, to a simpler two-peak shape at higher energies. The Fourier analysis shows a very complex harmonic component: up to 23 harmonics are clearly visible in the power spectrum, with a dominant first harmonic for low energy data, and a second one as the more prominent for energies greater than 15 keV. The aperiodic c...