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

TL;DR

This study analyzes BeppoSAX observations of Vela X-1, revealing spectral changes, a cyclotron resonance feature at 57 keV indicating a strong magnetic field, and differences from previous findings.

Contribution

First detailed spectral analysis of Vela X-1 with BeppoSAX showing a cyclotron line at 57 keV and revising earlier claims of a 27 keV feature.

Findings

Detected a cyclotron resonance at ~57 keV.

Observed spectral slope changes at ~10 keV and ~35 keV.

Found no evidence for the previously reported 27 keV line.

Abstract

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 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 ~ 10 keV, and a second one at ~ 35 keV. To mimic this behaviour we used a double power law modified by an exponential cutoff --- the so-called NPEX model --- to fit the whole 2-100 keV continuum. This functional is able to adequately describe the data, expecially 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 interpreted this feature as a cyclotron resonance, corresponding to a neutron star surface magnetic strength of 4.9 x 10^12 Gauss. The BeppoSAX data do not require the presence of a cyclotron resonance at ~ 27 keV as found in earlier works.