1997/11/19 by M. Orlandini, Orlandini, M., D. Dal Fiume +17 · 1 citation
Physics and Astronomy · #Astro and Planetary Science #Astrophysics (astro-ph) #FOS: Physical sciences #Solar and Space Plasma Dynamics #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9711215
6 pages, 4 PostScript figures. Uses l-aa.sty. Accepted for publication in Astronomy & Astrophysics, Main Journal
arxiv created 1997/11/19 · openalex publication_date 1997/11/19 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
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 1012 Gauss. The BeppoSAX data do not require the presence of a cyclotron resonance at ~ 27 keV as found in earlier works.