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Astron. Astrophys. 332, 121-126 (1998)

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

M. Orlandini 1, D. Dal Fiume 1, F. Frontera 1, G. Cusumano 2, S. Del Sordo 2, S. Giarrusso 2, S. Piraino 2, A. Segreto 2, M. Guainazzi 3 and L. Piro 4

1 Istituto Tecnologie e Studio Radiazioni Extraterrestri (TeSRE), C.N.R., Via Gobetti 101, I-40129 Bologna, Italy
2 Istituto di Fisica Cosmica ed Applicazioni dell'Informatica (IFCAI), C.N.R., Via La Malfa 153, I-90146 Palermo, Italy
3 BeppoSAX Scientific Data Center (SDC), Nuova Telespazio, Via Corcolle 19, I-00131 Roma, Italy
4 Istituto di Astrofisica Spaziale (IAS), C.N.R., Via Fermi 21, I-00044 Frascati, Italy

Received 18 September 1997 / Accepted 25 November 1997

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 [FORMULA], and a change in absorption of a factor [FORMULA]. 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 [FORMULA] keV, and a second one at [FORMULA] 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, especially the low intensity state. We found an absorption-like feature at [FORMULA] 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 [FORMULA] Gauss. The BeppoSAX data do not require the presence of a cyclotron resonance at [FORMULA] keV as found in earlier works.

Key words: binaries: eclipsing – stars: individual: HD 77581 – stars: neutron – pulsars: individual: (Vela X – 1) – X-rays: stars

Send offprint requests to: orlandini@tesre.bo.cnr.it

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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