A pulse phase-dependent spectroscopic study of Vela X–1 in the 8–100 keV band

¹ Istituto di Astrofisica spaziale e Fisica Cosmica, C.N.R., sezione di Palermo, via Ugo La Malfa, 153, 90146 Palermo, Italy
² Istituto di Astrofisica spaziale e Fisica Cosmica, C.N.R., sezione di Bologna, via Piero Gobetti, 101, 40129 Bologna, Italy

Corresponding author: A. La Barbera, nino.labarbera@pa.iasf.cnr.it

Received: 31 October 2002
Accepted: 23 December 2002

Abstract

We present a pulse phase resolved study of the 8–100 keV X–ray spectrum of the High Mass X–ray binary Vela X–1, observed by the BeppoSAX satellite. The continuum part of the spectrum is found to be strongly dependent on pulse phase and evidence for saturated and unsaturated Comptonization is found. We confirm the presence of a phase dependent absorption–like feature detected at ~55 keV that we interpret as due to cyclotron resonant scattering. The equivalent width of the feature ranges from ~10 to ~40 keV whereas its width varies from ~5 to ~10 keV. The energy of the cyclotron line, whose values range from ~50 to ~60 keV, is strongly correlated with the trend of the luminosity intensity of the pulsed profile. With regard to the early reported cyclotron absorption at ~25 keV we do not find strong evidence of it in our BeppoSAX observations.