Search for cyclotron lines in INTEGRAL/SPI spectra of Vela X-1 Original Research Article

The wind-accreting X-ray binary pulsar Vela X-1 has been observed during the two INTEGRAL Core Program observations of the Vela region in June–July and November–December 2003. Preliminary results on time averaged and time resolved spectra of INTEGRAL/SPI for the two observation epochs are presented. Time averages have been used in order to increase the signal to noise ratio, but cyclotron line features expected at ∼20–25 and ∼53 keV are without any doubt detected in the INTEGRAL/SPI spectra. This work shows that the detection of cyclotron feature as given in [Kretschmar, P., Staubert, R., Kreykenbohm, I., et al. In: Proceedings of 5th INTEGRAL Workshop, 2004.] should be taken with caution due to background modelling uncertainties. Further work using a better in-flight energy calibration and background monitoring is planed to improve the analysis.