2014MNRAS.440.1114W

Long-term hard X-ray monitoring observations of high-mass X-ray binary Vela X-1 from 2003-2011 have been carried out by the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL). I systematically analysed the average hard X-ray spectra of Vela X-1 from 3-200 keV, with the main aims of detecting cyclotron resonant scattering features and studying their pattern of variation with accreting luminosity and orbital phase. The cyclotron scattering lines of Vela X-1 at ∼ 22-27 and 49-57 keV are confirmed in the average spectra of Vela X-1. However, in flare states with hard X-ray luminosity higher than ∼ 5x10³⁶ erg/s (3-100 keV), the fundamental line cannot be detected. This feature suggests that a fan-like beam radiation pattern is expected in high-luminosity ranges of Vela X-1. Variations of the cyclotron line energies and depths are discovered, which may change with continuum spectral properties. Both cyclotron line energies show no correlations with X-ray luminosity. The fundamental line energy shows no significant correlations with photon index and cut-off energy, while the first harmonic energy shows a positive correlation with photon index and exponential cut-off energy. The energy ratio of two cyclotron lines, always higher than 2, has a weak correlation with photon index and cut-off energy. These relations support the idea that the X-ray spectral properties of accreting X-ray pulsars are affected by cyclotron resonance scattering. In the case of Vela X-1, the broader and deeper first harmonic would play the main role in causing the spectral variations. The positive correlations between the ratio of line width to energy and the corresponding depth for both lines support a cylindrical column accretion geometry in Vela X-1.