Phase resolved X-ray spectra of VELA X-1.

Combining simultaneous observations of Vela X-1 by the instruments HEXE and TTM on Mir-Kvant, we have analyzed pulse profiles and pulse phase resolved spectra over a broad energy band (2-120keV). The pulse profiles show the well known transition from a complex shape at low energies to a double peak structure at energies >20keV. The high-energy lightcurves exhibit strong pulse-to-pulse variations. We find indications for spectral features at ~23 and ~45keV in the phase averaged spectra, which can be interpreted as the fundamental and first harmonic cyclotron resonances, but we caution that the first feature coincides with the spectral cutoff of the continuum spectra used. Pulse phase resolved spectroscopy shows that the shape of the continuum spectrum varies significantly with pulse phase and that the cyclotron features are mainly visible in one of the two pulses seen at high energies.