VELA X-1 AS SEEN BY RXTE

We present results from four observations of the ac- creting X-ray pulsar Vela X-1 with the Rossi X-ray Timing Explorer (RXTE) in 1996 February. The light curves show strong pulse to pulse variations, while the average pulse profiles are quite stable, similar to previous results. Below 5 keV the pulse profiles display a complex, 5- peaked structure with a transition to a simple, double peak above 15 keV. We analyze phase-averaged, phase-resolved, and on-pulse minus off-pulse spectra. The best spectral fits were obtained using continuum models with a smooth high-energy turnover. In contrast, the commonly used power law with exponential cutoff introduced artificial features in the fit residuals. Using a power law with a Fermi-Dirac cutoff modified by photoelectric absorption and an iron line, the best fit spectra are still unaccept- able. We interpret large deviations around 25 and55 keV as fundamental and second harmonic cyclotron absorption lines. If this result holds true, the ratio of the line energies seems to be larger than 2. Phase resolved spectra show that the cyclotron lines are strongest on the main pulse while they are barely visible outside the pulses.