I. Kreykenbohm(1), P. Kretschmar(1,3), J. Wilms(1), R. Staubert(1), E. Kendziorra(1), D.E. Gruber(2), W.A. Heindl(2) and R.E. Rothschild(2)
(1)Institut für Astronomie und Astrophysik, Astronomie, Universität
Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2)Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093, USA
(3)INTEGRAL Science Data Center, 1290 Versoix, Switzerland
1999, Astronomy & Astrophysics, accepted
Abstract. We present results from four observations of the accreting 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 unacceptable. We interpret large deviations around ~25 and ~55 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.
Key words: Pulsars: individual: Vela X-1 - Stars: neutron - Stars: magnetic field - X-rays: stars
Paper (199k gzip'ed Postscript including figures)
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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de)
Last updated 16-Nov-1998