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Astron. Astrophys. 325, 623-630

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Phase resolved X-ray spectra of Vela X-1

P. Kretschmar1,5, H.C. Pan2, E. Kendziorra1, M. Maisack1, R. Staubert1, G. K. Skinner2, W. Pietsch3, J. Trümper3, V. Efremov4, and R. Sunyaev4

1Institut für Astronomie und Astrophysik - Astronomie, D-72076 Tübingen, Germany
2School of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
3Max-Planck-Institut für Extraterrestrische Physik, D-85470 Garching, Germany
4Space Research Institute, Moscow 117296, Russia
5Integral Science Data Center, CH-1290 Versoix, Switzerland

Received 14 October 1996 / Accepted 7 April 1997


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 (2-120 keV). The pulse profiles show the well known transition from a complex shape at low energies to a double peak structure at energies FORMULA 20 keV. The high-energy lightcurves exhibit strong pulse-to-pulse variations. We find indications for spectral features at ~ 23 and ~ 45 keV 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.

Key words: pulsars: individual: Vela X-1 - stars: magnetic fields - X-rays: stars

Send offprint requests to: Peter.Kretschmar@obs.unige.ch

© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 28, 1998