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Preprint 12b/99

TWO CYCLOTRON LINES IN VELA X-1?

I. Kreykenbohm (1), P. Kretschmar (1;3), J. Wilms (1), R. Staubert (1), W. A. Heindl (2), D. E. Gruber (2), R. E. Rothschild (2)

(1) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2) CASS, University of California, San Diego, La Jolla, CA 92093, U.S.A.
(3) INTEGRAL Science Data Center, ISDC, 1290 Versoix, Switzerland

To appear in: CONFERENCE PROCEEDINGS X-ray Astronomy '99, published in Astrophysical Letters and Communications, 1999,

Abstract. We present the spectral analysis of our 35 ksec continuous RXTE observation of the High Mass X-ray Binary Vela X-1. The spectral models used normally to describe pulsar spectra like a power law with an exponential cutoff or the NPEX model (Negative Positive EXponential), do not work for our data. Our successful model consists of two NPEX components: one is heavily absorbed, while the other is due to reflection. But we still find large deviations around ~21 keV and ~50 keV. We interpret the feature at ~55 keV as a cyclotron absorption line while the lower deviation might be due to instrument calibration problems around 25 keV in the PCA.

Key words: X-rays: stars - stars: neutron stars: pulsars: individual: Vela X-1

Paper (50k gzip'ed Postscript including figures)

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