EDP Sciences
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Volume 395, Number 1, November III 2002
Page(s) 129 - 140
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021181

A&A 395, 129-140 (2002)
DOI: 10.1051/0004-6361:20021181

Confirmation of two cyclotron lines in Vela X-1

I. Kreykenbohm1, W. Coburn2, J. Wilms1, P. Kretschmar3, 4, R. Staubert1, W. A. Heindl5 and R. E. Rothschild5

1  Institut für Astronomie und Astrophysik - Astronomie, Sand 1, 72076 Tübingen, Germany
2  Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, 94702-7450, USA
3  INTEGRAL Science Data Center, 6 ch. d'Écogia, 1290 Versoix, Switzerland
4  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85740 Garching, Germany
5  Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA

(Received 6 August 2001 / Accepted 9 August 2002)

We present pulse phase-resolved X-ray spectra of the high mass X-ray binary Vela X-1 using the Rossi X-ray Timing Explorer. We observed Vela X-1 in 1998 and 2000 with a total observation time of ~90 ksec. We find an absorption feature at $23.3^{+1.3}_{-0.6}~ke\kern -0.09em V$ in the main pulse, that we interpret as the fundamental cyclotron resonant scattering feature (CRSF) . The feature is deepest in the rise of the main pulse where it has a width of $7.6^{+4.4}_{-2.2}~ke\kern -0.09em V$ and an optical depth of 0.33+0.06-0.13. This CRSF is also clearly detected in the secondary pulse, but it is far less significant or undetected during the pulse minima. We conclude that the well known CRSF at $50.9^{+0.6}_{-0.7}~ke\kern -0.09em V$, which is clearly visible even in phase-averaged spectra, is the first harmonic and not the fundamental. Thus we infer a magnetic field strength of $B=2.6\times
10^{12}$ G.

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

Offprint request: I. Kreykenbohm, kreyken@astro.uni-tuebingen.de

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