Confirmation of two cyclotron lines in Vela X-1

• Published in: Astronomy and astrophysics (Berlin) 2002-11, Vol.395 (1), p.129-140
• Main Authors: Kreykenbohm, I., Coburn, W., Wilms, J., Kretschmar, P., Staubert, R., Heindl, W. A., Rothschild, R. E.
• Format: Article
• Language: English
• Subjects: stars: magnetic fields; stars: pulsars: individual: Vela X-1; X-rays: stars
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20021181

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.