Astronomy & Astrophysics

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Issue		A&A Volume 377, Number 3, October III 2001
Page(s)		925 - 944
Section		Formation, structure and evolution of stars
DOI		10.1051/0004-6361:20011122

A&A 377, 925-944 (2001)
DOI: 10.1051/0004-6361:20011122

The mass of the neutron star in Vela X-1

O. Barziv^{1, 2}, L. Kaper², M. H. Van Kerkwijk³, J. H. Telting⁴ and J. Van Paradijs^{2, 5}

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
² Astronomical Institute "Anton Pannekoek" , University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Isaac Newton Group of Telescopes, Netherlands Organisation for Scientific Research (NWO), Apartado 321, 38700 Santa Cruz de La Palma, Spain
⁵ Physics Department, University of Alabama, Huntsville, AL35899, USA

(Received 19 February 2001 / Accepted 6 August 2001 )

Abstract
We measured the radial-velocity curve of HD 77581, the B-supergiant companion of the X-ray pulsar Vela X-1, using 183 high-resolution optical spectra obtained in a nine-month campaign. We derive radial-velocity amplitudes for different lines and wavelength regions, and find all are consistent with each other, as well as with values found in previous analyses. We show that one apparent exception, an anomalously low value derived from ultra-violet spectra obtained with the International Ultraviolet Explorer, was due to an error in the analysis procedures. We re-analyse all IUE spectra, and combine the resulting velocities with the ones derived from the new optical spectra presented here, as well as those derived from optical spectra published earlier. As in previous analyses, the radial velocities show strong deviations from those expected for a pure Keplerian orbit, with root-mean-square amplitudes of ~7 kms^-1 for strong lines of Si4 and N3 near 4100 Å, and up to ~20 kms^-1 for weaker lines of N2 and Al3 near 5700 Å. The deviations likely are related to the pronounced line-profile variations seen in our spectra. Our hope was that the deviations would average out when a sufficient number of spectra were added together. It turns out, however, that systematic deviations as a function of orbital phase are present as well, at the 3 kms^-1 level, with the largest deviations occurring near inferior conjunction of the neutron star and near the phase of maximum approaching velocity. While the former might be due to a photo-ionisation wake, for which we observe direct evidence in the profiles of H $\delta$ and H $\alpha$ , the latter has no straightforward explanation. As a result, our best estimate of the radial-velocity amplitude, $K_{\rm opt}=21.7\pm1.6$ kms^-1, has an uncertainty not much reduced to that found in previous analyses, in which the influence of the systematic, phase-locked deviations had not been taken into account. Combining our velocity amplitude with the accurate orbital elements of the X-ray pulsar, we infer $M_{\rm ns}\sin^3{i}=1.78\pm0.15$ $M_\odot$ .

Key words: equation of state -- stars: binaries: eclipsing -- stars: early-type -- stars: fundamental parameters -- stars: neutron -- stars: pulsars: individual: Vela X-1

Offprint request: L. Kaper, lexk@astro.uva.nl

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