Issue |
A&A
Volume 377, Number 3, October III 2001
|
|
---|---|---|
Page(s) | 925 - 944 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20011122 | |
Published online | 15 October 2001 |
The mass of the neutron star in Vela X-1*,**
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
Corresponding author: L. Kaper, lexk@astro.uva.nl
Received:
19
February
2001
Accepted:
6
August
2001
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 Hδ and Hα, the latter has no straightforward
explanation. As a result, our best estimate of the radial-velocity
amplitude,
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
.
Key words: equation of state / stars: binaries: eclipsing / stars: early-type / stars: fundamental parameters / stars: neutron / stars: pulsars: individual: Vela X-1
© ESO, 2001
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