The orbit of the supergiant component of Vela X-1 derived from IUE radial velocities

et al. 2001

A&A

161

175

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 rootmean-square amplitudes of ∼7 km s −1 for strong lines of Si iv and N iii near 4100Å, and up to ∼20 km s −1 for weaker lines of N ii and Al iii near 5700Å. 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 km s −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, Kopt = 21.7 ± 1.6 km s −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 Mns sin 3 i = 1.78 ± 0.15 M .

Section: Introductionmentioning

confidence: 83%

Section: Reanalysis Of the Iue Spectramentioning

confidence: 99%

Section: Reanalysis Of the Iue Spectramentioning

confidence: 99%

See 1 more Smart Citation

The mass of the neutron star in Vela X-1

Barziv

Kaper

et al. 2001

A&A

161

175

“…As a consequence the radial velocities are biased systematically towards zero and the radial-velocity amplitude will be underestimated. A discrepancy of about 4 km s −1 had been found between the radial-velocity amplitude for the ultraviolet spectra of Vela X-1 analyzed by both groups (see Barziv et al 2001 andStickland et al 1997). The reason for the much larger discrepancy of 10 km s −1 for HD 153919 might be the weaker and broader photospheric lines in this star combined with the better signal-to-noise ratio which give more striking errors when interstellar and wind lines are set to zero.…”

Section: Comparison With Earlier Resultsmentioning

confidence: 96%

“…In a similar study of the high-mass X-ray binary HD 77581/Vela X-1 Barziv et al (2001) found a disagreement in the semi-amplitude of the radial velocity curve with the results of Stickland et al (1997) for this system. This strengthened our decision to re-analyse the ultraviolet spectra of HD 153919 again.…”

Section: Introductionmentioning

confidence: 86%

The radial velocity curve of HD 153919 (4U 1700-37) revisited

Hammerschlag-Hensberge

Kaper

2003

A&A

Abstract.We have re-analysed all available high-resolution ultraviolet IUE spectra of the high-mass X-ray binary HD 153919/4U 1700-37. The radial velocity semi-amplitude of 20.6 ± 1.0 km s −1 and orbital eccentricity of 0.22 ± 0.04 agree very well with the values obtained earlier from optical spectra. They disagree with earlier conclusions for the same data reduced by Heap & Corcoran (1992) and by Stickland & Lloyd (1993).

Neutron Star Mass Determinations

Black Holes in Binaries and Galactic Nuclei: Diagnostics, Demography and Formation

I review attempts made to determine the properties of neutron stars. The focus is on the maximum mass that a neutron star can have, or, conversely, the minimum mass required for the formation of a black hole. There appears to be only one neutron star for which there is strong evidence that its mass is above the canonical 1.4 solar masses, viz., Vela X-1, for which a mass close to 1.9 solar masses is found. Prospects for progress appear brightest for studies of systems in which the neutron star should have accreted substantial amounts of matter.Comment: 6 pages, 3 figures, to appear in Proc. ESO Workshop on "Black Holes in Binaries and Galactic Nuclei", Garching (Sept. 1999), L. Kaper, E.P.J. van den Heuvel, P.A. Woudt eds., Springer-Verla