Abstract
THE 6.9 mag B0.5 Ib supergiant HD77581 has been identified as the optical counterpart of the X-ray eclipsing binary system 3U0900–40 (Vela X-1)1,2, which eclipses with a period of 8.95 ± 0.02 d. The discovery of regular X-ray pulses in Vela X-l has also been reported3, with a mean pulse period of 282.9 s, modulated because of the radial velocity variation of the X-ray pulsar in its orbital motion. This makes Vela X-l the third X-ray binary system in which the orbits of both the optical and the X-ray component can be studied, and the following orbital parameters have been derived4, ex = 0.15±0.05, wx = 157° ± 24° and Kx = 268 ±12 km S−1. Analyses of the light curve of HD77581 have shown that the heating effect is too small to be detected2. Furthermore, the star is bright and the spectral lines are not very broad. This means that here the first relatively accurate direct mass determination of both the X-ray and the early-type supergiant components of an eclipsing X-ray binary becomes possible.
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VAN PARADIJS, J., HAMMERSCHLAG-HENSBERGE, G., VAN DEN HEUVEL, E. et al. Mass determination for the X-ray binary system Vela X-1. Nature 259, 547–549 (1976). https://doi.org/10.1038/259547a0
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DOI: https://doi.org/10.1038/259547a0
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