Nature 235, 271 - 273 (04 February 1972); doi:10.1038/235271b0

Identification of Cygnus X-1 with HDE 226868

C. T. BOLTON

David Dunlap Observatory, University of Toronto, Richmond Hill, Ontario

THE ninth magnitude BOIb star HDE 226868 is closely coincident with the position of Cygnus X-1 and its associated variable radio source¹. Dolan² has pointed out that Cyg X-1 appears to be a two component X-ray source. One component has a synchrotron spectrum, and the other has a thermal (or bremsstrahlung) spectrum. The latter component varies on a time scale of days in a way that suggests it is being eclipsed. Therefore I decided to take photographic spectra of HDE 226868 to look for velocity and spectrum variations that might be correlated with the X-ray fluctuations. I find that the velocity of the star varies with a period consistent with that of the X-ray variation. It may not be possible, however, to interpret the X-ray variations in terms of simple eclipses.

References

1.	Braes, L. L. E., and Miley, G. K., Nature, 232, 246 (1971).
2.	Dolan, J. F., Space Sci. Rev., 10, 830 (1969).
3.	Plavec, M., Pub. Astron. Soc. Pacific, 82, 957 (1970).
4.	Dolan, J. F., Nature, 233, 109 (1971).
5.	Matteson, J. L., Bull. Amer. Astron. Soc., 3, 456 (1971).
6.	Brini, D., Ciriegi, U., Fuligni, F., Moretti, E., and Vespignani, G., Astrophys. J., 149, 429 (1967).
7.	Agrawal, P. C., Gokhale, G. S., Iyengar, V. S., Kunte, P. K., Manchanda, R. K., and Sreekantan, B. V., Nature, 232, 38 (1971).
8.	Hiltner, W. A., and Mook, D., Ann. Rev. Astron. Astrophys., 8, 139 (1970).
9.	Kraft, R. P., and Demoulin, M., Astrophys. J. Lett., 150, L83 (1967).