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National Science Foundation. We thank Miss Elaine Webster The_ Nation~) Ra~!o Astronomy Observatory is operated by for assistance and Mrs Mary Jane Wade for background Associated Untvers1ties, Inc., under contract with the National material from an unfinished book by herself and Otto Struve. Science Foundation. C. M. WADE C. M . WADE R. M. HJELLMING R. M. HJELLMING National Radio Astronomy Observatory, Green Bank, National Radio Astronomy Observatory, West Virginia Green Bank, West Virginia Received December 31, 1971. Received January 3, 1972. ~ B~aes, _L. L. E., and Miley, G. K., Nature, 232,246 (1971). HJeltmmg, R. M., and Wade, C. M. Astrophys. J Lett 168 L2I 1 Wade, C. M., and Hjellming, R. M., Astrophys. J. Lett., 163, (1971). ' . . ., ' Ll05 (1971) . 3 J ., Brucato, R., Visvanathan, N., Lenning, H., and 2 ys. J. Lett. 168 Kristian, Hjellming, R. M., and Wade, C. M ., Astroph Sandage, A., Astrophys. J. Lett., 168, L91 (1971). Lll5 (1971). ' ' 4 3 Wade? C. M., Astroplzys. J., 162, 381 (1970). Wade, C. M., Astrophys. J., 162, 381 (1970). 5 4 Murdm, P., and Webster, B. L., Nature, 235, 37 (1972). Frieboes-Conde, H., Herczeg, T., and Hog, E. Astron. Astrophys. 6 Bolton, C. T., Bull. Amer. Astron. Soc., 3,458 (1971). 4, 78 (1970) . ' ' ; Struve, 0 ., Stellar Evolutio11, 204 (Princeton Univ. Press, 1950). Struve, 0., and Sahade, J., Pub/. Astron. Soc. Pacific 69 41 (1957). . ' ' ; Andrews, P. J., Astrophys. J., 147, 1183 (1967). Identification of X-1 Glushneva, I. N., and Esipov, V. F., Sov. Astron. (A. J.) 11 828 (l 968). . ' ' with HDE 226868 , 235, 271 (1972). ,: Wade, C. M., and Hjellming, R. M., Nature 226868 is closely D~vinney, E. J., Nature, 233, 1IO (1971). THE ninth magnitude BOlb HOE 11 Wilson, R. E., Nature, 234, 406 (1971). coincident with the position of -1 and its associated 1 2 variable radio source . Dolan 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 Position and Identification of the time scale of days in a way that suggests it is being eclipsed. Cygnus X-1 Radio Source Therefore I decided to take photographic spectra of HOE 1 2 226868 to look for velocity and spectrum variations that THE initial measurements • of the position of the radio might be correlated with the X-ray fluctuations. I find that the source associated with the X-ray source Cygnus X-1 were velocity of the star varies with a period consistent with that of uncertain by 3 to 5 arc s. The results were consistent with two 3 the X-ray variation. It may not be possible, however, to inter­ possible identifications with : the 8.9 mag. BO lb star pret the X-ray variations in terms of simple eclipses. HDE 226868 ( = BD + 34° 3815), and a 15 mag. red star 9 arcs The spectrograms were taken between mid-September and to the north-west of the B star. Because of the apparent mid-November 1971 with the 74-inch telescope of the David normality of the B star and possible signs of peculiarity in the Dunlap Observatory in Richmond Hill, Ontario. All except red star3, it was not clear which should be identified with the one were taken at a dispersion of 12 A mm_,, but because of radio source. Here, we report the resolution of the ambiguity the star's faintness and unfavourable weather conditions it was by an accurate determination of the radio position. necessary to use projected slit widths of 40 to 50 ~tm . These The observations were made at frequencies of 2,695 and observations have been supplemented by results from 5 to 8,085 MHz with the NRAO three-element interferometer at 40 A mm- ' plates taken in one night with the 90-inch reflector Green Bank from October 28 to October 31, 1971. The of the Steward Observatory of the University of Arizona. baseline lengths were 900, 1,800 and 2,700 m. Cygnus X-1 These plates were obtained and measured by Dr Roberta was observed concurrently with sixty other radio sources in a Humphreys, who communicated the results to us before pub­ programme of precise position measurements. The method has 4 lication. She also provided some additional observations of been described previously ; certain refinements, however, were emission features in the spectrum, and Mr David L. DuPuy introduced in the analysis to eliminate the principal sources of of the David Dunlap Observatory obtained UBV photometry systematic error. The procedures and results for the sixty of HOE 226868 on three nights. sources will be described in detail later. The results of the measures are given in The position of the Cygnus X-1 radio source, with its Table I and the velocity curve is plotted in Fig. I for a period standard errors, is found to be:

U1950= 19 h 56 m 28.87 s±0.02 s Table 1 Radial Velocity Measurements 81950= +35° 03 ' 55.0" ±0.3". Inter- The AGK2 position of HOE 226868 is Julian date V, Standard stellar Hl3 Disper- deviation K-line emission sion 1 U1950= 19 h 56 m 28.88 S (km s- ') (km s- ) (km s-') (km s-') fAmm-') 81950= +35° 03 ' 54.9". *24412I0.660 + 15 ± 9 -19.5 43 1213.665 -24 ± 9 -15.7 +176 12 The nearly perfect positional coincidence of the radio source tl217.588 -58 ±10 -14.3 + 39 12 and the star leaves no doubt that HOE 226868 is the correct +74.7 1224.581 -41 ± 7 -10.7 +154 12 identification. 1225.696 +33 ± 9 -15.6 12 During the three-day observing period, the flux density for 1237.579 +57 ± 15 -28.3 12 Cygnus X-1 was (l2±3)x 10-29 Wm 2 Hz- ' at 2,695 MHz 1245.555 -48 ±14 -12.6 12 and (15 ± 3) x 10 - 29 W m - 2 Hz-1 at 8,085 MHz. 1252.549 -62 ± 9 -15.6 12 5 6 1259.524 +56 ± 7 -14.4 12 It is now known • that HOE 226868 is a spectroscopic 12 6 1261.573 -15 ± 8 -17.2 binary with a period of 5.6 days. Bolton has found variable t]263.7 -65 ± 6 40 emission lines resembling those of P Cygni in high-dispersion spectra of this star. This strongly supports the identification * Unidentified emission feature at 1,.4519.JO A. of the radio and X-ray sources with the HDE 226868 binary t Uncertain identification of Hl3 emission. system. t Mean of five plates taken by Dr Roberta Humphreys.

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