Cygnus X-2

C. Chevalier, Observatoire de Meudon; S. A. Ilovaisky, Observatoire de Meudon and Centre d'Etudes Nucleaires de Saclay; and G. Branduardi and P. W. Sanford, Mullard Space Science Laboratory, report: "Analysis of photoelectric photometry of the optical candidate for Cyg X-2 (R.A. = 21h42m36s.91, Decl. = +38o05'27".9, equinox 1950.0; Giacconi et al. 1957, Astrophys. J. 148, L129) obtained at the Haute Provence Observatory and of x-ray observations of this source made with the Copernicus satellite in 1974 and 1975 indicates that the available data are consistent with a period of 13.6 days. The optical light curve, which shows intrinsic scatter, exhibits two maxima and two minima per period and has an amplitude of 0.4 magnitude in the B filter. The 1974 x-ray light curve, whose amplitude is of a factor of two, shows a minimum which coincides in phase with one of the optical minima. The 1975 x-ray curve, based on 12 days of observations in June, shows a smooth 45-percent flux modulation with an intensity maximum at midphase (phi = 0.5). Superposed on this are sharp x-ray 'dips' during which the source drops in intensity by as much as a factor of two. The mean optical brightness of Cyg X-2 in the B filter decreased from 1974 to 1975 by 0.3 magnitude, whereas the mean x-ray level increased by about 50 percent. Further correlated observations are in progress.