Observational constraints on the models for Cygnus X-3.

Observations of Cygnus X-3 made by the ANS and Uhuru satellites have shown significant X-ray flux variability on a time scale of hours. No spectral change was observed associated with these fluctuations. Significant intensity and spectral changes were found on a time scale of months with an inverse correlation between the source intensity and the temperature, and possibly the low-energy cutoff. The spectrum studies have confirmed the existence of an excess above the fitted continuum which can be interpreted as line emission of Fe XXIV and/or Fe XXV at about 6.7 keV. Phenomenological models for Cygnus X-3 are discussed; the observational data are best interpreted by assuming that a scattering and absorbing shell is symmetrically placed around the X-ray source.