Evidence for a 17-d periodicity from Cyg X-3

Abstract

Cyg X-3 (3U2030+40) has exhibited phenomena which are observationally unique among identified X-ray sources. The giant radio flare of 1972 is, perhaps, the most spectacular such anomaly, but there are others no less unusual. A wide variation in X-ray spectra has been observed, including the identification of X-ray emission lines at some times, and consistency with a black body at others¹. The approximately sinusoidal 4.8-h variation² is at a period far in excess of any rotation period which has been ascribed to the compact members of other binary sources, and at least four times shorter than any comparable orbital period. Models have been constructed which identify the 4.8-h variation with orbital period^3–5, acknowledging the peculiar geometries which could give rise to a smooth 4.8-h effect in observed X rays. We here present data indicating that a much longer periodicity of ∼17 d is also characteristic of Cyg X-3.