34.1 day periodicity in CYG X-3

Abstract

COS-B observations of a 34.1-day periodicity in the X-ray intensity of the X-ray source Cyg X-3 are discussed. Collimated proportional counter observations of the source were made in the 2-12 keV energy range for a total of approximately 100 days in three intervals of approximately one month each; data from two of these intervals allow the monitoring of long-term intensity variability. Counting rates averaged over each 4.8-h intensity cycle within the intervals are shown to exhibit an additional 34.1-day periodicity, the maximum time of which is in agreement with Ariel V data suggesting a 16.75-d periodicity. COS-B data support the hypothesis that the long-term periodicity is accounted for by orbital motion in a binary system with high eccentricity, with the 4.8-h modulation due to some rotational mechanism of the compact X-ray emitter. Finally, similarities between Cyg X-3 and the binary system Cir X-1 are pointed out.