The 78.4-day period of Cygnus XR-1

Abstract

CYGNUS XR-1/HDE226868 is a multiple star system in which the secondary is the best astronomical candidate for a black hole. The existence of a third body in the system could affect the mass derived for the secondary from the analysis of spec-troscopic radial velocity curves¹. Hence, the presence of a periodicity in the system other than that of the 5.6-d binary orbit has a major bearing on the identification of the secondary with a black hole, especially if such a periodicity can be related to the orbital period of a third body. The existence of a 78.4 (or 39.2)-d periodicity in the optical and low-energy X-ray wavelength region of the emission from this system has been reported by Kemp et al.^2,3. The existence of a 39.2-d modulation in the optical and low-energy X-ray luminosities has been questioned by Walker et al.⁴. We report here a search for a 78.4-d modulation in the high-energy X-ray flux observed with OSO 8 and in the U-band optical polarisation previously observed⁵, with negative results. We suggest, however, that if such modulation does exist, it is more likely to be related to the rotation of the free modes of oscillation of the primary than to the existence of a third body in the system.