Cygnus X-3: The recent X-ray period measurements and gamma-ray astronomy

Abstract

Our knowledge about the nature of Cyg X-3 is briefly reviewed, with particular emphasis on the lack of observational basis for some common assumptions about the object. Some consequences for gamma-ray astronomy are considered of the X-ray period measurements with EXOSAT, which indicate the presence of a large second period-derivative. If the orbital period variations are due to apsidal motion, then the gamma-ray ephemeris is predicted to differ from the X-ray ephemeris, with variable phase shifts of up to ~0.16, and in most models the peak separation in double-peaked gamma-ray light curves is also predicted to vary, by a similar amount.