Cygnus X-2

A. P. Smale, NASA/Goddard Space Flight Center and Universities Space Research Association; E. Kuulkers, Astrophysics Division, ESTEC, European Space Agency; and R. A. D. Wijnands, Astronomical Institute 'Anton Pannekoek', University of Amsterdam, report: "The public RXTE ASM lightcurve (covering the interval 1996 Feb. 22-Aug. 8) of the 9.8-day low-mass x-ray-binary Cyg X-2 shows a pronounced intensity modulation with a period of 77.0 +/- 1.0 days and full amplitude of 40 percent. A shallower (20 percent) secondary minimum is also observed. The spectral hardness and source intensity are anticorrelated on this cycle. We find that this long-term periodicity can be detected independently in the HEASARC archival datasets from Vela 5B (see also Smale and Lochner 1992, Ap.J. 395, 582) and the Ariel V ASM. From these three datasets we derive a single overall ephemeris from 1969 to 1996 of HJD 2442202.5 (+/- 5.0) + 77.13 (+/- 0.05) E (where phase zero is the primary minimum). The occurrence of different intensity states in the Einstein, EXOSAT and Ginga data from Cyg X-2 (Kuulkers et al. 1996, A.Ap. 311, 197) is also consistent with this 77.1-day periodicity, which we tentatively associate with the precession of a tilted accretion disk in the system."