Long-Term Temporal Variability of Cygnus X-2

Observations of Cyg X-2, spanning 5 yr and using six different instruments on three satellites (OSO 8, HEAO 1, and Einstein), are presented. Irregular changes in intensity of up to 60% on time scales ranging from minutes to days are seen. The source appears to have long-term low-luminosity states during which dips lasting ≡3 days occur. When these long dips occur, they always appear at the same phase in the optically determined 9.8 day orbital period. There are three distinct types of short-term dips distinguished by differing correlations between spectral hardness and intensity. Both the temporal and the spectral variations are consistent with a scenario in which emission from an intrinsic thermal source surrounded by a hot accretion disk corona is modified by instabilities in a tilted accretion disk.