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P.T. Boyd (JCA/UMBC & LHEA/GSFC), A.P. Smale (NASA Headquarters), M.J. Tripicco (SSAI & LHEA/GSFC)
The low-mass X-ray binary Z-source Cygnus X-2 displays a high amplitude, non-periodic long-term X-ray modulation with a timescale on the order of five to ten times its 9.8 day orbital period. The long-term modulation is often attributed to a warped, precessing accretion disk, however if this model is correct then the precession period is not stable. There are now (June 2004) just over 200 pointed RXTE observations of the Cyg X-2, spanning more than 3000 days. These observations cover fairly evenly the range of observed RXTE count rates (600-2400 counts/s/PCU). We use this collection of observations to investigate the evolution of the Z-shape in the color-color diagram and search for correlations with the long-term modulation. We also compare the time variability of the X-ray components of a common spectral model for Cyg X-2 with the long-term flux variability and the Z-shape evolution.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.