High-speed Keck II and RXTE spectroscopy of Cygnus X-2 - I. Three X-ray components revealed by correlated variability

We have performed simultaneous X-ray and optical spectroscopic observations of the low-mass X-ray binary Cygnus X-2. We have used a new data system attached to the Low Resolution Imaging Spectrograph (LRIS) instrument on Keck II to obtain spectra with a mean time resolution of 72.075 ms, simultaneous with pointed X-ray observations using the Proportional Counter Array (PCA) onboard RXTE. In this paper, we have analysed the variability in both wavebands on time-scales of 16 s. During our observations Cygnus X-2 covered all three branches of the Z-curve, allowing us to study how the changes in X-ray spectral state affect the optical emission. As the optical flux rises, the X-ray intensity first rises on the horizontal branch (0 < Sz < 1) but then falls on the normal branch (1 < Sz < 2) and flaring branch (2 < Sz < 3), where Sz is a rank number characterizing the position on the Z-curve. This linear increase in the optical flux with Sz indicates the optical flux is a good predictor of the accretion rate (possibly normalized by its own long-term average) inferred from the Z-state Sz. We have used this correlation to decompose the total X-ray count rate into three distinct spectral components.