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

K. O'Brien·Keith Horne·Richard Gomer·J. B. Oke·M. van der Klis
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Abstract

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 instrument on Keck II to obtain spectra with a mean time resolution of 72.075 milliseconds, simultaneous with pointed X-ray observations using the PCA onboard XTE. In this paper, we have analysed the variability in both wavebands on timescales of 16 seconds. 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<S_z<1) but then falls on the Normal Branch (1<S_z<2) and Flaring Branch (2<S_z<3), where S_z is a rank number characterising the position on the Z-curve. This linear increase in the optical flux with S_z 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 S_z. We have used this correlation to decompose the total X-ray count-rate into three distinct spectral components.

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K. O'Brien, Keith Horne, Richard Gomer, J. B. Oke, M. van der Klis. 2004-02-05. High-speed Keck II and RXTE spectroscopy of Cygnus X-2: (I) Three X-ray components revealed by correlated variability. https://doi.org/10.1111/j.1365-2966.2004.07667.x

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