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

K O'Brien, Keith Douglas Horne, Richard H Gomer, JB Oke, M van der Klis

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9 Citations (Scopus)


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<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 characterizing 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.

Original languageEnglish
Pages (from-to)587-595
Number of pages9
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Publication statusPublished - 11 May 2004


  • binaries : close
  • stars : individual : Cyg X-2
  • X-rays : binaries
  • V1341 CYGNI
  • GX 17+2
  • X-2
  • MASS


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