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

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.

arXiv comment10 pages, 7 figures, accepted for publication in MNRAS, corrected problem with references in previous version

References in corpus (8)

• The Optical Light Curves of Cygnus X-2 (V1341 Cyg) and the Mass of its Neutron Star
• The radio luminosity of persistent X-ray binaries
• The Discovery of a State Dependent Hard Tail in the X-ray Spectrum of the Luminous Z-source GX 17+2
• A Possible Explanation for the "Parallel Tracks" Phenomenon in Low-Mass X-Ray Binaries
• The mass of the neutron star in Cyg X-2 (V1341 Cyg)
• On the Spectral Evolution of Cygnus X-2 along its Color-Color Diagram
• GX 17+2: X-ray spectral and timing behaviour of a bursting Z source
• Normal-Branch Quasi-Periodic Oscillations during the High Intensity State of Cygnus X-2

Cited by in corpus (9)

• Heating in an Extended Accretion Disk Corona along the Z-Pattern in Cyg X-2
• Echoes from the companion star in Sco X-1
• Evolution of cross-correlation and time lag of Cyg X-2 along the branches
• Period--luminosity relation for persistent LMXBs in the near-infrared
• Evolution of cross-correlation and time lag of 4U 1735-44 along the branches
• Optical spectroscopy and Doppler tomography of Cygnus X-2
• Kepler K2 Observations of Sco X-1: Orbital Modulations and Correlations with Fermi GBM and MAXI
• The properties of cross-correlation and spectra of the low-mass X-ray binary 4U 1608-52
• Links Between Optical and X-ray Light in Cygnus X-2