X-ray Spectral States and High-Frequency QPOs in Black Hole Binaries
  The high-frequency quasi-periodic oscillations in the X-ray emission of black hole binaries and candidates are briefly examined with respect to the question of X-ray states. We use the prescription of McClintock and Remillard (2005), in which X-ray spectra and power density spectra are used to differentiate three main states of active accretion: "thermal", "hard", and "steep power-law". For the sources that exhibit pairs of HFQPOs with frequencies that scale in a 3:2 ratio, the QPO detections are associated with the steep power-law state. Furthermore, there is a clear trend in which the oscillation at twice the fundamental frequency is seen when the source has high X-ray luminosity, while the QPO at three times the fundamental frequency is seen at lower luminosity. 
R. A. Remillard (MIT Kavli Institute for Astrophysics and Space   Research)