Are There Three Peaks in the Power Spectra of GX 339-4 and Cyg X-1?
  Among the variability behaviour exhibited by neutron star systems are the so-called ``horizontal branch oscillations'' (HBO, with frequencies ~50 Hz), the ``lower-frequency kHz quasi-periodic oscillation'' (QPO) and the ``upper-frequency kHz QPO'', with the latter two features being separated in frequency by an amount comparable to, but varying slightly from, the suspected spin-frequency of the neutron star. Recently, Psaltis, Belloni, & van der Klis (1999) have suggested that there exists a correlation between these three frequencies that, when certain identifications of variability features are made, even encompasses black hole sources. We consider this hypothesis by reanalyzing a set of GX 339-4 observations. The power spectral density (PSD) constructed from a composite of 7 separate, but very similar, observations shows evidence for three broad peaks in the PSD. If the peak frequencies of these features are identified with ``QPO'', then their frequencies approximately fit the correlations suggested by Psaltis, Belloni, & van der Klis (1999). We also reanalyze a Cyg X-1 observation and show that the suggested QPO correlation may also hold, but that complications arise when the ``QPOs'' (which, in reality, are fairly broad features) are considered as a function of energy band. These fits suggest the existence of at least three separate, independent physical processes in the accretion flow, a hypothesis that is also supported by consideration of the Fourier frequency-dependent time lags and coherence function between variability in different energy bands. 
Michael A. Nowak