2000ApJ...531..537S

Utilizing observations obtained with the Rossi X-Ray Timing Explorer, we examine correlations between the properties of 0.08-22 Hz variable-frequency quasi-periodic oscillations (QPOs) and the X-ray spectral parameters for the black hole candidates XTE J1550-564 and GRO J1655-40. The spectra were fitted to a model including a multitemperature blackbody disk and a power-law component. We find that the QPO frequency and amplitude are well correlated with the spectral parameters, although the correlations found for XTE J1550-564 are generally opposite to those for GRO J1655-40. There is one exception: both sources exhibit a general increase in the QPO frequency as the disk flux increases (or as the rate of mass accretion through the disk increases). In addition, these QPOs are observed only when the power-law component contributes more than 20% of the 2-20 keV flux, which indicates that both the disk and the power-law components are linked to the QPO phenomenon.