  Utilizing observations obtained with the Rossi X-ray Timing Explorer, we
examine correlations between the properties of 0.08-22 Hz variable frequency
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
multi-temperature 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 exeption: Both sources exhibit a general
increase in the QPO frequency as the disk flux increases (or as the mass
accretion rate 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.
