  We investigated the relation between the centroid frequency of the
quasi-periodic oscillation observed in the power density spectra of a sample of
galactic black-hole candidates with the power-law photon index obtained from
spectral fits. Our aim is to avoid inner accretion disk radius determination
directly from spectral fits, given the uncertainties of the absolute values
obtained in that way, but to base our analysis on the likely association of QPO
frequency to a characteristic radius. We used archival RXTE data of GRS
1915+105 and published parameters for GRO 1655-40, XTE J1550-564, XTE J1748-288
and 4U 1630-47. While for low values of the QPO frequency, the two parameters
are clearly correlated for each source, there is evidence for a turnoff in the
correlation above a characteristic frequency, different for different sources.
We discuss the possible nature of this turnoff.
