Quasi-periodic oscillations in black hole candidates as an indicator of transition between low and high states

By comparing positions on a spectral color-color diagram from 10 black hole candidates (BHCs) observed with Ginga (1354-64, 1826-24, 1630-47, LMC X-1, LMC X-3, GS 2000+25, GS 2023+33, GS 1124-68, Cyg X-1, and GX 339-4) with t he observed broadband noise (BBN) (0.001-64 Hz) and quasi-periodic oscillat ion (QPO) variability, we find that the "very high state" is spectrally int ermediate to the soft/high state and hard/low state. We find a transition p oint in spectral hardness where the dependence of the BHC QPO centroid freq uency (of GS 1124-68 and GX 339-4) on spectral hardness switches from a cor relation to an anticorrelation; where the BBN variability switches from hig h state to low state; and where the spectral hardness of the QPO relative t o that of the BBN variability is a maximum. This coincidence of changing be havior in both the QPO and the broadband variability leads us to hypothesiz e that the QPO is due to interaction between the physical components which dominate the behaviors of BHCs when they occupy the hard/low and soft/high states. We conclude that these QPOs should be observed from BHCs during tra nsition between these two states. Comparison with QPO and BBN behavior obse rved during the 1996 transition of Cyg X-1 supports this hypothesis. We als o report 1-3 Hz QPOs observed in GS 2000+25 and Cyg X-1 in the hard/low sta te, and we compare these to the QPOs observed in GS 1124-68 and GX 339-4.