We present timing and spectral analysis of ~2.3 Ms of Rossi X-Ray Timing Explorer (RXTE) archival data from Cyg X-1. Using a generic Comptonization model we reveal that the spectrum of Cyg X-1 consists of three components: a thermal seed photon spectrum, a Comptonized part of the seed photon spectrum, and the iron line. We find a strong correlation between the 0.1-20 Hz frequencies of quasi-periodic oscillations (QPOs) and the spectral index. The presence of two spectral phases (states) is clearly seen in the data when the spectral indices saturate at low and high values of QPO frequencies. This saturation effect was discovered earlier in a number of black hole candidate (BHC) sources, and now we strongly confirm this phenomenon in Cyg X-1. In the soft state this index-QPO frequency correlation shows a saturation of the photon index Γ~2.1 at high values of the low frequency νL. The saturation level of Γ~2.1 is the lowest value found yet in BHCs. The bolometric luminosity does not show clear correlation with the index. We also show that Fe Kα emission line strength (equivalent width [EW]) correlates with the QPO frequency. The EW increases from 200 eV in the low/hard state to 1.5 keV in the high/soft state. The observational correlations revealed compel us to propose a scenario for the spectral transition and iron line formation that occur in BHC sources. We also present the spectral state (power-law index) evolution for 8 yr of Cyg X-1 observations by RXTE.
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