We present timing and spectral analysis of \sim 2.2 Ms of Rossi X-ray Time 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 quasiperiodic oscillations ( QPOs ) and the spectral index .
Presence of two spectral phases ( states ) are 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 \Gamma \sim 2.1 at high values of the low frequency \nu _ { L } .
The saturation level of \Gamma \sim 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 K _ { \alpha } 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 which occur in BHC sources .
We also present the spectral state ( power-law index ) evolution for eight years of Cyg X-1 observations by RXTE .