  We present timing and spectral analysis of ~ 2.2 Ms of 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 ~ 2.1 at high values
of the low frequency \nu_{L}. The saturation level of Gamma~ 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.
