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L. Titarchuk (GMU/NRL), N. Shaposhnikov (GSFC/USRA)
We present timing and spectral analysis of ~ 2.2 Ms of Rossi X-ray Time Explorer RXTE archival data from Cyg X-1. Using generic Comptonization model we reveal that the spectrum of Cyg X-1 consists of three components: a thermal seed photon spectrum, a Comptonized component of the seed photon spectrum and iron line. We find a strong correlation between 0.1-20 Hz frequencies of quasiperiodic oscillations (QPOs) and the spectral power-law 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 ~ 2.1 at high values of the low frequency. The saturation level of the photon index ~ 2.1 is the lowest value found yet in BHCs. The data show three spectral states : low/hard, intermediate, and high/soft (steep power-law) states when QPO frequency (features) are detected. The bolometric luminosity does not show clear correlation with the index in the low/hard state and it gradually increases with the spectral index during the intermediate state. Then the luminosity decreases with the index when no QPO is detected. We also show that the presence of Fe Kalpha emission-line strengths (EW) and its correlation with the QPO frequency, when EW increases from 200 eV in the low/hard state to 1.5 keV in the high/soft state.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.