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[31.15] The primary drivers of spectral variability in Cyg X-1

A. Zdziarski (N. Copernicus Astr.\ Ctr., Warsaw, Poland), L. Wen (MIT)

We study Cyg X-1 observations by the RXTE/ASM, RXTE/PCA, and CGRO/BATSE. We find that correlations seen in the data between X-ray intensity and hardness ratios can be interpreted in terms of simple physical models. In the hard/low state, the main driver of the variability appears to be a variable flux of soft seed photons cooling a hot thermal plasma. An increase of the soft flux increases the measured X-ray flux but, at the same time, it reduces the plasma temperature, which, in turn, softens the X-ray spectrum, explaining the observed negative correlation between the X-ray hardness and the X-ray flux. We discuss possible accretion scenarios fitting this variability pattern. On the other hand, the X-ray spectrum in the soft/high state consists of a disk blackbody and a high-energy tail. We model the observed spectral variability by variable non-thermal power supplied to flares above an optically-thick, stable, accretion disk. An increase of the non-thermal power results in an increase of the tail amplitude without changing its shape, which explains then the observed positive correlation between the X-ray hardness and the X-ray flux.