Short-time-scale correlations between line and continuum fluxes in Cygnus X-1

We present the results of 16-s time-scale spectral fits for Cygnus X-1 in the soft state and in the transition state, using a Comptonized blackbody plus an iron line. On these time-scales, we find that that the continuum source flux can vary by factors 2-3 and that the iron-line intensity appears to track these changes well, i.e. the inferred equivalent width of the line remains constant to within the errors. We also find no significant changes in the seed (blackbody) photon temperature, while the properties of the Comptonizing corona clearly do vary, with the spectral hardness and flux generally being correlated. The corona therefore seems to be the overall driver for the rapid time-scale variability observed in the soft and transition states. These results are consistent with the Fourier resolved spectroscopy results of Gilfanov, Churazov & Revnivtsev that indicate the iron line shows rapid flux variations while the blackbody component does not, and suggest that the iron-line flux, in fact, tracks continuum changes down to very short time-scales. We extend this work by showing that not only the variability amplitudes, but also the phases of the iron line and continuum components are identical. We note that the short-time-scale variability properties of the soft and transition states are actually not very different from those of the hard state, suggesting that the corona is the main cause of rapid variability in that state too, and hence that the mechanism responsible for the corona is similar in all three states.