Short term X-ray rms variability of Cyg X-1

A linear dependence of the amplitude of broadband noise variability on flux for GBHC and AGN has been recently shown by Uttley & McHardy (2001). We present the long term evolution of this rms-flux-relation for Cyg X-1 as monitored from 1998-2002 with RXTE. We confirm the linear relationship in the hard state and analyze the evolution of the correlation for the period of 1996-2002. In the intermediate and the soft state, we find considerable deviations from the otherwise linear relationship. A possible explanation for the rms-flux-relation is a superposition of local mass accretion rate variations.