Upper panels: spectral model parameters obtained from the PCA data in the 8–40 keV energy range; F_PL(8 − 40   keV) is the 8–40 keV flux in units of 10^-10   erg   s^-1   cm^-2, Γ the PL photon index. Different symbols mark in all panels the three phases of the oubursts discussed in Sect. 3. Bottom left panel: fractional rms computed from the background subtracted RXTE/PCA light curves in the 2–25 keV energy range as a function of time. Bottom right panel: fractional rms vs. intensity. The average count rate is background-subtracted and refers to PCU2. Arrows indicate temporal evolution.