Examples of characteristic power spectra. The solid gray lines show the Lorentzian profiles, and the dashed gray lines a cutoff power law Left: PSD from an observation with one of the hardest spectra in our data set. f  ×  PSD(f) vanishes for low frequencies and shows a double-humped shape, with no requirement for a power law in addition to the two Lorentzians. There is a slight indication for a third Lorentzian profile with a peak frequency between about 40 − 80 Hz. Middle: PSD obtained from a transitional state, requiring an additional power law at low frequencies. Right: PSD in the soft state, with yet stronger contribution of the power law. This sequence of power spectra shows that the Lorentzian peaks shift to higher frequencies as the X-ray spectrum becomes softer.