Best-fit spectral parameters as a function of flux for the main peak (phase interval 0.1–0.5) and the secondary peak (phase interval 0.6–1.0) of the pulse profile. The data points and the linear fit (solid line) corresponding to the main peak are shown in black. The data corresponding to the secondary peak are shown in magenta. As in previous figures, the INTEGRAL observations are marked with circles, the RXTE observations – with triangles. For the upper plot, E_rolloff was fixed to 16.0 keV (see text). For the bottom plot all fit parameters are kept free. The corresponding values for the linear fit and the significance are shown in Table 3.