Contours at 90% confidence level (Δχ² = 4.61 for two parameters) between the photon index Γ and the high energy cut-off (E_c, top panel) and the reflection fraction (middle). Contours in the bottom panel refer to the high energy cut-off and the reflection fraction. All the contours have been computed with the column density, the photon index, the high energy cut-off, the reflection fraction, and the xillver normalisation free to vary in all the observations.