χ² contour plots for the Vela X-1 double-peaked sample fitted with an absorbed cutoff power-law model including a blackbody component (model a, see Table 3). The ellipses are χ²-contours for two parameters (N_H and Γ). The contours correspond to (the projections of this contour to the parameter axes correspond to the 68% uncertainty for one parameter of interest), (68% uncertainty for two parameters of interest), and (90% uncertainty for two parameters of interest). The respective orbital phase bins are indicated.