Top panel: observed peak energy E_p vs. the power-law index α of the PDS (15−150 keV) for a sample of GRBs with known redshift and well-constrained time–averaged energy spectrum. The shaded area highlights the BAT energy passband. Bottom panel: same plot in the intrinsic plane, i.e. where the peak energy E_p,i refers to the GRB comoving frame. Circles (triangles) correspond to pl (bpl) model. Median errors are shown (top right). The dashed line shows the case α = 2.