Top panel: α_pow distribution for 55 GRBs with relative uncertainties smaller than 0.3. Mid panel: α_cpl distribution for 77 GRBs with relative uncertainties smaller than 0.5. Bottom panel: alternatively to the cpl model, we show the −α_band distribution for 31 GRBs for which the band function provides an acceptable fit, although not significantly better than the cpl. The vertical dotted and dashed lines show the cases α = 2/3 (synchrotron death line) and α = 3/2 (cooling death line). In each panel dashed distributions show the corresponding best-fitting Gaussian functions.