Power-law decay indices before (α₁, in red) and after the break (α₂, in blue) for the BPL subsample (see Sect. 3.2). The green shaded area highlights the region that can be explained with the standard afterglow model (assuming the slow-cooling regime and a constant circumburst medium; e.g. Sari et al. 1998), that is 0.75 < α₁ < 2.5. All the values we find for α₁ cannot be explained with this model, suggesting the need for an alternative interpretation (see Sect. 3.3 for further details).