Evolution of β with time for individual GRBs. For every GRB we considered the “correct” spectral index as selected in Table C.8, hence β can be β_op,X or β_X depending on the chosen SED fitting function, a single power-law or a broken power-law. Blue dotted lines: the initial spectral slope is steeper than the final spectral slope. Red dotted lines: the initial spectral slope is flatter than the final spectral slope. Light blue stars: steep decay data. Magenta dots: plateau data. Orange squares: normal decay data. Black: only one SED is available for these GRBs and precisely during the steep decay (stars), the plateau (dots), and normal decay (squares). Inset: the same as the principal plot, but in the rest frame.