Evolution of the best fit spectral parameters for each GRB displaying a low-energy break. First panel: light curve of the GRB, in the energy range 8–900 keV. The vertical lines mark the time bins selected for the time-resolved analysis. Panels below: temporal evolution of all best fit parameters (of the 2SBPL or SBPL function, according to which model fits best the spectrum of the time bin). While all the parameters of the SBPL fit are shown as red points, different colours have been used to represent the parameters of the 2SBPL fit. In particular, from top to bottom, second panel: photon indices α₁ (yellow points) and α₂ (green points) of the 2SBPL function, and α (red points) of the SBPL function. Third panel: two photon indices β (in purple for the 2SBPL function and in red for SBPL). Fourth panel: E_break (blue points) and E_peak (green points) for 2SBPL, and E_peak for SBPL (red points). Bottom panel: ratio E_peak/E_break.