Consequences on energetics of a steepening of the low-energy slope in the “Band” function. Left: the normalized “Band” function is plotted for β = −2.25 and α = −1.5 (standard value for synchrotron radiation in fast cooling regime) or α = −1 (mean value observed in GRB spectra). The dashed area corresponds to the fraction f_e of the energy that should be removed by any process leading to such a change of the low-energy photon index. Here f_e ≃ 19%. Center: the fraction f_e is now plotted as a function of β, assuming a low-energy photon index changing from α = −1.5 to −1. Right: the fraction f_e is now plotted as a function of α, when the photon index is changing from α = −1.5 to α. We assume β = −2.25. These three figures are plotted assuming that the process responsible for the change of the slope α does not affect the value of the break energy E_b nor the high-energy tail of the spectrum.