a)Left panel: lags between the low energy band [10–20] keV and a moving band [E-1.25E] as a function of E (in keV) for a pulse with t_p/ C = 0.3 s and a Band spectrum (n = 1 in Eq. (4)) with E_p = 500 keV, α = −1, β = −2.3 for three different cases of spectral evolution: δe_p = −0.5 and δa = δb = 0 (dotted line), 0.1 (full line), and 0.2 (dashed line); b) right panel: same as a) for the case δe_p = −0.5 and δa = δb = 0.1 (full line), but also with a modified Band spectrum with n = 2 (dotted line) and n = 0.67 (dashed line). The full line with no break at 500 keV corresponds to a power-law spectrum with α = −1 and an exponential cut-off.