a)Left panel: lags between the two bands, [100–150] and [200–250] keV, as a function of the peak energy of the pulse spectrum. The spectral indices at low and high energy are α = −1 and β = −2.3. The curves correspond to the same three cases of spectral evolution considered in Fig. 2; 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 that continuously rises at low E_p corresponds to a power-law spectrum with α = −1 and an exponential cut-off.