a)Left panel: lag-luminosity relation for α = −1, β = −2.3, t_p/ C = 0.3 s, δe_p = 0.5, and δa = δb = 0 (dotted line), 0.1 (full line), and 0.2 (dashed line). The full line that extends to large lag values at low luminosity corresponds to a power-law spectrum with α = −1 and an exponential cut-off; 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).