Top panel: simulated light curves: the thick black curve represents the one at low energies; the others show three different examples of higher energy data. The left-hand curves show the case when leading sides are delayed with respect to the low energy, but the trailing sides are the same in two cases and slightly different in the third one. In the right-hand curves, different shifts between the pulse maxima and decays are introduced. Bottom panel: the cross-correlation functions for the simulated light curves normalised to the values of zero lag. Colours are the same as the corresponding simulated data.