z-transformed discrete correlation functions (zDCFs) between the light curves of J1109 extracted in the soft and hard X-ray energy bands from XMM-Newton EPIC pn (top panel) and NuSTAR FPMA+FPMB (bottom panel) data using a time step of 2 s (see Sect. 6.2.1 for the definitions adopted for the energy bands). The zDCFs have been normalised so as to account for Poisson noise. Positive time delays correspond to higher energy photons lagging lower energy photons. For plotting purpose, an extended range of time delays is shown, and uncertainties on the values have been omitted.