Ensemble structure function for the sources studied in this work. Blue points represent the averaged SF values and take advantage only of XMM-Newton observations, while the red point is obtained also using data from ROSAT. The red point refers to 220 AGN pointed by both the satellites and is computed cutting the XMM-Newton fluxes to the ROSAT flux threshold as discussed in the text. The yellow, black circled point is computed without removing the XMM-Newton fluxes fainter than the threshold, and it strongly overestimates the amount of variability. The black dashed line is the contribution of the noise (as defined in Eq. (2)) for the whole investigated time-lag interval. Small dots represent the variations for the individual pairs of measurements contributing to the SF and the colours emphasise the different subsamples used during the SF calculation.
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