log ₁₀(S_o/S_f) plotted against the log of the total counts, C_T, measured from the optimised aperture. The grey points indicate the data and include only clean (SUM_FLAG = 0), point-like (EP_EXTENT = 0) detections. The red line links measurements of the average log ₁₀(S_o/S_f), in bins sampling the range in C_T, for cases where −1 < log ₁₀(S_o/ S_f) < 1. The blue line is similar but is for the subset of data where, additionally, the background rate is >10^-8 cts s^-1 (sub-pixel)^-2 (sub-pixels have side lengths of 0.05′′). The lower X-axis limit reflects the minimum threshold of 100 total counts in the optimised extraction aperture, imposed for extracting XMM source spectra; the plot is otherwise truncated for clarity.