Integrated photon flux or 90% CL upper limit (UL) with respect to point-like emission at the center of the ROIs versus the number of co-added ROIs. The stacking is performed for simulated ROI samples that contain central sources with photon indices −2.0, −2.4, −2.8 and −3.2. For the analysis, power-law spectra are used that apply the same fixed photon indices as in the corresponding simulations. The stacking is also performed for ROIs that contain central sources with photon index −2.0 using a test source with photon index −2.4 in the analysis. The grey and blue shaded areas correspond to the statistical uncertainties on the integrated flux values. All central sources are simulated with an integrated flux of 7.5 × 10^-10 ph/(cm² s), indicated by the dashed line.