Number of events that come from Fermi sources as a function of the radius of the search circle (red data points). Error bars indicate fluctuations in the background when determining the signal from the sources. Average number of background photons is shown with green line. Vertical line at 0.1 degree is the radius of the 68% containment circle of Fermi PSF. The vertical line at 0.17 degree shows the position of the dip in the probability shown in Fig. 1. The horizontal line indicates the asymptotic number of photons from all the sources.