Example for the dependence of F(E) given by Eq. (10) on the cutoff energy E. The example is based on the HGPS data for the source HESS J1858+020. The maximum likelihood estimate for an exponential cutoff in the γ-ray spectrum (Eq. (3) with β = 1) is indicated with a green dashed line. The blue and the magenta lines indicate the critical value F_crit and the inferred lower limit on E_cut,  γ at 90% CL. The magenta region is the excluded range of cutoff energies.