Dependence of the model spectrum on cut-off energy and age of the supernova remnant (left panel) and uncertainty in the flux of galactic cosmic rays (middle panel). The right-hand panel shows the spectrum for the case that two of the eight photons with energy > 1 PeV can be attributed to the microquasar Cygnus X-3. In addition to a change in spectral shape, the energy required to match the normalisation of the data increases for lower cut-off energies and older supernova remnants. The displayed models require 4.5, 6, and 2.1 × 10⁵⁰ erg in non-thermal protons for an age of 80 kyr and a cut-off energy of 1 PeV and 3 PeV, respectively.