The γ-ray flux of the 5 shells considered. The color code goes from the inner to the outer shell: red, green, blue, yellow, magenta. The black line is the total flux from the molecular cloud, i.e. the sum of the 5 shells, and can be compared with Fig. 10 of Aharonian & Atoyan (1996). The two panels show the example for a continuous accelerator, with D₁₀ = 10²⁶ cm² s^-1 (slow diffusion, left) and D₁₀ = 10²⁸ cm² s^-1 (fast diffusion, right). In the case of fast diffusion, the spectrum is similar for all the shells, in particular the blue and yellow lines are coincident. The accelerator age is 10⁴ years.