Evolution of the spectrum slope under various conditions. Because the exact quantity of energy injected at each pulse is only weakly constrained, this quantity is left as a free parameter, which can vary between 10⁵⁶ and 10⁵⁷ erg. The corresponding range of values obtained are represented by the colored surfaces, while the dotted (dashed) line represents the case where a total CR energy of 10⁵⁶ erg (10⁵⁷ erg) is injected from the CS at t = 0. The left panel shows the influence of the propagation mode (i.e. isotropic or anisotropic). The right panel shows the influence of the source intermittence. The results of our two simulations with intermittent source, i.e. with multiple injections separated in time by period Δt are presented for Δt = 1 Myr and Δt = 3 Myr. The simulation with single anisotropic injections is also represented by the solid line for comparison.