Exploration of the internal shock parameter space (1). All cases fulfilling the conditions of (i) transparency; (ii) radiative efficiency; and (iii) synchrotron dominance at low-energy (see text) are plotted with black dots in three planes: E_p,obs vs. α (top-left panel); ℰ_ic/ℰ_syn vs. α (bottom-left panel); E_p,obs vs ℰ_ic/ℰ_syn (top-right panel). The ratio ℰ_ic/ℰ_syn does not take into account the fraction of high energy photons that are suppressed by γγ annihilation. In addition, the cases where the synchrotron spectrum peaks in the gamma-ray range (10   keV ≤ E_p,obs ≤ 10   MeV) and the low-energy photon index is in the range −3/2 ≤ α ≤ −1 (resp. −1 ≤ α ≤ −2/3) are plotted in red (resp. blue).