Light curves at 1 TeV (top panel) and spectra at light-curve peak (bottom panel) of the moderate reference case (see Table 6) for varying external medium densities n_ext. The case fitting the afterglow of GW 170817 corresponds to n_ext = 4.25 × 10⁻³ cm⁻³. Each line is labelled with the multiplication factor applied to the reference n_ext. The peak times at which spectra are calculated are 123, 60, 26, 12, and 0.55 days. The dotted line in the top panel corresponds to the assumed CTA sensitivity in Sect. 5.