UHE γ-ray (ε_γ ≳ 10¹⁸ eV) injection model. Pair echo synchrotron emission flux assuming L_γ,iso = 10⁵⁴ erg s⁻¹, E_p,iso ≃ 6 × 10⁵¹ erg, f_esc → 1. The dashed vertical line corresponds to the characteristic energy of synchrotron radiation at a time Δt ≃ 192 days after the initial trigger. The dashed vertical line marks the synchrotron photon energy for which the characteristic pair echo time spread is Δt_syn ≃ 192 days.