Comparison between the time-averaged SEDs of the secondary cascade output (pair echo emission, color solid lines) and the MAGIC (black dash-dotted lines) and the CTA-North (black dotted lines) sensitivity curves for three different observational times (respectively from left to right: 3 h, 6 h, 9 h). Two different cases are shown: GRB 221009A, z = 0.15 (upper panel) and a scaled GRB 221009A with redshift z = 1 (bottom panel).