Time evolution of the expected ‘pair echo’ and registered GRB 221009A signal. Top: comparison of measured GRB 221009A Fermi/LAT signal in the 0.1 − 10 GeV energy range with the models of the pair echo emission for different IGMF correlation length and strength combinations, stemming from the LHAASO measurements of its multi-TeV flux. For the time intervals with no significant detection (TS < 9), the 95% confidence level upper limits are given. Short timescale fluctuations visible in the curves are numerical artifacts of the ‘echo’ signal calculation. Bottom: GRB 221009A detection test statistics (TS). Horizontal lines denote the TS = 25 and TS = 9 levels, roughly reflecting the 5σ and 3σ detection significances correspondingly.