Constraints on the intergalactic magnetic field from Fermi/LAT observations of the 'pair echo' of GRB 221009A

Delayed 'pair-echo' signal from interactions of very-high-energy gamma rays in the intergalactic medium can be used for the detection of the intergalactic magnetic field (IGMF). We used the data of the Fermi/LAT telescope coupled with LHAASO observatory measurements to confirm the presence of IGMF along the line of sight to the gamma-ray burst GRB 221009A. Comparing the Fermi/LAT measurements with the expected level of the pair-echo flux, set by the multi-TeV LHAASO detection, we derived a lower bound 10(-19) G on the IGMF with correlation length l larger than 1 Mpc, improving as l(-1/2) for shorter correlation lengths. This provides an independent verification of the existence of a lower bound on IGMF in the voids of the large-scale structure, previously derived from the observations of active galactic nuclei.