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

Abstract

Delayed `pair-echo' signal from interactions of very-high-energy s 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 burst 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 $ $ G on the IGMF with correlation length $ l $ larger than 1 Mpc, improving as $ l^ $ 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.