ИСТИНА

Primary very high energy (VHE, E>100 GeV) gamma-rays from distant (redshift z>0.1) extragalactic gamma-ray sources are partially absorbed on extragalactic background light (EBL) photons by means of the pair production process \gamma\gamma\rightarrow e^{+}e^{-} with the subsequent formation of intergalactic electromagnetic cascades through inverse Compton scattering of secondary electrons (mostly on cosmic microwave background (CMB) photons). These electrons get deflected in the extragalactic magnetic field (EGMF); therefore, the parameters of the observable gamma-ray flux are sensitive to the EGMF strength and structure. The weakest EGMF (magnetic field strength B<100 aG) could, in principle, be probed through the observation of gamma-ray bursts with imaging atmospheric Cherenkov telescopes (IACTs) and space gamma-ray telescopes such as Fermi-LAT. The gamma-ray burst GRB 190114C was detected with the MAGIC IACTs in the 200 GeV – 1 TeV energy range, for the first time allowing us to estimate the observable intensity of cascade gamma-rays robustly. In this work we present detailed calculations of the observable cascade signal for various assumptions about the strenght of the EGMF. We perform an analysis of the publicly available dataset of the Fermi-LAT telescope and derive upper limits on the intensity of the intergalactic pair echo. We show that the sensitivity of Fermi-LAT is not sufficient to obtain any constraints on the EGMF parameters. However, next-generation space gamma-ray observatories would be able to detect pair echoes from GRBs similar to GRB 190114C for the EGMF strength below 1-10 aG.