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

¹ Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa City, Chiba 277-8582, Japan
e-mail: vovk@icrr.u-tokyo.ac.jp
² Université Libre de Bruxelles, CP225 Boulevard du Triomphe, 1050 Brussels, Belgium
³ Université de Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁴ Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland

Received: 28 June 2023
Accepted: 22 October 2023

Abstract

Delayed ‘pair-echo’ signal from interactions of very-high-energy γ 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 γ-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.