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

TL;DR

This study uses Fermi/LAT and LHAASO data to set a lower bound on the intergalactic magnetic field strength along the line of sight to GRB 221009A, confirming its presence in cosmic voids.

Contribution

It provides the first combined analysis of Fermi/LAT and LHAASO data to constrain the intergalactic magnetic field using gamma-ray observations of a specific GRB.

Findings

Lower bound of 10^{-19} G on IGMF strength established.

Correlation length dependence of the bound derived as l^{-1/2}.

Independent verification of IGMF existence in cosmic voids.

Abstract

Delayed "pair echo" signal from interactions of very-high-energy gamma rays in the intergalactic medium can be used for detection of the inter-galactic magnetic field (IGMF). We use the data of Fermi/LAT telescope coupled with LHAASO observatory measurements to confirm the presence of IGMF along the line of sight to the gamma-ray burst GRB221009A. Comparing the Fermi/LAT measurements with the expected level of the pair echo flux, set by the multi-TeV LHAASO detection, we derive 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 existence of a lower bound on IGMF in the voids of the Large Scale Structure, previously derived from the observations of active galactic nuclei.