Constraints on the intergalactic magnetic field from Fermi-LAT observations of GRB 221009A

TL;DR

This paper uses Fermi-LAT observations of GRB 221009A to set the most stringent limits to date on the intergalactic magnetic field strength, constraining it to be greater than 2.5 x 10^{-17} G for large coherence lengths.

Contribution

It provides the tightest constraints on the intergalactic magnetic field using gamma-ray burst observations, avoiding assumptions required by previous methods.

Findings

Constrained the IGMF to be > 2.5 x 10^{-17} G at 95% confidence.

Achieved limits comparable to blazar-based constraints.

Method avoids assumptions about source duty cycle and plasma instabilities.

Abstract

A cosmological origin of the magnetic fields in large scale structures of the Universe would require a non-negligible magnetic field in cosmic voids, which, however, remains undetected. Gamma-ray emission from gamma-ray bursts (GRBs) offers the opportunity to indirectly probe such an intergalactic magnetic field (IGMF), as gamma rays interact with cosmic radiation fields, producing electron-positron pairs, and initiate an electromagnetic cascade. The deflection of the pairs in the IGMF results in a time-delayed signal at GeV energies. Using observations with the Fermi Large Area Telescope of the GRB 221009A, we are able to derive the most stringent constraints to date from the non-observation of the cascade and rule out magnetic fields B < 2.5 x 10^{-17} G at 95% confidence level for a coherence length larger than 1 Mpc. Our results are comparable to limits obtained from blazar observations but do not suffer from assumptions on the duty cycle of the gamma-ray source or whether inverse-Compton scattering losses dominate over the development of plasma instabilities.