Constraints on the intergalactic magnetic field strength from $\gamma$-ray observations of GRB 221009A

TL;DR

This paper uses gamma-ray observations of the exceptionally bright GRB 221009A to set a new lower limit on the strength of intergalactic magnetic fields by simulating cascade emissions and comparing with observational flux limits.

Contribution

It introduces a novel constraint on the intergalactic magnetic field using TeV gamma-ray data from a gamma-ray burst, expanding the methods beyond blazar observations.

Findings

Lower limit on IGMF strength: B ≥ 10^{-18.5} G

Simulation of cascade emissions with ELMAG code

Independent constraint from a new class of TeV sources

Abstract

Characteristics of the cascade gamma-ray signal resulting from very-high-energy gamma-ray sources, such as gamma-ray bursts, can be used to constrain the strength and structure of intergalactic magnetic fields (IGMF). There has been a debate on whether GRB 190114C, the first gamma-ray burst with observed TeV photons, can constrain the IGMF. Recently, LHAASO detected the brightest-of-all-time GRB 221009A, which has much larger energy in TeV band and the spectrum extends to energy above 10 TeV, providing an unprecedented opportunity to studying IGMF. We perform a Monte-Carlo simulation of the cascade process with the public ELMAG code, considering the TeV data of GRB 221009A observed by LHAASO. By comparing the resulting cascade emission with the flux limit obtained from Fermi-LAT observations, we infer a limit of $B\ge 10^{-18.5}\rm G$ for IGMF. Though this limit may not be as strong as the limit from blazars, it serves as an independent constraint on IGMF from a new class of TeV sources.