A delayed 400 GeV photon from GRB 221009A and implication on the intergalactic magnetic field

TL;DR

This paper reports the detection of a delayed 400 GeV photon from GRB 221009A, suggesting it is likely due to cascade emission caused by intergalactic magnetic fields, with implications for understanding gamma-ray propagation.

Contribution

It provides the first detection of a delayed 400 GeV photon from this GRB and analyzes its origin, favoring cascade emission over Synchrotron Self-Compton models.

Findings

Delayed 400 GeV photon detected by Fermi LAT.

Cascade emission explanation is more probable than SSC.

Probabilities of detecting such photons are quantified.

Abstract

Large High Altitude Air Shower Observatory has detected $0.2-13$ TeV emission of GRB 221009A within 2000 s since the trigger. Here we report the detection of a 400 GeV photon, without accompanying prominent low-energy emission, by Fermi Large Area Telescope in this direction with a 0.4 days' delay. Given an intergalactic magnetic field strength of about $4 \times 10^{-17}$ G, which is comparable to limits from TeV blazars, the delayed 400 GeV photon can be explained as the cascade emission of about 10 TeV gamma rays. We estimate the probabilities of the cascade emission that can result in one detectable photon beyond 100 GeV by Fermi Large Area Telescope within $0.3-1$ days is about 2$\%$ whereas it is about 20.5$\%$ within $0.3-250$ days. Our results show that Synchrotron Self-Compton explanation is less favored with probabilities lower by a factor of about $3-30$ than the cascade scenario.