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

The Large High Altitude Air Shower Observatory (LHAASO) detected 0.2–13 TeV emission from GRB 221009A within 2000 s of the trigger. This paper reports the Fermi Large Area Telescope's (LAT) detection of a delayed 400 GeV photon, lacking accompanying low-energy emission, approximately 0.4 days post-trigger. Assuming an intergalactic magnetic field (IGMF) strength around 4 × 10⁻¹⁷ G (consistent with limits from TeV blazars), the delayed photon is explained as cascade emission from ~10 TeV gamma rays. The probabilities of such cascade emission resulting in one detectable photon above 100 GeV within 0.3–1 days and 0.3–250 days are estimated at ~2% and ~20.5%, respectively. The Synchrotron Self-Compton (SSC) explanation is less likely, with probabilities 3–30 times lower.

Zi-Qing Xia, Yun Wang, Qiang Yuan, Yi-Zhong Fan