Speaker
Description
The delayed GeV−TeV cascade emission from extragalactic TeV−PeV sources are regarded as an ideal probe of the inter-galactic magnetic fields. Recently, LHAASO has detected ~10 TeV emission of the extraordinary powerful GRB 221009A within ~2000 s after the burst. Here we report the detection of a ~400 GeV photon, without accompanying prominent low-energy emission, by Fermi-LAT in the direction of GRB 221009A with the time delay of 0.4 days. Such a hard spectrum can be generated from electromagnetic cascades initiated by early primary ~10 TeV photons in the intergalactic space. An inter-galactic magnetic field strength of BIGMF∼4e-17 G, comparable to limits from TeV blazars, can naturally account for the arrival time of the ~400 GeV photon as well as the HAWC non-detection. Such a strength will be stringently tested by the upcoming CTA.
