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Very-high-energy (VHE) photons around TeV energies from a gamma-ray burst(GRB) jet will play an essential role in the multi-messenger era, with a fairfraction of the events being observed off-axis to the jet. We show thatdifferent energy photons (MeV and TeV photons in particular) arrive fromdifferent emission zones for off-axis observers even if the emission radius isthe same. The location of the emission region depends on the jet structure ofthe surface brightness, and the structures are generally different at differentenergies, mainly due to the attenuation of VHE photons by electron-positronpair creation. This off-axis zone-shift effect does not justify the usualone-zone approximation and also produces a time-delay of VHE photons comparableto the GRB duration, which is crucial for future VHE observations, such as bythe Cherenkov Telescope Array.

Off-axis MeV and very-high-energy gamma-ray emissions from structured gamma-ray burst jets