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Some gamma-ray bursts (GRBs) have an afterglow in the tera-electronvolt (TeV)band, but the early onset of this afterglow has not been observed. We reportobservations with the Large High Altitude Air Shower Observatory of the brightGRB 221009A, which serendipitously occurred within the instrument field ofview. More than 64,000 photons (above 0.2~TeV) were detected within the first3000 seconds. The TeV photon flux began several minutes after the GRB trigger,then rose to a flux peak about 10 seconds later. This was followed by a decayphase, which became more rapid at $\sim 650\,{\rm s}$ after the peak. Theemission can be explained with a relativistic jet model with half-opening angle$\sim 0.8^\circ$, consistent with the core of a structured jet. Thisinterpretation could explain the high isotropic energy of this GRB.

A tera-electronvolt afterglow from a narrow jet in an extremely bright gamma-ray burst 221009A