Detection of 2 MeV Spectral Bump and ∼40 MeV Emission Line from GRB 221009A

Gamma-ray bursts (GRBs) are the most energetic transients in the Universe, but prompt MeV–sub-GeV observations remain challenging with current gamma-ray observatories. Here, using unsaturated HEPP-H measurements on board CSES-01 and cross calibrating with likewise unsaturated GECAM-C data, we reconstruct the continuous spectral evolution of GRB 221009A over 2–255 MeV. We identify an unusual spectral bump at ∼2 MeV in the first light-curve peak. We also confirm a narrow emission line whose energy decreases from ∼40 MeV following a t −1 evolution, and whose flux shows a “rise-then-fall” profile, implying weak emission from the central jet. These findings hint that a magnetically dominated, central-empty jet exists in the brightest of all time GRB.