Abstract
The emission region of γ-ray bursts (GRBs) is poorly constrained. The uncertainty on the size of the dissipation site spans over 4 orders of magnitude (1012–1017 cm) depending on the unknown energy composition of the GRB jets. The joint multiband analysis from soft X-rays to high energies (up to ∼1 GeV) of one of the most energetic and distant GRBs, GRB 220101A (z = 4.618), allows us to make an accurate distinction between prompt and early afterglow emissions. The enormous amount of energy released by GRB 220101A (E iso ≈ 3 × 1054 erg) and the spectral cutoff at MeV observed in the prompt emission spectrum constrain the parameter space of the GRB dissipation site. We put stringent constraints on the prompt emission site, requiring 700 < Γ0 < 1160 and R γ ∼ 4.5 × 1013 cm. Our findings further highlight the difficulty of finding a simple self-consistent picture in the electron–synchrotron scenario, favoring instead a proton–synchrotron model, which is also consistent with the observed spectral shape. Deeper measurements of the time variability of GRBs, together with accurate high-energy observations (MeV–GeV), would unveil the nature of the prompt emission.
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