2024ApJ...968L...5W

Gamma-ray bursts (GRBs) have long been considered potential sources of ultrahigh-energy cosmic rays (UHECRs; with energy ≳10¹⁸ eV). In this work, we propose a novel model generating MeV emission lines in GRBs, which can constrain the properties of heavy nuclei that potentially exist in GRB jets. Specifically, we find that relativistic hydrogen-like high-atomic-number ions originating from the β decay of unstable nuclei and/or the recombination entrained in the GRB jet can generate narrow MeV emission lines through the de-excitation of excited electrons. This model can successfully explain the MeV emission line observed in the most luminous GRB ever recorded, GRB 221009A, with suitable parameters including a Lorentz factor γ ∼ 820-1700 and a total mass of heavy nuclei M_tot ∼ 10²³-10²⁶ g. Especially, the emission line broadening can be reasonably attributed to both the expansion of the jet shell and the thermal motion of nuclei, naturally resulting in a narrow width (σ_line/E_line ≲ 0.2) consistent with the observation. Furthermore, we predict that different GRBs can exhibit lines in different bands with various evolving behaviors, which might be confirmed with further observations. Finally, our model provides indirect evidence that GRBs may be one of the sources of UHECRs.