A Novel Model for the MeV Emission Line in GRB 221009A

Gamma-ray bursts (GRBs) have long been considered potential sources of ultra-high-energy cosmic rays (UHECRs; with energy ≳ 10^18 eV). In this work, we propose a novel model generating MeV emission lines in GRB, 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∼ 10^23 - 10^26 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.