Search

Gamma-ray bursts (GRBs) have long been considered potential sources of ultrahigh-energy cosmic rays (UHECRs; with energy ≳10 ^18 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 ∼ 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.

Ultrarelativistic jets are believed to play an important role in producing prompt emission and afterglow of gamma-ray bursts (GRBs), but the nature of the jet is poorly known owing to the lack of decisive features observed in the prompt emission. The discovery of an emission line evolving from about 37 to 6 MeV in the brightest-of-all-time GRB 221009A provides an unprecedented opportunity to probe GRB jet physics. The time evolution of the central energy of the line with power-law index −1 is naturally explained by the high-latitude curvature effect. Under the assumption that the line emission is generated in the prompt emission by e ^± pair production, cooling, and annihilation in the jet, we can strictly constrain jet physics with observed line emission properties. We find that the radius of the emission region is r ≳ 10 ^16 cm. The narrow line width of ∼10% requires that the line emission occurs within 10% of the dynamical time, which further implies short timescales of pair cooling to the nonrelativistic state and pair annihilation, as well as a slightly clumpy emission region. If the jet’s Lorentz factor is Γ ≳ 400, the fast cooling requirement needs an energy density of magnetic field in the jet much larger than that of prompt gamma rays, i.e., a magnetically dominated jet. The temporal behavior of line flux suggests some angle dependence of line emission. We also discuss the difficulties of other scenarios for the observed emission line.

The observation of a kilonova AT2017gfo associated with the gravitational wave event GW170817 provides the first strong evidence that neutron star mergers are dominant contributors to the production of heavy r -process elements. Radioactive gamma-ray lines emitted from neutron star merger remnants provide a unique probe for investigating the nuclide composition and tracking its evolution. In this work, we studied the gamma-ray line features arising from the radioactive decay of heavy nuclei in the merger remnants based on the r -process nuclear reaction network and the astrophysical inputs derived from numerical relativity simulations. The decay chain of ${}_{50}^{126}$ Sn ( T _1/2 = 230 kyr) → ${}_{51}^{126}$ Sb ( T _1/2 = 12.35 days) → ${}_{52}^{126}$ Te (stable) produces several bright gamma-ray lines with energies of 415, 667, and 695 keV, making it the most promising decay chain during the remnant phase. The photon fluxes of these bright gamma-ray lines reach ∼10 ^−5 γ cm ^−2 s ^−1 for Galactic merger remnants with ages less than 100 kyr, which can be detected by the high energy resolution MeV gamma-ray detectors like the MASS mission.