Candidate Spectral Lines at 6 and 15 MeV from GRB 200829A and GRB 160905A

Abstract

The discoveries of the evolving spectral line in gamma-ray burst (GRB) 221009A provide valuable information for understanding its extreme physical environment and emission mechanisms. In this work, we perform a comprehensive search for 3941 GRBs recorded in the Fermi/Gamma-ray Burst Monitor (GBM) from 2008 July to 2025 February; although no confirmed spectral lines are found, two spectral line candidates at ∼15 and ∼6 MeV in GRB 160905A and GRB 200829A are identified, respectively. Interestingly, the latter’s rest-frame line energy ∼13 MeV at redshift z = 1.25 is similar to that of GRB 221009A at z = 0.151. If this is true, it implies that they may share a common physical origin, possibly linked to blueshifted electron–positron annihilation processes within relativistic jets. Finally, this comprehensive search reveals no >5σ significant line features across the GBM GRB sample, with only two candidates at ∼3σ level. These results place meaningful constraints on the peculiarity and possible origin of the spectral line observed in GRB 221009A.