Spectral analysis of long gamma-ray bursts as an instrument to investigate the early Universe

This paper presents an analysis of the spectrum of the long and bright gamma-ray burst GRB 180720B with the aim of identifying features in the spectral region associated with nuclear resonances. The resonant gamma-ray absorption method (GRAM) is applied to investigate these features, accounting for the possible presence of absorbers along the line of sight between the GRB source and the observer. The resulting spectral energy distribution indicates the presence of an absorption-like feature around 300 MeV. Based on previous studies of gamma-ray interactions with matter, we suggest that the observed feature may correspond to the delta-resonance, either scattered by one or multiple intervening absorbers, or represented a blend of two separate absorption lines at different energies, merged due to spectral broadening effects.