A self-consistent explanation of the MeV line in GRB 221009A unveils a dense circumstellar medium

GRB 221009A is the brightest gamma-ray burst (GRB) observed to date, and its afterglow has been characterized with unprecedented detail at TeV energies by LHAASO. Quite puzzlingly, it is also the most energetic GRB known. Among the questions posed by this mysterious source, however, the sheer energetics are hardly the most intriguing; an unprecedented, narrow, luminous emission line at around 10 MeV has been uncovered by a detailed spectral analysis of /GBM data immediately following the brightest peak in the GRB prompt emission and the peak of the TeV afterglow. As noted in the discovery article, the temporal evolution of the line properties can be explained as being due to high-latitude emission from a geometrically thin, relativistically expanding shell where annihilation of a large number of electron--positron pairs took place. We show that this interpretation yields stringent constraints on the properties of such a shell, which point to a process that happens at radii typical of external shocks. We then demonstrate that the shell could have been the blastwave associated with the GRB precursor, with the line arising after pair loading of the blastwave as it was illuminated by the bright and hard radiation of the GRB main event. The scenario, which also explains the abrupt initial rise of the LHAASO afterglow, requires the progenitor of the GRB to have been surrounded by a circumstellar medium (CSM) extending out to a few 10^ Fermi 15 , , with a density n_ ∼ 10^ cm ext 8 -10^ 9 , reminiscent of those found from studies of type IIn supernovae. provide a precious clue to the nature of the progenitor of this peculiar GRB, which could also be present in other bursts that feature a long quiescence followed by a bright emission episode with a hard spectrum. cm^ -3 The consequences of such a CSM on the dynamics and emission of the external shock are yet to be fully explored. If future, more detailed work confirms the compatibility of the GRB 221009A afterglow emission with our scenario, this will