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

Abstract

GRB~221009A has been 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 riddles posed by this mysterious source, however, the sheer energetics are hardly the most intriguing: an unprecedented emission line at around 10 MeV has been uncovered by a detailed spectral analysis of Fermi/GBM data immediately following the brightest peak in the GRB prompt emission and the peak of the TeV afterglow. The temporal evolution of the line properties can be explained as 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 shell, that 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 such blastwave as it was illuminated by the bright and hard radiation of the GRB. 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 circum-stellar medium (CSM) extending out to a few 10^15 cm, with a density 10^8-10^9 cm-3 reminiscent of those found in Type IIn supernovae. 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 will confirm the compatibility of the GRB 221009A afterglow with our scenario, this will 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.