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

TL;DR

This paper explains the unprecedented 10 MeV emission line in GRB 221009A through a dense circum-stellar medium and pair annihilation in a relativistically expanding shell, linking it to the GRB's progenitor environment.

Contribution

It presents a self-consistent model connecting the MeV line to pair annihilation in a shell influenced by a dense circum-stellar medium, revealing new insights into the GRB's environment and progenitor.

Findings

The 10 MeV line is explained by pair annihilation in a relativistic shell.

The circum-stellar medium density is estimated at 10^8-10^9 cm^-3.

The model constrains the shell's radius to be typical of external shocks.

Abstract

GRB~221009A has been the brightest gamma-ray burst (GRB) observed to date, and its afterglow has been characterised 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, narrow, luminous emission line at around 10 MeV has been uncovered by a detailed spectral analysis of \textit{Fermi}/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 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 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 circum-stellar medium (CSM) extending out to a few $10^{15}\,\mathrm{cm}$, with a density $n_\mathrm{ext}\sim 10^{8}-10^{9}\,\mathrm{cm^{-3}}$ reminiscent of those found from studies of Type IIn supernovae. This provides 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.