MeV absorption in gamma-ray bursts as a probe of their progenitor environments

A small fraction of X-ray photons from gamma-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon--photon absorption between the incoming MeV γ-rays and the back-scattered X-rays generates electron--positron pairs, enriching the surrounding medium with leptons. We investigated how these back-scattered photons modify the prompt GRB spectrum through γ-γ absorption. In a dense and pair-loaded environment, the emerging spectra exhibit a broad absorption feature, whose profile is sensitive to the low-energy spectral index, α. In particular, spectra with α > -1 develop a pronounced, saddle-shaped absorption between 1 and 100 MeV (rest frame). Such external MeV absorption could account for the spectral curvature seen in some bright GRBs and may point to a dense circumstellar medium (CSM) around their progenitor stars --- consistent with early observations of core-collapse supernovae. In this scenario, the blast wave caused by the GRB is expected to start off with a relatively low Lorentz factor and undergo an acceleration phase when traversing the large density drop at the interface between the dense CSM and the surrounding medium. The impact of these nontrivial dynamics on the afterglow emission is yet to be explored.