2019ApJ...880L..27D

MAGIC's observations of late sub-TeV photons from GRB 190114C enable us, for the first time, to determine the details of the emission process in a GRB afterglow and to pin down the physical parameters, such as the bulk Lorentz factor and the Lorentz factor of the emitting electrons as well as some of the microphysical parameters. We find that the sub-TeV emission is synchrotron-self-Compton radiation produced at the early afterglow stage. Combining the sub-TeV and X-ray observations we narrow uncertainties in the conditions inside the emitting zone, almost eliminating them for some parameters. Seventy seconds after the trigger the external shock had a Lorentz factor ≃100, and the electrons producing the observed sub-TeV radiation had a Lorentz factor ≃10⁴, so that the sub-TeV radiation originates from Comptonization of X-ray photons at the border between the Thomson and Klein-Nishina regimes. The inferred conditions within the emitting zone are at odds with theoretical expectations unless one assumes moderate (with τ ≃ 2) absorption of sub-TeV photons inside the source. With this correction the conditions are in good agreement with predictions of the pair-balance model, but are also acceptable for generic afterglow model as one of many possibilities. The different temporal evolution of the inverse Compton peak energy of these two models opens a way to discriminate between them once late-time detection in the TeV range become available.