  The MAGIC observations of late sub-TeV photons from GRB190114C 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 an Inverse
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 forward shock had Lorentz factor $\simeq 100$ and the
electrons producing the observed sub-TeV radiation had a Lorentz factor $\simeq
10^4$, so that the sub-TeV radiation originates from Comptonization of X-ray
photons at the border between Thomson and Klein-Nishina regimes. The inferred
conditions within the emitting zone are at odds with theoretical expectations
unless one assumes moderate (with $\tau \simeq 2$) 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. However, the temporal
evolution of the Inverse Compton peak energyis very different in these two
models. This opens a way to discriminate between the models once late-time
detection in the TeV range become available.
