  Long-duration GRBs are the most luminous sources of electromagnetic radiation
known in the Universe. Their initial prompt flashes of MeV gamma rays are
followed by longer-lasting afterglow emission from radio waves to GeV gamma
rays. Emission at TeV energies had been theoretically predicted, but never
confirmed by observations. Here we report the detection of a huge signal from
GRB 190114C in the TeV energy range by the MAGIC imaging atmospheric Cherenkov
telescopes. Starting one minute after the onset of the burst, gamma rays in the
energy range 0.2 -1 TeV were observed at more than 50 sigma level. This allowed
us to study the spectral and temporal development of the GRB, revealing a new
emission component in the afterglow with a power comparable to that of the
synchrotron component. We found a second peak in the spectral energy
distribution of the GRB at an energy of few hundred GeVs. Our modeling, based
on the data from the two dozen space- and ground-based instruments that
followed GRB 190114C at multiple wavelengths, supports the explanation that the
second peak is due to the Inverse Compton radiation mechanism. The two-peaked
structure of the spectral energy distribution allows us to constrain some of
the key physical parameters of the GRB as the bulk Lorentz factor, minimal
electron energy, the ratio of the radiation to magnetic field density. Recently
also the H.E.S.S. imaging atmospheric Cherenkov telescope reported on a 5 sigma
gamma-ray signal from the GRB 180720B, measured in the afterglow phase, 10
hours after the onset of the explosion. These observations prove that the GRBs
are more powerful than assumed until recently. Because the observed GRBs did
not show peculiar properties, we believe that from now on detection of
gamma-ray signal from GRB afterglows at very high energies will become one of
the standard observations.
