High Energy Emission

The first observations of GeV emission from GRBs were obtained by EGRET (the Energetic Gamma-Ray Experiment Telescope, [38]) on board the *Compton Gamma Ray Observatory* in flight 1991–2000 [39,40]. The duration of high energy emission was often longer than the emission at keV, and showed a distinct temporal evolution [41,42].

The *Fermi* LAT instrument (the Large Area Telescope; [43]) is sensitive to *γ*-rays in the energy range ∼30 MeV to ≥300 GeV. Since its launch in 2008, it asserted several new observational characteristics at energies >100 MeV [44]: (i) Many of the bright GRBs could not be fitted with commonly used models consisting of the low- and high-energy power law, and an additional power law component was required to fit the high-energy portion of the spectrum abd (ii) the emission above 100 MeV tends to be delayed with respect to emission at lower (sub-MeV) energies. When high energy emission was detected, it started during the prompt phase in >60% of the cases. Given the *Fermi* LAT field of view, this fraction may be even higher; (iii) the high energy emission lasts systematically longer than the sub-MeV prompt emission, and the high energy flux often follows a power law decay ∼t −1 . Recently, the two Imaging Atmospheric Cherenkov Telescopes, MAGIC and H.E.S.S. telescopes, reported the observations of the very high energy emission [6,7]. The *γ*-rays from GRB 190114C were observed in the energy range 0.2–1 TeV starting 57 s after the burst onset. The prompt emission duration of this event was ∼116 s by *Fermi* GBM and ∼362 s by *Swift* BAT. The observed very high energy emission was associated with the inverse Compton component in the afterglow phase, however the contribution from the prompt emission at early times could not be excluded [45].
