The Gamma Ray Burst ( GRB ) 180720B is one of the brightest events detected by the Fermi satellite and the first GRB detected by the H.E.S.S .
telescope above 100 GeV .
We analyse the Fermi ( GBM and LAT ) and Swift ( XRT and BAT ) data and describe the evolution of the burst spectral energy distribution in the 0.5 keV–10 GeV energy range over the first 500 seconds of emission .
We reveal a smooth transition from the prompt phase , dominated by synchrotron emission in a moderately fast cooling regime , to the afterglow phase whose emission has been observed from the radio to the GeV energy range .
The LAT ( 0.1–100 GeV ) light curve initially rises ( F _ { LAT } \propto t ^ { 2.4 } ) , peaks at \sim 78 s , and falls steeply ( F _ { LAT } \propto t ^ { -2.2 } ) afterwards .
The peak , which we interpret as the onset of the fireball deceleration , allows us to estimate the bulk Lorentz factor \Gamma _ { 0 } \sim 150 ( 300 ) under the assumption of a wind-like ( homogeneous ) circum-burst medium density .
We derive a flux upper limit in the LAT energy range at the time of H.E.S.S .
detection , but this does not allow us to unveil the nature of the high energy component observed by H.E.S.S .
We fit the prompt spectrum with a physical model of synchrotron emission from a non-thermal population of electrons .
The 0–35 s spectrum after its EF ( E ) peak ( at 1–2 MeV ) is a steep power law extending to hundreds of MeV .
We derive a steep slope of the injected electron energy distribution N ( \gamma ) \propto \gamma ^ { -5 } .
Our fit parameters point towards a very low magnetic field ( B ^ { \prime } \sim 1 G ) in the emission region .