SIMBAD references

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, at around ten hours after the trigger time. We analysed the Fermi (GBM and LAT) and Swift (XRT and BAT) data and describe the evolution of the burst spectral energy distribution in the 0.5keV-10GeV energy range over the first 500s 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 gigaelectronvolts energy range. The LAT (0.1-100GeV) light curve initially rises (F_LAT∝t^2.4), peaks at ∼78s, and falls steeply (F_LAT∝t^–2.2) afterwards. The peak, which we interpret as the onset of the fireball deceleration, allows us to estimate the bulk Lorentz factor Γ₀∼150(300) under the assumption of a circum-burst medium with a wind-like (homogeneous) density profile. 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 0-35s spectrum after its EF(E) peak (at 1-2MeV) is a steep power law extending to hundreds of megaelectronvolts. We derive a steep slope of the injected electron energy distribution N(γ)∝γ^–5. Our fit parameters point towards a very low magnetic field (B'∼1G) in the emission region.