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Gamma-ray bursts (GRBs) comprise of short, bright, energetic flashes ofemission from extragalactic sources followed by a longer afterglow phase ofdecreased brightness. Recent discoveries of very-high-energy (VHE, $\gtrsim100$ GeV) afterglow emission from GRB 180720B and GRB 190829A by H.E.S.S. haveraised questions regarding the emission mechanism responsible. We interpretthese observed late-time emission to be the result of inverse Compton emissionof ultra-relativistic electrons in the GRB blastwave in an external radiationfield, i.e., external Compton (EC), considering both the wind and interstellarmedium scenarios. We present predictions of multiwavelength light curves andenergy spectra, ranging from optical to VHE, and include the synchrotron andsynchrotron self-Compton (SSC) radiation mechanisms as well. We corrected theEC and SSC model for the $\gamma$-ray attenuation by absorption of photonsthrough their interaction with the extragalactic background light (EBL). Wecompared our results to multiwavelength data and found that EC gives asatisfactory fit for a given set of fixed model parameters for GRB 180720B,whereas SSC result in a better fit for GRB 190829A. For both GRBs a windenvironment is preferred over constant density inter-stellar medium, and theCosmic Microwave Background as the external radiation field. However, with moredata and an effective optimisation tool we can find a more robust fit of themodel, implying better constraints on the GRB environment and the particleenergy requirements for the emission observed at late times. This hasconsequences for future observations of GRBs at these extreme energies.

Very high-energy gamma rays from GRB 180720B and GRB 190829A with external Compton emission