2023MNRAS.525.1630F

The Second Gamma-ray Burst Catalogue (2FLGC) was announced by the Fermi Large Area Telescope (Fermi-LAT) Collaboration. It includes 29 bursts with photon energy higher than 10 GeV. Gamma-ray burst (GRB) afterglow observations have been adequately explained by the classic synchrotron forward-shock model, however, photon energies greater than 10 GeV from these transient events are challenging, if not impossible, to characterize using this afterglow model. Recently, the closure relations (CRs) of the synchrotron self-Compton (SSC) forward-shock model evolving in a stellar wind and homogeneous medium was presented to analyse the evolution of the spectral and temporal indexes of those bursts reported in 2FLGC. In this work, we provide the CRs of the same afterglow model, but evolving in an intermediate density profile (∝r^–k) with 0 ≤ k ≤ 2.5, taking into account the adiabatic/radiative regime and with/without energy injection for any value of the electron spectral index. The results show that the current model accounts for a considerable subset of GRBs that cannot be interpreted in either stellar-wind or homogeneous afterglow SSC model. The analysis indicates that the best-stratified scenario is most consistent with k = 0.5 for no-energy injection and k = 2.5 for energy injection.