Closure relations of synchrotron self-Compton from reverse shock and <i>Fermi</i>-LAT GRBs

ABSTRACT Synchrotron radiation from the reverse- and forward-shock regions typically describes the evolution of temporal and spectral features given by the closure relations (CRs) during the late and long-/short-lasting emission in the afterglow phase of gamma-ray bursts (GRBs). Although synchrotron photons are restricted to keV and a few MeV energies, the synchrotron self-Compton (SSC) mechanism can disperse them above hundreds of MeV energies. We present the CRs of the SSC process radiated from the reverse-shock region for the case of a thick and thin shell, considering that the reverse shock lies in the adiabatic regime and evolves in an environment with a homogeneous and stratified medium. We analyse these CRs with the spectral and temporal characteristics of the bursts described in the second Fermi-LAT GRB catalogue (2FLGC) and found that (i) the thin shell case is preferred over a thick shell and a constant-density medium over a stellar-wind environment, (ii) bursts with an atypical and hard spectral index could be successfully described by this scenario in different cooling conditions, (iii) the early optical flash and GeV emission exhibited in GRB 160625B and 180720B were generated from the same accelerated region and electron population, concluding that LAT emission originated during the early afterglow, and (iv) the maximum synchrotron energy radiated from the reverse-shock scenario could explain only a few photons with the exception of a pair of bursts, so that scattered photons by the SSC process must be required.