An explanation of GRB Fermi-LAT flares and high-energy photons in stratified afterglows

Abstract

ABSTRACT The second Fermi/LAT gamma-ray burst (GRB) Catalogue (2FLGC) spanning the first decade of operations by the Large Area Telescope (LAT) collaboration was recently released. The closure relations of the synchrotron forward shock (FS) model are not able to reproduce a sizeable portion of the afterglow-phase light curves in this collection, indicating that there may be a large contribution from some other mechanism. Recently, synchrotron self-Compton (SSC) light curves from the reverse shock (RS) regions were derived in the thick- and thin-shell regimes for a constant-density medium, and it was demonstrated that analytical light curves could explain the GeV flare observed in several bursts from 2FLGC, including GRB 160509A. Here, we generalize the SSC RS scenario from the constant density to a stratified medium, and show that this contribution helps to describe the early light curves exhibited in some Fermi/LAT-detected bursts. As a particular case, we model a sample of eight bursts that exhibited a short-lasting emission with the synchrotron and SSC model from FS and RS regions, evolving in a stellar-wind environment, constraining the microphysical parameters, the circumburst density, the bulk Lorentz factor, and the fraction of shock-accelerated electrons. We demonstrate that the highest energy photons can only be described by the SSC from the FS region.