Nissim Fraija, Jorge Alexis Montes, Sara Fraija-Castellanos, María Magdalena González

Gamma-ray bursts (GRBs) are among the most luminous transients in the Universe and constitute prime targets for multimessenger studies, particularly in connection with gravitational-wave events. The detection of very-high-energy (TeV) photons from GRBs would provide valuable constraints on the physical conditions in the outflow, including the bulk Lorentz factor, circumburst density, radiation processes, and microphysical parameters. The possible detection of TeV emission temporally associated with an optical-infrared kilonova (KN), as suggested for GRB 160821B, presents a challenge to standard synchrotron self-Compton scenarios. In this work, we explore an alternative mechanism in which TeV photons are produced during the afterglow phase via external inverse Compton (EIC) scattering. In this scenario, electrons accelerated in the reverse shock upscatter seed photons originating from the KN. We derive the corresponding EIC light curves and spectra for a reverse shock evolving in the thin-shell regime within a constant-density medium, and apply the model to GRB 160821B. We further constrain the parameter space for TeV detectability by incorporating the high KN luminosity observed in AT2017gfo, as well as flux upper limits reported by H.E.S.S. and HAWC. We find that TeV emission is more likely under conditions of very low magnetic energy fraction, $ε_{\rm B_r} \lesssim 10^{-6}$, combined with a bright KN and relatively low redshift. This mechanism predicts TeV photons on timescales of hours to a few days after the burst.