The impact of kilonova seed photons on GRB VHE emission

Over the last several years, there have been a number of GRBs with very high energy (VHE) emission in excess of 100 GeV, and even above 1 TeV, detected. In several instances, synchrotron seed photons do not fully explain the emission observed, suggesting the presence of other seed photon sources to up-scatter. In this work, we consider the kilonova as a source of seed photons for up-scattering in the afterglow. We model the kilonova as a thermal source injecting into the back of a GRB fireball, evolved using a shell model, and with the electron and photon populations updated via a kinetic solver. We find that VHE emission from weaker afterglows, such as those found in short GRBs, can be affected by such seed photons, with the kilonova seed photons mitigating the loss of synchrotron photons on the VHE emission when afterglow parameters are varied. We also find that VHE emission in structured jets, due to weaker synchrotron emission at their wings, can also benefit from this supply of seed photons, especially when viewed off-axis. We apply this model to GRB 170817A, and show that its VHE spectral flux is higher than expected in previous models for the first 100 d, though still below the detection threshold.