John P. Hope | Searcharxiv

arXiv2026

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.

astro-ph.HE↗

arXiv2025

TeV afterglow emission from a structured GRB jet using the kinetic approach

Recent years have seen a growing sample of TeV emission detections in gamma-ray burst afterglows, as well as an increasing role for structured jets in afterglow modelling. Using a kinetic approach, we show that the structure of an afterglow jet impacts its TeV emission, with jets where the energy falls off more sharply with angle showing a decrease in Inverse Compton (IC) peak flux relative to synchrotron peak flux. We use a modified version of the code katu, to which we have added adiabatic expansion and a fully self-consistent treatment of IC cooling both for the electron and photon populations. We compare our results to the semi-analytical code afterglowpy, finding a good agreement with our model except at early times off axis where the effects of baryon loading are important. We compare electron cooling in the cases where there is no IC cooling, Thomson cooling and an inclusion of Klein-Nishina effects, finding that the spectra can only be distinguished if the Compton potential is significantly increased. We obtain a similar cooling rate compared to semi-analytical solutions, with some small difference at the transition from IC to synchrotron dominated cooling. Finally, we use best-fit parameters determined by afterglowpy and re-scaled for our model to reproduce the light curves of GRB 170817A. For our choice of parameters, we find that GRB 170817 would not have been detected in the TeV domain if seen on-axis, even by the upcoming Cherenkov Telescope Array Observatory.

astro-ph.HE↗