Kinetic Modelling of Gamma-Ray Bursts with Structured Jets

Abstract

Gamma-ray bursts are among the most powerful transient events in the universe. They are characterised by their initial burst of prompt emission in the gamma-ray and X-ray bands, followed by a longer--lived broadband afterglow in the following hours and days. While much research has been done on GRBs over the last 60 years, improvements in detection capabilities have allowed for the discovery of more recent GRBs that raise questions that simpler models are unable to answer. Two particular, and connected, issues are the energetic structure of the GRB jet and the very high energy (VHE) (> 100 GeV) emission that has been seen in a number of GRBs. Structured jets have grown in importance since the discovery of GRB 170817A, a multi-messenger event that was best fitted with a structured, off-axis (relative to the jet normal) jet. VHE emission gives rise to several problems to overcome, including attenuation from extragalactic background light, Klein-Nishina effects in Inverse Compton scattering and generating a sufficient seed photon population to power the emission.

In this thesis, we consider how we can apply a kinetic code to study these two issues. We first consider the historical background and morphology of GRBs, including their progenitors and their prompt/afterglow emission. The three key features of modelling GRBs are also outlined; the jet dynamics, the radiative processes that generate the emission and the translation of the emission to observed flux. Each of these are expanded on, including the types of structured jets we consider, the emission mechanisms to consider and how this can be combined with the flux calculations to produce the spectra and light curves seen in GRBs. Following this, the numerical approach is outlined, including the blazar kinetic code katu and the changes made to it to use it for GRBs.

It is shown that the updated code is able to reproduce the expected GRB emission when compared with afterglowpy, as well as accurately model the effects of IC emission and cooling, including with Klein-Nishina effects. The code is applied to GRB 170817A, where it shows that there was likely VHE emission, but that it was too weak to be detected either on or off-axis, despite the relative closeness of the GRB in question. Katu is then used to study external Compton emission when the kilonovae associated with short GRBs are accounted for, including for a series of different jet structures and observer angles. This is also applied to GRB 170817A, but despite a significant boost in TeV emission, it still is unable to be seen by current or future Cherenkov telescopes. Finally, the code is further extended to consider prompt and reverse shock emission. The latter is shown to have a potential impact on VHE emission if treated as an external Compton source. The thesis concludes with some future outlook and final remarks.

Date of Award	22 Apr 2026
Original language	English
Awarding Institution	University of Bath
Sponsors	Science and Technology Facilities Council
Supervisor	Hendrik Van Eerten (Supervisor), Patricia Schady (Supervisor) & David Tsang (Supervisor)