Gamma-Ray Burst Afterglows in the Multi-Messenger Era

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Gamma-ray bursts (GRBs) are the most luminous events in the Universe, and are capable of powering relativistic jets which interact with the surrounding interstellar medium. Two variants of GRBs are relevant for this research; long GRBs, which are driven by the death of supermassive stars, and short GRBs, which are caused by neutron star mergers. Because of gravitational waves generated by such events, coincident observations can be made on the same objects. This was demonstrated in 2017, when the gravitational wave event GW170817 was used to triangulate and study the gamma-ray burst GRB170817A, which was subsequently seen by optical telescopes. This approach allows for the study of GRB which do not point directly at Earth (off-axis), allowing for the full jet structure to be potentially viewed.Current models of afterglow emission which invoke synchrotron radiation do not explain many features which are found in observed afterglow light curves. Furthermore, energies in the range of TeV have been detected in both long (GRB190114C) and short (GRB160821B) GRBs, suggesting that Compton upscattering may also occur and should be accounted for in models. Off-axis studies of the jet also allows for better studies of its structure and the non-thermal acceleration occuring in different parts of the jet.For this project, I will be making use of several different packages to simulate the jet afterglow of GRBs. Firstly, numerical jet simulations using the moving-mesh hydrodynamic code GAMMA, developed by Ayache for his PhD in Bath, will be performed to study the long-term evolution of structured jets. Using the ScaleFit model fitting tool, a broadband analysis of observational data wil then be done and compared to the results of these simulations. Secondly, the kinetic code Katu, developed by Jimenez for his PhD in Bath, along with the python module afterglowpy, will be used to study the various emission zones found within the jet structure. Combined together, a fuller theoretical picture of the afterglow jet will be obtained. While archival data is available if no observations are successful during the PhD, it is hoped with the next LIGO run, along with the new Japanese gravitational detector KAGRA and the Cherenkov Telescope Array (CTA) coming online in the second half of 2022, that new data will be become available, allowing the model to be tested and refined.The shock acceleration in the jet and the impact of Compton upscattering will be considered in trying to reconcile theoretical models with the observed light-curve. The effect of the jet environment will also be studied, with the edge of the jet being considered to see if it could seed the photons necessary for Compton upscattering. This research will also investigate if short and long GRBs are comparable by studying any similarities or differences in the jet. Publications on the structured jet simulations, the TeV emissions and the application of both to develop and compare the model to observational data will be released as the PhD progresses.This research is in astrophysics, which is one of the research areas of the STFC.