Gamma-ray bursts as multi-messenger probes
2025/09/29

The first and so far only multimessenger observation of a binary neutron star (BNS) merger in 2017 marked the beginning of gravitational wave (GW) multimessenger astrophysics. For the first time, it became possible to study heavy element nucleosynthesis through the spectral features of a kilonova (KN) and to monitor an off-axis gamma-ray burst (GRB) afterglow, thereby accessing the structure of its ultra-relativistic jet. However, no other joint detection of GWs and electromagnetic (EM) radiation has occurred since. The first part of this thesis focuses on off-axis afterglow observations as a means to probe jet structure. GRB afterglow data from the 2017 event provided direct evidence that at least a fraction of GRB jets possess a lateral structure in energy and velocity, as had been hypothesized since the early 2000s. Specifically, I investigate the potential link between jet lateral structure and the plateau phase observed in a subset of GRB X-ray light curves, which may arise when the jet is viewed from a near-core line of sight, according to models proposed in the literature. To explore this, I constructed a dataset of well-sampled optical and X-ray afterglow light curves from Swift data, with the goal of testing predictions from a state-of-the-art structured jet model. I present a detailed analysis of two GRBs as case studies, highlighting the challenges of reproducing both X-ray and optical light curves with a single, physically consistent set of model parameters. The second part of the thesis looks ahead to the promising future of the field, when third-generation (3G) GW interferometers will lead to a dramatic increase in the number of BNS detections. I evaluate the role of Integral Field and Multi-Object Spectroscopy (IFS and MOS) in addressing the challenges of detecting, identifying, and characterizing EM counterparts of BNS mergers in the 3G era. To this end, I perform simulations of observations with the Wide-field Spectroscopic Telescope (WST), determining its detection capabilities and assessing its impact on counterpart searches. I discuss possible observing strategies, potential limitations and mitigation approaches, and outline the main challenges to be addressed. This thesis aims to contribute to the scientific paths opened by the 2017 multimessenger event. It seeks to advance our understanding of EM counterparts by extracting new insights from existing GRB datasets and it underscores the importance of building new adapted EM facilities and developing optimized follow-up strategies well in advance of the 3G GW detector era, in order to fully exploit the discovery potential of BNS multimessenger observations.