Signatures of relativistic jets in the prompt and afterglow emission of gamma-ray bursts

This thesis investigates whether early-time X-ray emission in gamma-ray bursts can be explained within a geometrical framework governed by relativistic jet structure, without invoking prolonged central-engine activity. A forward-modelling and fitting pipeline is developed to test directly against the data two physically motivated components: prompt high-latitude emission (HLE) and synchrotron forward-shock (FS) afterglow emission. To the best of our knowledge, this is the first implementation of this combined framework for direct fits to plateau GRBs, and the first systematic data-driven test of an HLE interpretation for the X-ray extended emission of short GRBs. The framework is applied to two problems: X-ray plateaus in long GRBs and extended emission in short GRBs. For plateau GRBs, a unified structured-jet interpretation is tested, in which the X-ray plateau is attributed to structured-jet HLE and the contemporaneous optical emission to a structured FS component. Modelling of GRB 061121 and GRB 100814A shows that structured-jet HLE can reproduce the X-ray steep-decay/plateau/post-plateau evolution and broadly match the observed photon-index trends. However, these solutions underpredict the optical flux, whereas FS solutions fitted to the optical data overproduce the X-ray emission around plateau onset. The minimal structured-jet HLE+FS framework therefore does not provide a self-consistent unified description of these events. For short GRBs with extended emission, a top-hat HLE model is adopted. In four standard cases, it reproduces the main XRT light-curve morphology and is often consistent with the observed spectral evolution and with independent extrapolations to the BAT band. Two more complex bursts, GRB 080503 and GRB 211227A, instead favour a second prompt-like episode. Overall, extended emission in short GRBs emerges as a promising testbed for curvature-tail models, while plateaus in long GRBs require a more flexible multi-component picture.