Jet-Ejecta Interactions in Neutron Star Mergers : How jets interacting with the surroundings affect multi-wavelength observations

In August 2017 a gravitational wave signal from a binary neutron star merger was detected for the first time, and it was followed by several electromagnetic detections that spanned the whole electromagnetic spectrum. Those observations proved the ejection of two dynamical components: mildly-relativistic neutron rich ejecta and an ultra-relativistic jet. The first one produced a rapidly evolving thermal transient powered by the radioactive decay of the freshly synthesized r-process nuclei and subsequent recombination, named "macronova" or "kilonova". The second component powered instead a gamma-ray burst consisting of an early (and anomalous) gamma-ray signal ≈1.7 second after the merger followed by a long lasting multi-wavelength afterglow. These two dynamical components are typically observed and modelled independently, but when a jet is produced and propagates through the surrounding environment the properties of both may change significantly because of the interaction. However the observational consequences of such interaction are still unclear. In this thesis I present a comprehensive introduction to the properties of jets and post-merger ejecta, with the aim to show how 3D hydrodynamic simulations form a key instrument in investigating their interaction. I present the results of some of the first simulations of jets propagating within a realistic post-merger wind, and how the properties of the macronova are significantly affected. As an example, this work shows how a jet can "punch-away" a fraction of high-opacity material at early times, causing the macronova becoming brighter and bluer for on-axis observers in the first few days following the merger. Moreover such interaction represents the main contribution to the final shape of the jet, that has a crucial impact in the late time emission for off-axis observers. While looking forward for future detections, this thesis aims to show how crucial is a detailed understanding of the environment surrounding the merger and how, in case of a joint observation of a macronova and a jet, should be taken into account how the interaction might have an impact on the inferred properties of the system