Binary Neutron Star mergers in the multi-messenger era: from astrophysics to cosmology

In August 2017, the groundbreaking observation of GW170817 marked the first- ever identification of a binary neutron star merger, accompanied by the detection of a Gravitational Wave (GW) and a gamma-ray burst (GRB). The GRB exhibited prompt gamma-ray emission and an afterglow across radio, optical, and X-ray bands, originating from a relativistic jet formed post-merger at an angle of 20-30 degrees from its axis. This PhD thesis joins the GW and afterglow datasets of GW170817 through simultaneous fitting using Bayesian statistics. The key parameters shared between the two models, namely the inclination of the binary system (or viewing angle) and the luminosity distance, facilitate this joint analysis. The afterglow dataset includes a broadband light curve and the motion of the relativistic jet centroid over time. The thesis delves into significant challenges in both astrophysics and cosmology. In astrophysics, the investigation focuses on understanding the angular structure and energetics of the jet, influenced by its opening angle. The analysis of the GW and afterglow light curves reveals a jet opening angle and energetics consistent with population studies. Considering the centroid motion results instead in higher energetics compared to population studies. The jet’s structure is discerned through the early rising phase of the afterglow, indicating a structured jet with an energetic core and less energetic wings. In the realm of cosmology, GWs emerge as a potent tool. The GW luminosity distance enables an independent estimation of the Hubble constant (H0), crucial for addressing the Hubble tension. While a sole GW fit lacks the precision required for H0 , incorporating the afterglow light curve and centroid motion improves precision threefold. However, this precision has not yet reached the level of other cosmological H0 estimations but may do so in the future. Additionally, the thesis explores future perspectives on new binary neutron star events.