Gravitational wave signature of binary neutron star mergers

Abstract

This work is focused on the analysis of the gravitational-waves emitted by binary neutron star systems when they merge to form either a Black Hole or a remnant neutron star which will eventually collapse to a Black Hole in a delayed time. This research is based on the use of general relativistic numerical simulations that represent the only tool available to study the evolution of binary neutron star systems through all the phases of their dynamics from the coalescence to the merger and eventually the post-merger.

In particular, the effect of the equation of state, the total mass of the system and the mass ratio on the post-merger phase of the gravitational-waves signal has been analysed, evaluating the properties when the system lives long enough after the merger. The focus of this research is on binary neutron star systems which form a (hyper-)massive remnant after the merger that lives long enough (at least 5 ms) and eventually collapse to a Black Hole. The spectral features of the gravitational-waves signal in the post-merger phase have been deeply investigated and compared to the literature also using publicly available data from other groups.

Emphasis has been given to the models that have a longer post-merger phase where additional inertial modes may appear due to the presence of instabilities in the remnant depending on the equation of state of the neutron stars. For that purpose, different techniques of time-frequency analysis has been used in order to study the peculiarities of the gravitational wave signal during this phase of the evolution.

The main interest for this research comes from the fact that binary neutron star mergers are an important target for Earth-based interferometric detectors such as LIGO, Virgo and KAGRA. The detection of GW170817 and its electromagnetic counterpart has been widely recognised as the beginning of multimessenger astrophysics, providing key evidence to long-standing issues in relativistic astrophysics and adding stronger constraints on neutron stars parameters. Complementary information on the internal structure of neutron stars can be obtained through the observation of the post-merger GW signal, which is still an open problem in multimessenger astrophysics. For that purpose, numerical simulations of binary neutron star mergers are an essential tool in the recognition process of GW signal in the detectors.

This work has been performed only using publicly available codes such as LORENE for the initial data setup and the Einstein Toolkit for the system evolution in order to make all the results presented here reproducible.