Gravitational-waves signal from binary neutron star merger simulations with different equation of state and mass ratio

Abstract

This work is focused on the determination of the gravitational-waves signal emitted by binary neutron stars when they finally merge to form either a Black Hole or a remnant neutron star that will, likely, eventually also collapse to Black Hole. This research is based on the use of general relativistic numerical simulations that are the only tool available to study the evolution of a binary neutron star system through its coalescent, merger and post-merger phase.

In particular, the gravitational signal emitted by different initial binary neutron star configurations has been analysed, evaluating the effects on the signal due to the total mass, the mass ratio, the equation of state and the initial stellar separation. The research focused on the post-merger phase, were analytical descriptions of the GW signal are still absent, both in the case when a (hyper)massive neutron star or a black hole surrounded by an accretion disk is formed. The gravitational waves phase evolution, the radiated energy and angular momentum and the post-merger gravitational waves spectrum have been determined. In particular, in the case of the post-merger gravitational signal, various possible interpretation of its spectral features have been analysed performing a close comparison with the recent literature.

Emphasis has been given to analysing some sources of systematic errors, such as the initial data, the orbital eccentricity, the finite-resolution errors in the time evolution determined by the choice of different numerical methods, and the gravitational waves extraction methodology. For the latter, several data analysis techniques were developed, applied and extensively tested on the simulation data.

The main interest for this research topic comes from the fact that binary neutron star mergers are the main target for Earth-based gravitational waves interferometric detectors, after the recent first detection of a gravitational signal from binary black hole mergers. They are characterized by a rich phenomenology, which includes microphysical effects and electromagnetic emissions. In particular, the most interesting challenge is to constraint the equation of state of the nuclear matter inside the neutron star core, which is still unknown from a theoretical point of view. In order to recognize a GW signal inside the detectors noise and perform source parameters estimation from it, the comparison with theoretical models coming from numerical simulations is a necessary and essential tool.

This work has a central point on the study of binary neutron star simulations with public codes, in particular the \codename{The Einstein Toolkit} and the \codename{LORENE} library. All the code enhancements for the binary initial data and evolution, the parameter files, and the post-processing scripts developed for this work have been made publicly available, making all the results presented here reproducible, following the simple instructions described in the appendix.