Postmerger Gravitational-Wave Signatures of Phase Transitions in Binary Mergers

TL;DR

This paper introduces a novel gravitational-wave signature of a delayed phase transition to quark matter in neutron star mergers, characterized by two distinct postmerger frequencies, offering a promising observational marker for quark matter formation.

Contribution

The study presents the first simulation-based evidence of a delayed phase transition in neutron star mergers, revealing a unique gravitational-wave signature with two distinct frequencies.

Findings

Identification of a 'delayed phase transition' producing a hypermassive hybrid star.

Detection of two separate fundamental gravitational-wave frequencies post-merger.

Potential for this signature to serve as a clear indicator of quark matter formation.

Abstract

With the first detection of gravitational waves from a binary system of neutron stars, GW170817, a new window was opened to study the properties of matter at and above nuclear-saturation density. Reaching densities a few times that of nuclear matter and temperatures up to $100\,\rm{MeV}$, such mergers also represent potential sites for a phase transition (PT) from confined hadronic matter to deconfined quark matter. While the lack of a postmerger signal in GW170817 has prevented us from assessing experimentally this scenario, two theoretical studies have explored the postmerger gravitational-wave signatures of PTs in mergers of binary systems of neutron stars. We here extend and complete the picture by presenting a novel signature of the occurrence of a PT. More specifically, using fully general-relativistic hydrodynamic simulations and employing a suitably constructed equation of state that includes a PT, we present the occurrence of a "delayed PT", i.e. a PT that develops only some time after the merger and produces a metastable object with a quark-matter core, i.e. a hypermassive hybrid star. Because in this scenario, the postmerger signal exhibits two distinct fundamental gravitational-wave frequencies -- before and after the PT -- the associated signature promises to be the strongest and cleanest among those considered so far, and one of the best signatures of the production of quark matter in the present Universe.