Signatures of quark-hadron phase transitions in general-relativistic neutron-star mergers

TL;DR

This paper presents the first general-relativistic simulations of neutron star mergers including quark matter, revealing distinct gravitational-wave signatures and collapse behaviors indicative of quark-hadron phase transitions at high densities.

Contribution

It introduces a novel simulation approach that incorporates quark effects in neutron star mergers and identifies observable signatures of phase transitions in gravitational waves.

Findings

Quark-hadron phase transitions produce distinct post-merger gravitational-wave signals.

Presence of a hot, dense quark core affects the collapse and ringdown signals.

Phase transition signatures can be linked to properties in the QCD phase diagram.

Abstract

Merging binaries of neutron stars are not only strong sources of gravitational waves, but also have the potential of revealing states of matter at densities and temperatures not accessible in laboratories. A crucial and long-standing question in this context is whether quarks are deconfined as a result of the dramatic increase in density and temperature following the merger. We present the first fully general-relativistic simulations of merging neutron stars including quarks at finite temperatures that can be switched off consistently in the equation of state. Within our approach, we can determine clearly what signatures a quark-hadron phase transition would leave in the gravitational-wave signal. In particular, we show that if the conditions are met for a phase transition to take place at several times nuclear saturation density, they would lead to a post-merger signal considerably different from the one expected from the inspiral, that can only probe the hadronic part of the equations of state, and to an anticipated collapse of the merged object. We also show that the phase transition leads to a very hot and dense quark core that, when it collapses to a black hole, produces a ringdown signal different from the hadronic one. Finally, in analogy with what is done in heavy-ion collisions, we use the evolution of the temperature and density in the merger remnant to illustrate the properties of the phase transition in a QCD phase diagram.