Binary neutron star mergers as a probe of quark-hadron crossover equations of state

TL;DR

Future gravitational wave observations from binary neutron star mergers can reveal the properties of quark-hadron crossover equations of state, which are modeled to be stiffer than purely hadronic EOSs and influence postmerger gravitational wave signals.

Contribution

This study performs the first simulations of binary neutron star mergers using parametrized quark-hadron crossover EOSs, revealing their unique gravitational wave signatures.

Findings

QHC EOSs soften at crossover densities but remain stiffer than hadronic EOSs.

Postmerger neutron stars have longer lifetimes with QHC EOSs.

Characteristic GW frequencies exhibit a dual nature indicating strongly interacting quark matter.

Abstract

It is anticipated that the gravitational radiation detected in future gravitational wave (GW) detectors from binary neutron star (NS) mergers can probe the high-density equation of state (EOS). We perform the first simulations of binary NS mergers which adopt various parametrizations of the quark-hadron crossover (QHC) EOS. These are constructed from combinations of a hadronic EOS ($n_{b} < 2~n_0$) and a quark-matter EOS ($n_{b} >~5~n_0$), where $n_{b}$ and $n_0$ are the baryon number density and the nuclear saturation density, respectively. At the crossover densities ($2~ n_0 < n_{b} < 5~ n_0$) the QHC EOSs continuously soften, while remaining stiffer than hadronic and first-order phase transition EOSs, achieving the stiffness of strongly correlated quark matter. This enhanced stiffness leads to significantly longer lifetimes of the postmerger NS than that for a pure hadronic EOS. We find a dual nature of these EOSs such that their maximum chirp GW frequencies $f_{max}$ fall into the category of a soft EOS while the dominant peak frequencies ($f_{peak}$) of the postmerger stage fall in between that of a soft and stiff hadronic EOS. An observation of this kind of dual nature in the characteristic GW frequencies will provide crucial evidence for the existence of strongly interacting quark matter at the crossover densities for QCD.