Merger and post-merger of binary neutron stars with a quark-hadron crossover equation of state

TL;DR

This study uses fully general-relativistic simulations to analyze how quark-hadron crossover equations of state affect gravitational wave signals from binary neutron star mergers, revealing potential observational signatures of quark matter.

Contribution

First simulation of binary neutron star mergers with quark-hadron crossover equations of state, highlighting their impact on gravitational wave spectra and potential for observational discrimination.

Findings

QHC EOSs show a peak in sound speed, stiffening in the transition region.

Lower-mass mergers have lower post-merger GW frequency ($f_2$) compared to purely hadronic EOSs.

High-mass mergers' $f_2$ depends on the sound-speed peak height, affecting GW signal interpretation.

Abstract

Fully general-relativistic binary-neutron-star (BNS) merger simulations with quark-hadron crossover (QHC) equations of state (EOSs) are studied for the first time. In contrast to EOSs with purely hadronic matter or with a first-order quark-hadron phase transition (1PT), in the transition region QHC EOSs show a peak in sound speed, and thus a stiffening. We study the effects of such stiffening in the merger and post-merger gravitational (GW) signals. Through simulations in the binary-mass range $2.5 < M/M_{\odot} < 2.75$, characteristic differences due to different EOSs appear in the frequency of the main peak of the post-merger GW spectrum ($f_2$), extracted through Bayesian inference. In particular, we found that (i) for lower-mass binaries, since the maximum baryon number density ($n_{\rm max}$) after the merger stays below $3\text{--}4$ times the nuclear-matter density ($n_0$), the characteristic stiffening of the QHC models in that density range results in a lower $f_2$ than that computed for the underlying hadronic EOS and thus also than that for EOSs with a 1PT, (ii) for higher-mass binaries, where $n_{\rm max}$ may exceed $4\text{--}5 n_0$ depending on the EOS model, whether $f_2$ in QHC models is higher or lower than that in the underlying hadronic model depends on the height of the sound-speed peak. Comparing the values of $f_2$ for different EOSs and BNS masses gives important clues on how to discriminate different types of quark dynamics in the high-density end of EOSs and is relevant to future kilohertz GW observations with third-generation GW detectors.