Merger and Postmerger of Binary Neutron Stars with a Quark-Hadron Crossover Equation of State

Fully general-relativistic binary-neutron-star (BNS) merger simulations with quark-hadron crossover (QHC) equations of state (EOS) are studied for the first time. In contrast to EOS with purely hadronic matter or with a first-order quark-hadron phase transition (1PT), in the transition region QHC EOS show a peak in sound speed and thus a stiffening. We study the effects of such stiffening in the merger and postmerger gravitational (GW) signals. Through simulations in the binary-mass range 2.5 < M/M circle dot < 2.75, characteristic differences due to different EOS appear in the frequency of the main peak of the postmerger GW spectrum (f2), extracted through Bayesian inference. In particular, we found that (i) for lower-mass binaries, since the maximum baryon number density (nmax) after the merger stays below 3-4 times the nuclear-matter density (n0), the characteristic stiffening of the QHC models in that density range results in a lower f2 than that computed for the underlying hadronic EOS and thus also than that for EOS with a 1PT; (ii) for higher-mass binaries, where nmax may exceed 4-5n0 depending on the EOS model, whether f2 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 f2 for different EOS and BNS masses gives important clues on how to discriminate different types of quark dynamics in the high-density end of EOS and is relevant to future kilohertz GW observations with third-generation GW detectors.

Automated summary

This study investigates fully general-relativistic binary-neutron-star merger simulations with quark-hadron crossover equations of state for the first time and analyzes the characteristic differences in the gravitational wave signals. The results are important for discriminating between different types of quark dynamics and future kilohertz gravitational wave observations.