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_{⊙}<2.75, characteristic differences due to different EOS appear in the frequency of the main peak of the postmerger 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_{max}) after the merger stays below 3-4 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 EOS with a 1PT; (ii) for higher-mass binaries, where n_{max} may exceed 4-5n_{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 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.