Signature of hadron-quark crossover in binary-neutron-star mergers

TL;DR

This study uses numerical simulations of binary-neutron-star mergers to identify observational signatures of the hadron-quark crossover, highlighting differences in gravitational-wave signals and remnant behavior that could be detected by future gravitational-wave observatories.

Contribution

It introduces a method to distinguish hadron-quark crossover from phase transition scenarios in neutron-star mergers using gravitational-wave and electromagnetic signals.

Findings

Crossover softens the EoS, leading to earlier black hole formation.

Sudden shutdown of gravitational waves indicates crossover.

Remnant lifetime depends on total system mass.

Abstract

We study observational signatures of the hadron-quark crossover in binary-neutron-star mergers by numerical-relativity simulations with various mass configurations. We employ two equations of state (EoSs) for matter consistent with inference from the observational data. In the crossover scenario the EoS is softened in a density realized in binary-neutron-star mergers and is smoothly continued to quark matter. In the phase transition scenario without crossover, the EoS remains stiff and a first-order phase transition takes place in a density out of reach of mergers. A GW170817-like system forms a remnant massive neutron star in both scenarios, and it collapses into a black hole only in the crossover scenario due to the softening while gravitational-wave emission is strong. This difference is clearly reflected in the sudden shutdown of gravitational waves. For a given EoS, the lifetime of the merger remnant is determined primarily by the total mass of the system. Identifying these features in a variety of future events with the next generation of ground-based gravitational-wave detectors will enable us to clarify details of hadron-quark transition. The mass of the accretion disk surrounding the remnant black hole is affected not only by the lifetime of the remnant but also by the mass ratio of the system. Electromagnetic emission associated with the disk outflow will also be useful for detailed investigation of the hadron-quark transition.