Signatures of Quark-Hadron Phase Transitions in General-Relativistic Neutron-Star Mergers

Merging binaries of neutron-stars are not only strong sources of gravitational waves, but also have the potential of revealing states of matter at densities and temperatures not accessible in laboratories. A crucial and long-standing question in this context is whether quarks are deconfined as a result of the dramatic increase in density and temperature following the merger. We present the first fully general-relativistic simulations of merging neutron-stars including quarks at finite temperatures that can be switched off consistently in the equation of state. Within our approach, we can determine clearly what signatures a quark-hadron phase transition would leave in the gravitational-wave signal. We show that if after the merger the conditions are met for a phase transition to take place at several times nuclear saturation density, they would lead to a postmerger signal considerably different from the one expected from the inspiral, that can only probe the hadronic part of the equations of state, and to an anticipated collapse of the merged object. We also show that the phase transition leads to a very hot and dense quark core that, when it collapses to a black hole, produces a ringdown signal different from the hadronic one. Finally, in analogy with what is done in heavy-ion collisions, we use the evolution of the temperature and density in the merger remnant to illustrate the properties of the phase transition in a QCD phase diagram.

Figure 1

Snapshots on the equatorial plane at three representative times of the evolution of the low-mass binary. For each snapshot, the left-hand part of the panel reports the temperature $T$, while the right-hand part reports the quark fraction $Y_{\text{quark}}$. The green lines show contours of constant number baryon density in units of the nuclear saturation density $n_{\mathrm{sat}}$. Note that a PT takes place only shortly before the HMNS collapses to a black hole (cf. right-hand panel).

Figure 2

Same as Fig. 1 but on the meridional plane. The top (bottom) panels show simulations with the ${\mathrm{CMF}}_Q$ (${\mathrm{CMF}}_H$) EOSs. The snapshot refers to the high-mass binary at a time shortly before the collapse to a black hole.

Figure 3

Evolution of the maximum normalized baryon number density (diamonds) and temperature (circles) after the merger for the low-mass binary with the ${\mathrm{CMF}}_Q$ EOS. Different times of the evolution are represented with a color code, together with the quark fraction $Y_{\text{quark}}$. The gray shaded area shows the first-order PT region.

Figure 4

Properties of the GW emission for the low- (left-hand panels) and high-mass binaries (right-hand panels). The top panels report the strain $h_{+}^{22}$ for the two EOSs, together with the instantaneous GW frequency $f_{\mathrm{GW}}$ (semitransparent lines). The bottom panels show the phase difference $\mathrm{\Delta }\mathrm{\Phi }$ between the two signals. The inset in the top right-hand panel highlights the differences in the ringdown.