Postmerger Gravitational-Wave Signatures of Phase Transitions in Binary Mergers

With the first detection of gravitational waves from a binary system of neutron stars GW170817, a new window was opened to study the properties of matter at and above nuclear-saturation density. Reaching densities a few times that of nuclear matter and temperatures up to 100 MeV, such mergers also represent potential sites for a phase transition (PT) from confined hadronic matter to deconfined quark matter. While the lack of a postmerger signal in GW170817 has prevented us from assessing experimentally this scenario, two theoretical studies have explored the postmerger gravitational-wave signatures of PTs in mergers of a binary system of neutron stars. We here extend and complete the picture by presenting a novel signature of the occurrence of a PT. More specifically, using fully general-relativistic hydrodynamic simulations and employing a suitably constructed equation of state that includes a PT, we present the occurrence of a “delayed PT,” i.e., a PT that develops only some time after the merger and produces a metastable object with a quark-matter core, i.e., a hypermassive hybrid star. Because in this scenario, the postmerger signal exhibits two distinct fundamental gravitational-wave frequencies—before and after the PT—the associated signature promises to be the strongest and cleanest among those considered so far, and one of the best signatures of the production of quark matter in the present Universe.

Figure 1

Schematic overview of the instantaneous characteristic gravitational-wave frequency and how its evolution can be used to classify the different scenarios associated with a hadron-quark PT.

Figure 2

Evolution of the central rest-mass density for the four BNS configurations we have simulated. Blue-shaded regions mark the different phases of the EOS and apply to the DPT and PTTC scenarios only since the NPT binaries are always purely hadronic.

Figure 3

Strain $h_{+}^{22}$ (top) and its spectrogram (bottom) for the four BNSs considered. In the top panels the different shadings mark the times when the HMNS core enters the mixed and quark phases (cf. Fig. 2); the NPT models are always purely hadronic. In the bottom panels, the white lines trace the maximum of the spectrograms, while the red lines show the instantaneous gravitational-wave frequency.

Figure 4

Comparison within a DPT scenario between the low-mass BNSs either undergoing a PT (left portions) or not (right portions). Shown are the rest-mass density (left panel) and temperature (right panel) at $t=12.85\mathrm{ms}$. The light-blue and dark-blue contours mark, respectively, the onset of the mixed phase and of the pure-quark phase, where the EOS is stiffened. Note that the “hot ring” falls in the mixed-phase region, where the EOS is softened.