Binary neutron star mergers as a probe of quark-hadron crossover equations of state

It is anticipated that the gravitational radiation detected in future gravitational wave (GW) detectors from binary neutron star (NS) mergers can probe the high-density equation of state (EOS). We perform the first simulations of binary NS mergers which adopt various parametrizations of the quark-hadron crossover (QHC) EOS. These are constructed from combinations of a hadronic EOS ($n_b<2n_0$) and a quark-matter EOS ($n_b>5n_0$), where $n_b$ and $n_0$ are the baryon number density and the nuclear saturation density, respectively. At the crossover densities ($2n_0<n_b<5n_0$) the QHC EOSs continuously soften, while remaining stiffer than hadronic and first-order phase transition EOSs, achieving the stiffness of strongly correlated quark matter. This enhanced stiffness leads to significantly longer lifetimes of the postmerger NS than that for a pure hadronic EOS. We find a dual nature of these EOSs such that their maximum chirp GW frequencies $f_{\max }$ fall into the category of a soft EOS while the dominant peak frequencies ($f_{\mathrm{peak}}$) of the postmerger stage fall in between that of a soft and stiff hadronic EOS. An observation of this kind of dual nature in the characteristic GW frequencies will provide crucial evidence for the existence of strongly interacting quark matter at the crossover densities for QCD.

Figure 1

The EOSs studied in this work are shown in $P$ v. $\rho$ (in multiples of nuclear saturation density) here. The inset shows mass-radius (${\mathrm{M}}_{\odot }$ and km, respectively) relations for raw QHC19 B-D EOSs (solid lines) and their parametrized counterparts (dashed with same color).

Figure 2

Evolution of maximum rest-mass density vs time for several equations of state employed in the present work. The blue band indicates QHC-crossover densities (note: SLy does not have a crossover; however, indicating the crossover density is suggestive of the need to account for quark matter in hadronic EOSs). In all these cases, the NSs start in the crossover density range (2–5 $n_0$) followed by a rise in density, leading to a collapse to a black hole (in all except the bottom-right panel). The bottom-right case (QHC19D 1.399) does not form a black hole within the simulation time.

Figure 3

Top panel shows $f_{\max }$ vs the dimensionless tidal deformability (${\mathrm{\Lambda }}^{1/5}$) along with the universality relations suggested in previous work [76, 77] as labeled. Lower panel shows $f_{\mathrm{peak}}$ vs pseudoaverage rest-mass density $(2M_0/R_{\max }^3{)}^{1/2}$. For each EOS, increasing sizes of the symbols indicates increase in $M_0$ as listed in Table 2.