Neutron Star Equation of State in Light of GW190814

The observation of gravitational waves from an asymmetric binary opens the possibility for heavy neutron stars, but these pose challenges to models of the neutron star equation of state. We construct heavy neutron stars by introducing nontrivial structure in the speed of sound sourced by deconfined QCD matter, which cannot be well recovered by spectral representations. Their moment of inertia, Love number, and quadrupole moment are very small, so a tenfold increase in sensitivity may be needed to test this possibility with gravitational waves, which is feasible with third generation detectors.

Figure 1

Parameterized speed of sound (a) as a function of baryon density. The individual colors indicate different choices for the functional form of $c_s^2$, as explained in the Supplemental Material. The horizontal lines denote the pQCD limit of $c_s^2=1/3$ and the causal limit $c_s^2=1$. Observe that all speed of sounds remain causal in the regime of interest. EOSs resulting from the parameterized speed of sounds (b). The shaded region corresponds to the 90% confidence region reported by the LVC in [1]. Mass-radius curves resulting from the new EOSs (c). Observe that all neutron star sequences reach a maximum mass of at least $M_{\max }\ge 2.5M_{\odot }$. The recently inferred mass of the small compact object in GW190814 is shown in the gray band and the radius extracted from NICER observations of the isolated pulsar PSR $\mathrm{J}0030+0451$ is shown in the blue square.

Figure 2

(a) $\mathrm{\Gamma }(p)≔[(\epsilon +p)/p]dp/d\epsilon$ as a function of $\log (p/p_0)$, with $p_0$ the pressure when the baryon density equals half nuclear saturation, $p$ the pressure, and $\epsilon$ the energy density, and (b) mass-radius curves. In both panels, we use two members of the EOS family we showed in Fig. 1 (dashed red and cyan lines, following the same color and line styles as in that figure), as well as spectral EOS fits to these members using the LIGO prior range (dotted lines). The spectral fit is able to reproduce $\mathrm{\Gamma }$ on average, but it misses the sharp features in the speed of sound. These missed features lead to large deviations in observables, such as in the mass-radius curves.

Figure 3

Dimensionless moment of inertia $I/M^3$ (a), tidal deformability parameter $\mathrm{\Lambda }$ (b), and rotational quadrupole moment $Q/(M^3{a}^2)$ (c) versus mass for our EOS family. The red dots indicated the values of $I/M^3$, $\mathrm{\Lambda }$, and $Q/(M^2{a}^2)$ for a $2.5M_{\odot }$ star. Observe that the tidal deformability decreases with mass, and in particular, it drops to $\mathrm{\Lambda }<35$ for $M>2.5M_{\odot }$.