Effect of the symmetry energy on the secondary component of GW190814 as a neutron star

The secondary component of GW190814 with a mass of 2.50–2.67 $M_{\odot }$ may be the lightest black hole or the heaviest neutron star ever observed in a binary compact object system. To explore the possible equation of state (EOS), which can support such a massive neutron star, we apply the relativistic mean-field model with a density-dependent isovector coupling constant to describe the neutron-star matter. The acceptable EOS should satisfy some constraints: the EOS model can provide a satisfactory description of the nuclei; the maximum mass $M_{\text{TOV}}$ is above 2.6 $M_{\odot }$; the tidal deformability of a canonical 1.4 $M_{\odot }$ neutron star ${\mathrm{\Lambda }}_{1.4}$ should lie in the constrained range from GW170817. In this paper, we find that nuclear symmetry energy and its density dependence play a crucial role in determining the EOS of neutron-star matter. The constraints from the mass of 2.6 $M_{\odot }$ and the tidal deformability ${\mathrm{\Lambda }}_{1.4}={616}_{-158}^{+273}$ (based on the assumption that GW190814 is a NS-BH binary) can be satisfied for the slope of symmetry energy $L\le 50$ MeV. Even including the constraint ${\mathrm{\Lambda }}_{1.4}={190}_{-120}^{+390}$ from GW170817 which suppresses the EOS stiffness at low density, the possibility that the secondary component of GW190814 is a massive neutron star cannot be excluded in this study.

Figure 1

Isovector coupling $g_{\rho }\left(n_b\right)$ as a function of the baryon number density $n_b$ for different NL3L parameter sets. The horizontal line indicates the density-independent isovector coupling $g_{\rho }\left(n_0\right)$ in the original NL3 model.

Figure 2

Symmetry energy $S$ (a) and energy per nucleon $E/A$ in pure neutron matter (PNM) and symmetry nuclear matter (SNM) (b) as a function of the baryon number density $n_b$ for different parameter sets. There is no difference in $E/A$ of SNM for all parametrizations, whereas the results of $S$ and $E/A$ of PNM show different density dependence with the same value at saturation density $n_0$.

Figure 3

Pressures as a function of the baryon mass density ${\rho }_b$ (a) and number density $n_b$ (b) obtained using different parameter sets. The green shaded area represents the joint constraints from GW170817 and GW190814 (assumed to be a NS-BH merger) [4].

Figure 4

Proton fraction $Y_p$ in neutron-star matter as a function of the baryon number density $n_b$ for different parameter sets.

Figure 5

Neutron-star masses as a function of the radius $R$ (a) and the central density $n_c$ (b) for different parameter sets. The colored bands indicate the mass measurements of PSR J1614-2230 [49, 50, 51], PSR J0348+0432 [52], and PSR J0740+6620 [5], while the mass constraint from GW190814 [4] is shown in yellow color. The grey shaded area represents the constraints from NICER [57, 58]. The excluded region of $R_{1.4}>13.6$ km inferred from GW170817 [29] is also indicated.

Figure 6

Tidal deformability as a function of the neutron-star mass for different parameter sets. The constraint $70\le {\mathrm{\Lambda }}_{1.4}\le 580$ is inferred from the analysis of GW170817 [54], while $458\le {\mathrm{\Lambda }}_{1.4}\le 889$ is achieved based on the assumption that GW190814 is a NS-BH binary [4]. The horizontal line represents the upper limit of ${\mathrm{\Lambda }}_{1.4}\le 800$ deduced from GW170817 [2].