Strong correlations of neutron star radii with the slopes of nuclear matter incompressibility and symmetry energy at saturation

We examine the correlations of neutron star radii with the nuclear matter incompressibility, symmetry energy, and their slopes, which are the key parameters of the equation of state (EoS) of asymmetric nuclear matter. The neutron star radii and the EoS parameters are evaluated using a representative set of 24 Skyrme-type effective forces and 18 relativistic mean field models, and two microscopic calculations, all describing $2M_{\odot }$ neutron stars. Unified EoSs for the inner-crust–core region have been built for all the phenomenological models, both relativistic and nonrelativistic. Our investigation shows the existence of a strong correlation of the neutron star radii with the linear combination of the slopes of the nuclear matter incompressibility and the symmetry energy coefficients at the saturation density. Such correlations are found to be almost independent of the neutron star mass in the range $0.6\text{–}1.8M_{\odot }$. This correlation can be linked to the empirical relation existing between the star radius and the pressure at a nucleonic density between one and two times saturation density, and the dependence of the pressure on the nuclear matter incompressibility, its slope, and the symmetry energy slope. The slopes of the nuclear matter incompressibility and the symmetry energy coefficients as estimated from the finite nuclei data yield the radius of a $1.4M_{\odot }$ neutron star in the range $11.09\text{–}12.86$ km.

Figure 1

Neutron star mass in $M_{\odot }$ as a function of the radius in km (left) and central density in ${\mathrm{fm}}^{-3}$ (right) for a representative set of RMF (blue) and Skyrme (red) models, and microscopic (green) calculations.

Figure 2

Radii $R_{1.0}$ (left) and $R_{1.4}$ (right) of $1.0M_{\odot }$ and $1.4M_{\odot }$ neutron stars versus the EoS parameters $K_0,M_0$, and $L_0$, obtained using our sets of RMF (blue triangles) and Skyrme (red diamonds) models, together with the BHF and the APR (green stars) calculations.

Figure 3

Neutron star radii $R_{1.0}$ (left) and $R_{1.4}$ (right) versus the linear correlations $K_0+\alpha L_0$ (top) and $M_0+\beta L_0$ (bottom), using a set of RMF (blue triangles), Skyrme (red diamonds), and BHF+APR (green stars) calculations.

Figure 4

Correlation coefficients between the neutron star radii and several EoS parameters as a function of the neutron star mass. The EoS parameters “$b$” denote $K_0,L_0$, and the linear combination $K_0+\alpha L_0$ in the top panel, and $M_0,L_0$, and $M_0+\beta L_0$ in the bottom panel.

Figure 5

$M_0$ as a function of $R_{1.4}$ for $L_0=40$, 60, and 80 MeV, as obtained from the multiple linear regression. The gray shaded region indicates the constraint on $M_0$ derived in Ref. [62].