Radius and equation of state constraints from massive neutron stars and GW190814

Motivated by the unknown nature of the $2.50\text{–}2.67M_{\odot }$ compact object in the binary merger event GW190814, we study the maximum neutron star mass based on constraints from low-energy nuclear physics, neutron star tidal deformabilities from GW170817, and simultaneous mass-radius measurements of PSR J0030+045 from NICER. Our prior distribution is based on a combination of nuclear modeling valid in the vicinity of normal nuclear densities together with the assumption of a maximally stiff equation of state at high densities, a choice that enables us to probe the connection between observed heavy neutron stars and the transition density at which conventional nuclear physics models must break down. We demonstrate that a modification of the highly uncertain suprasaturation density equation of state beyond 2.64 times normal nuclear density is required in order for chiral effective field theory models to be consistent with current neutron star observations and the existence of $2.6M_{\odot }$ neutron stars. We also show that the existence of very massive neutron stars strongly impacts the radii of $\approx 2.0M_{\odot }$ neutron stars (but not necessarily the radii of $1.4M_{\odot }$ neutron stars), which further motivates future NICER radius measurements of PSR $\mathrm{J1614}-2230$ and PSR J0740+6620.

Figure 1

Mass and radius probability distributions for the (top-left) prior with smooth high-density extrapolation, (top-right) prior with maximally stiff high-density extrapolation, (bottom-left) posterior not supporting $\approx 2.6M_{\odot }$ neutron stars, and (bottom-right) posterior supporting $\approx 2.6M_{\odot }$ neutron stars. The green [66] and blue [65] contours represent the NICER 68% (solid) and 95% (dashed) credibility bands.

Figure 2

Probability distribution for the maximum mass of a non-rotating neutron star (red) as a function of the transition density $n_t$ to the maximally stiff high-density equation of state. Also shown is the 95% credibility band (blue) for the maximum mass of a neutron star with spin $\chi =0.6$. The green dashed line lies at the central value, $M=2.59M_{\odot }$, of the GW190814 secondary.

Figure 3

Radius distributions for a $1.4M_{\odot }$ neutron star (left) and $2.0M_{\odot }$ neutron star (right) under the two assumptions that the GW190814 secondary was a black hole (blue) or a neutron star (red).

Figure 4

Probability distributions for the tidal deformability versus mass under the two assumptions ${\mathcal{L}}_{\mathrm{s}}^{\mathrm{NS}}\left(\theta \right)$ (left) and ${\mathcal{L}}_{\mathrm{s}}^{\mathrm{BH}}\left(\theta \right)$ (right). The blue and green contours show the 68% (solid) and 95% (dashed) marginal likelihoods associated with the primary and secondary in GW170817, respectively. Inset: probability distributions for ${\mathrm{\Lambda }}_{1.4}$ in the two competing scenarios.