What if the neutron star maximum mass is beyond $\sim 2.3M_{\odot }$?

Abstract

By assuming the formation of a black hole soon after the merger event of GW170817, Shibata et al. updated the constraints on the maximum mass ($M_{\text{max}}$) of a stable neutron star within $\lesssim$ 2.3 $M_{\odot }$, but there is no solid evidence to rule out $M_{\text{max}}>2.3M_{\odot }$ from the point of both microphysical and astrophysical views. In order to explain massive pulsars, it is naturally expected that the equation of state (EOS) would become stiffer beyond a specific density. In this paper, we consider the possibility of EOSs with $M_{\text{max}}>2.3M_{\odot }$, investigating the stiffness and the transition density in a polytropic model. Two kinds of neutron stars are considered, i.e., normal neutron stars (the density vanishes on gravity-bound surface) and strange stars (a sharp density discontinuity on self-bound surface). The polytropic model has only two parameter inputs in both cases: (${\rho }_{\mathrm{t}}$, $\gamma$) for gravity-bound objects, while (${\rho }_{\mathrm{s}}$, $\gamma$) for self-bound ones, with ${\rho }_{\mathrm{t}}$ the transition density, ${\rho }_{\mathrm{s}}$ the surface density and $\gamma$ the polytropic exponent. In the matter of $M_{\text{max}}>2.3M_{\odot }$, it is found that the smallest ${\rho }_{\mathrm{t}}$ and $\gamma$ should be $\sim 0.50{\rho }_0$ and $\sim 2.65$ for normal neutron stars, respectively, whereas for strange star, we have $\gamma >1.40$ if ${\rho }_{\mathrm{s}}>1.0{\rho }_0$ and ${\rho }_{\mathrm{s}}<1.58{\rho }_0$ if $\gamma <2.0$ (${\rho }_0$ is the nuclear saturation density). These parametric results could guide further research of the real EOS with any foundation of microphysics if a pulsar mass higher than $2.3M_{\odot }$ is measured in the future. We also derive rough results of common neutron star radius range, which is $9.8\mathrm{km}<{\mathrm{R}}_{1.4}<13.8\mathrm{km}$ for normal neutron stars and $10.5\mathrm{km}<{\mathrm{R}}_{1.4}<12.5\mathrm{km}$ for strange stars.

7 pages, 6 figures

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• Symmetry energy effect on the secondary component of GW190814 as a neutron star
• Compact stars in the Einstein dark energy model
• A roadmap to strange star
• Quark cores in extensions of the MIT Bag model
• Fully general-relativistic simulations of isolated and binary strange quark stars