2020MNRAS.499.4526W

By assuming the formation of a black hole soon after the merger event of GW170817, the maximum mass of non-rotating stable neutron star, M_TOV ≃ 2.3 M_☉, is proposed by numerical relativity, but there is no solid evidence to rule out M_TOV > 2.3 M_☉ from the point of both microphysical and astrophysical views. It is naturally expected that the equation of state (EOS) would become stiffer beyond a specific density to explain massive pulsars. We consider the possibility of EOSs with M_TOV > 2.3 M_☉, investigating the stiffness and the transition density in a polytropic model, for two kinds of neutron stars (i.e. gravity-bound and strong-bound stars on surface). Only two parameters are input in both cases: (ρ_t, γ) for gravity-bound neutron stars, while (ρ_s, γ) for strong-bound strange stars, with ρ_t the transition density, ρ_s the surface density, and γ the polytropic exponent. In the matter of M_TOV > 2.3 M_☉ for the maximum mass and 70 <= Λ_1.4 <= 580 for the tidal deformability, it is found that the smallest ρ_t and γ should be ∼0.50 ρ₀ and ∼2.65 for neutron stars, respectively, whereas for strange star, we have γ > 1.40 if ρ_s > 1.0 ρ₀ (ρ₀ 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 2.3 M_☉ is measured in the future, especially for an essential comparison of allowed parameter space between gravity-bound and strong-bound compact stars.