On the sound velocity bound in neutron stars

Abstract

It has been suggested in the literature that the sound velocity of the nuclear matter vs exceeds the so-called sound velocity bound vs≤c/3 at high density, where c is the speed of light. In this paper, we revisit the issue of neutron star properties, comparing current measurements of mass, radius, and tidal deformability with 105 different equations of state. These equations are parametrized at low density and saturate the sound velocity bound beyond twice the saturation density. This allows us to conservatively determine the maximum mass compatible with observed neutron star properties and the sound velocity bound. We find that majority of the models are eliminated by the incompatibility with the observations and, especially, the recently detected massive pulsar (2.35±0.17M⊙) is hardly realized by our simulations. Our study strongly supports that the sound velocity bound is not satisfied in nature.