Speed of Sound and Phase Transitions in Neutron Stars Indicated by the Thick Neutron Skin of $^{208}$Pb

TL;DR

This paper investigates how the speed of sound in neutron star matter, influenced by the thick neutron skin of lead-208, reveals phase transitions and constrains the internal structure of neutron stars, linking nuclear experiments with astrophysical observations.

Contribution

It demonstrates that specific structures in the speed of sound are necessary to reconcile nuclear experiment results with neutron star observations, highlighting the role of phase transitions.

Findings

Exotic speed of sound structures are needed to match observations.

Peak speed of sound is constrained near twice nuclear saturation density.

Early strong phase transitions are favored in neutron star models.

Abstract

The speed of sound is a novel probe of equation of state and phase transitions in dense cores of neutron stars. Recently nuclear experiments extracted a surprising thick neutron skin of $^{208}$Pb, causing tensions to reproduce the tidal deformability in gravitational-wave observations. This work finds that exotic structures in the speed of sound with a small softening slope followed by a steep-rising peak are required to reconcile the thick neutron skin of $^{208}$Pb with astronomical observations of neutron stars. Furthermore, the peak of speed of sound is narrowly constrained around two times the nuclear saturation density with the thick neutron skin. Consequently early and strong first-order phase transitions are comparatively more favorable.