Investigating Signatures of Phase Transitions in Neutron-Star Cores

TL;DR

This paper investigates whether current astrophysical observations of neutron stars can reveal signatures of phase transitions in their cores, using a broad modeling approach of the speed of sound without assuming specific physics.

Contribution

It employs a physics-agnostic modeling of the speed of sound to explore possible signatures of phase transitions in neutron star cores from observational data.

Findings

Some equations of state show non-trivial structures in sound speed.

Current observations do not necessitate a phase transition in neutron star cores.

Multiple equations of state fit the data equally well.

Abstract

Neutron stars explore matter at the highest densities in the universe such that their inner cores might undergo a phase transition from hadronic to exotic phases, e.g., quark matter. Such a transition could be associated with non-trivial structures in the density behavior of the speed of sound such as jumps and sharp peaks. Here, we employ a physics-agnostic approach to model the density dependence of the speed of sound in neutron stars and study to what extent the existence of non-trivial structures can be inferred from existing astrophysical observations of neutron stars. For this, we exhaustively study different equations of state, including as well those with explicit first-order phase transitions. We obtain a large number of different EoSs which reproduce the same astrophysical observations and obey the same physical constraints such as mechanical stability and causality. Among them, some have non-trivial structures in the sound speed while others do not. We conclude that astrophysical information to date do not require the existence of a phase transition to quark matter in the density range explored in the core of neutron stars.