Constraining the speed of sound inside neutron stars with chiral effective field theory interactions and observations

TL;DR

This paper combines chiral effective field theory calculations with astrophysical observations to constrain the speed of sound inside neutron stars, revealing it likely exceeds the conformal limit and reaches close to the speed of light at high densities.

Contribution

It introduces physically motivated models for the speed of sound at high density and uses observational data to constrain neutron star core properties, extending microscopic calculations.

Findings

The speed of sound likely exceeds the conformal limit in neutron star cores.

Neutron star radii are estimated between 10 and 14 km for typical masses.

The minimum radius for a 1.4 solar mass neutron star is about 8.4 km.

Abstract

The dense matter equation of state (EOS) determines neutron star (NS) structure but can be calculated reliably only up to one to two times the nuclear saturation density, using accurate many-body methods that employ nuclear interactions from chiral effective field theory constrained by scattering data. In this work, we use physically motivated ansatzes for the speed of sound $c_S$ at high density to extend microscopic calculations of neutron-rich matter to the highest densities encountered in stable NS cores. We show how existing and expected astrophysical constraints on NS masses and radii from X-ray observations can constrain the speed of sound in the NS core. We confirm earlier expectations that $c_S$ is likely to violate the conformal limit of $c_S^2\leq c^2/3 $, possibly reaching values closer to the speed of light $c$ at a few times the nuclear saturation density, independent of the nuclear Hamiltonian. If QCD obeys the conformal limit, we conclude that the rapid increase of $c_S$ required to accommodate a $2 $ M$_\odot$ NS suggests a form of strongly interacting matter where a description in terms of nucleons will be unwieldy, even between one and two times the nuclear saturation density. For typical NSs with masses in the range $1.2-1.4~$ M$_\odot$, we find radii between $10$ and $14$ km, and the smallest possible radius of a $1.4$ M$_{\odot}$ NS consistent with constraints from nuclear physics and observations is $8.4$ km. We also discuss how future observations could constrain the EOS and guide theoretical developments in nuclear physics.