Large and massive neutron stars: Implications for the sound speed in dense QCD

TL;DR

This study explores how recent measurements of massive neutron stars constrain the maximum sound speed in their dense cores, suggesting it must be significantly high if the stars are large, based on chiral effective field theory calculations.

Contribution

It links observational data of neutron star radii to theoretical limits on the sound speed in dense QCD matter, providing new bounds based on recent astrophysical measurements.

Findings

Minimum sound speed squared increases with neutron star radius.

Stars with radius ≥13 km imply a minimum sound speed squared ≥0.562.

Results depend on the validity of chiral EFT up to twice nuclear saturation density.

Abstract

The NASA telescope NICER has recently measured x-ray emissions from the heaviest of the precisely known two-solar mass neutron stars, PSR J0740+6620. Analysis of the data [Miller et al., Astrophys. J. Lett. 918, L28 (2021); Riley et al., Astrophys. J. Lett. 918, L27 (2021)] suggests that PSR J0740+6620 has a radius in the range of $R_{2.0} \approx (11.4-16.1)$ km at the $68\%$ credibility level. In this article, we study the implications of this analysis for the sound speed in the high-density inner cores by using recent chiral effective field theory ($\chi$EFT) calculations of the equation of state at next-to-next-to-next-to-leading order to describe outer regions of the star at modest density. We find that the lower bound on the maximum speed of sound in the inner core, $\textbf{min}\{c^2_{s, {\rm max}}\}$, increases rapidly with the radius of massive neutron stars. If $\chi$EFT remains an efficient expansion for nuclear interactions up to about twice the nuclear saturation density, $R_{2.0}\geqslant 13$ km requires $\textbf{min}\{c^2_{s, {\rm max}}\} \geqslant 0.562$ and $0.442$ at the $68\%$ and $95\%$ credibility level, respectively.