On the compactness of neutron stars

TL;DR

This paper derives a model-independent lower limit on neutron star radii based on recent observational constraints, challenging existing dense matter models and emphasizing the importance of causality in the equation of state.

Contribution

It introduces a method to determine the neutron star equation of state from mass-radius templates without assumptions on core composition.

Findings

Lower limit on 1.4 solar mass neutron star radius is approximately 10.7 km.

Analysis supports smaller radii than purely nucleonic models suggest.

Method ensures causality is not violated in the derived equation of state.

Abstract

Recent progress in the determination of both masses and radii of neutron stars are starting to place stringent constraints on the dense matter equation of state. In particular, new theoretical developments together with improved statistical tools seem to favor stellar radii that are significantly smaller than those predicted by models using purely nucleonic equations of state. Given that the underlying equation of state must also account for the observation of $2M_{\odot}$ neutron stars, theoretical approaches to the study of the dense matter equation of state are facing serious challenges. In response to this challenge, we compute in a model-independent way the underlying equation of state associated with an assumed mass-radius template similar to the "common radius" assumption used in recent studies. Once such a mass-radius template is adopted, the equation of state follows directly from the implementation of Lindblom's algorithm; assumptions on the nature or composition of the dense stellar core are not required. By analyzing mass-radius profiles with a maximum mass consistent with observation and common radii in the 8 to 11 km range, a lower limit on the stellar radius of a $1.4M_{\odot}$ neutron star of $R_{\rm NS}\!\gtrsim\!10.7$ km is obtained in order to prevent the equation of state from violating causality.