Neutron Star vs Quark Star in the Multimessenger Era

TL;DR

This study uses multi-messenger astrophysical data and theoretical calculations to compare neutron stars and quark stars, finding strong evidence favoring neutron stars and revealing differences in their internal sound velocities.

Contribution

It provides the first Bayesian model-agnostic comparison of neutron stars and quark stars using combined observational and theoretical data, highlighting key differences in their internal physics.

Findings

Neutron stars are strongly favored over quark stars with a Bayes factor of 11.5.

The squared sound velocity peaks around 0.5c^2 at 3.5 times nuclear saturation density in neutron stars.

In heavy neutron stars, the sound velocity and trace anomaly approach conformal limits, unlike in quark stars.

Abstract

Neutron stars (NSs) which could contain exotic degrees of freedom in the core and the self-bound quark stars (QSs) made purely of absolutely stable deconfined quark matter are still two main candidates for the compact objects observed in pulsars and gravitational wave (GW) events in binary star mergers. We perform a Bayesian model-agnostic inference of the properties of NSs and QSs by combining multi-messenger data of GW170817, GW190425, PSR J0030+0451, PSR J0740+6620, PSR J1614-2230, PSR J0348+0432 as well as ab initio calculations from perturbative quantum chromodynamics and chiral effective field theory. We find the NS scenario is strongly favored against the QS scenario with a Bayes factor of NS over QS $\mathcal{B}^\text{NS}_\text{QS} = 11.5$. In addition, the peak of the squared sound velocity $c_s^2 \sim 0.5c^2$ around $3.5$ times nuclear saturation density $n_0$ observed in the NS case disappears in the QS case which suggests that the $c_s^2$ first increases and then saturates at $c_s^2 \sim 0.5c^2$ above $\sim 4n_0$. The sound velocity and trace anomaly are found to approach the conformal limit in the core of heavy NSs with mass $M \gtrsim 2M_{\odot}$, but not in the core of QSs.