Properties of First-Order Hadron-Quark Phase Transition from Inverting Neutron Star Observables

TL;DR

This study uses neutron star observational data to constrain the properties of the hadron-quark phase transition, revealing lower bounds on the speed of sound in quark matter and correlations between transition parameters.

Contribution

It introduces a method to invert neutron star data within the CSS model to constrain phase transition parameters and their correlations.

Findings

Lower bounds on quark matter speed of sound squared ($c_{QM}^2$) are established.

An approximately linear correlation between transition density and transition strength is identified.

Twin star configurations are deemed unlikely based on current data.

Abstract

By inverting the observational data of several neutron star observables in the three dimensional parameter space of the constant speed of sound (CSS) model while fixing all hadronic Equation of State parameters at their currently known most probable values, we constrain the three parameters of the CSS model and their correlations. Using two lower radius limits of $R_{2.01}=11.41$ km and $R_{2.01}=12.2$ km for PSR J0740+6620 obtained from two independent analyses using different approaches by the Neutron Star Interior Composition Explorer (NICER) Collaboration, the speed of sound squared $c_{\rm QM}^2$ in quark matter is found to have a lower limit of $0.35$ and $0.43$ in unit of $c^2$, respectively, above its conformal limit of $c_{\rm QM}^2<1/3$. An approximately linear correlation between the first-order hadron-quark transition density $\rho_t$ and its strength $\Delta\varepsilon$ is found. Moreover, the presence of twin star is deemed improbable by the present work.