[Paper Review] Constraining the properties of neutron-star matter with observations

Eemeli Annala, Tyler Gorda+3 more|arXiv (Cornell University)|Apr 2, 2019

Pulsars and Gravitational Waves ResearchPhysics and Astronomy3 citations

TL;DR

This paper uses model-independent constraints from neutron star mass-radius observations and gravitational wave data to demonstrate that quark matter cores are not exotic but likely common in massive neutron stars. By interpolating the speed of sound in neutron star matter and applying physical consistency conditions, the study finds that quark matter cores are nearly universal in stars above 1.4 solar masses, especially when the speed of sound approaches the conformal limit.

ABSTRACT

In this conference-proceedings contribution, we review recent advances in placing model-independent constraints on the properties of cold and dense QCD matter inside neutron stars. In addition to introducing new bounds for the Equation of State, we explain how these results may be used to make robust statements about the physical phase of strongly interacting matter in the centers of neutron stars of different masses. Our findings indicate that the existence of quark-matter cores inside massive neutron stars appears to be a very common feature of the allowed Equations of State, and should not be considered an exotic or unlikely scenario.

Motivation & Objective

• To determine whether quark matter cores in neutron stars can be constrained model-independently using astrophysical observations.
• To investigate the physical conditions under which quark matter emerges in neutron star cores, independent of specific theoretical models.
• To assess whether current observations already allow definitive statements about the phase of matter in neutron star interiors.
• To explore the implications of quark matter cores for neutron star stability, structure, and observable signatures such as gravitational wave ringdown or seismology.

Proposed method

• Construct a family of neutron star equation of state (EoS) models using a linear interpolation of the speed of sound squared ($c_s^2$) across intermediate densities.
• Apply physical consistency conditions: causality ($c_s^2 \leq 1$), thermodynamic stability, and energy conditions.
• Incorporate low-density limits from chiral effective field theory (CET) and high-density limits from perturbative QCD (pQCD).
• Generate large ensembles of EoSs by sampling free parameters in the speed-of-sound interpolation, ensuring all EoSs are physically viable.
• Use Tolman-Oppenheimer-Volkoff (TOV) equations to compute neutron star masses and radii from each EoS.
• Compare predicted mass-radius relations with observational constraints from $2M_\odot$ neutron stars and gravitational wave detections.

Experimental results

Research questions

• RQ1 Can current neutron star observations constrain the presence of quark matter cores in neutron star interiors without relying on specific theoretical models?
• RQ2 What conditions on the speed of sound and transition type (first-order vs. crossover) lead to the formation of quark matter cores in massive neutron stars?
• RQ3 How does the presence of a quark matter core affect the maximum mass and radius of neutron stars?
• RQ4 Is the existence of quark matter cores in $2M_\odot$ neutron stars a generic feature of viable EoSs, or does it require fine-tuned parameters?
• RQ5 What observable signatures might quark matter cores produce, such as enhanced damping in gravitational wave ringdown or seismic echoes?

Key findings

• Quark matter cores are not exotic but are a common feature in massive neutron stars, especially when the speed of sound approaches the conformal limit ($c_s^2 \approx 1/3$).
• For EoSs that respect the conformal bound ($c_s^2 \leq 1/3$), two-solar-mass neutron stars always contain large quark cores of approximately 6.5 km in radius.
• If quark matter is present in $2M_\odot$ stars, the maximum stable neutron star mass cannot exceed approximately 2.25$M_\odot$, indicating a destabilizing effect from the quark core.
• Quark matter cores are absent only under very specific conditions: a strong first-order transition combined with a very high speed of sound ($c_s^2 \geq 0.7$) and large latent heat ($\geq 130$ MeV/fm³).
• The speed of sound in neutron star matter must be at least 0.7 in the core for quark matter to be excluded, which is a stringent constraint from current observations.
• The presence of quark matter cores may lead to observable effects such as enhanced bulk viscosity, shock wave reflections at the quark-hadron interface, and potential seismic echoes in neutron star ringdowns.