Evidence for quark-matter cores in massive neutron stars (1903.09121v2)

Abstract: The theory governing the strong nuclear force, Quantum Chromodynamics, predicts that at sufficiently high energy densities hadronic nuclear matter undergoes a deconfinement transition to a new phase of quarks and gluons. Although this has been observed in ultrarelativistic heavy-ion collisions, it is currently an open question whether quark matter exists inside neutron stars. By combining astrophysical observations and theoretical ab-initio calculations in a model-independent way, we find that the inferred properties of matter in the cores of neutron stars with mass corresponding to 1.4 solar masses are compatible with nuclear model calculations. However, the matter in the interior of maximally massive, stable neutron stars exhibits characteristics of the deconfined phase, which we interpret as evidence for the presence of quark-matter cores. For the heaviest reliably observed neutron stars with masses of about two solar masses, the presence of quark matter is found to be linked to the behaviour of the speed of sound c_s in strongly interacting matter. If the conformal bound (c_s)^2 < 1/3 is not strongly violated, massive neutron stars are predicted to have sizable quark-matter cores. This finding has important implications for the phenomenology of neutron stars, and affects the dynamics of neutron star mergers with at least one sufficiently massive participant.

• The paper demonstrates that model-independent methods constrained by astrophysical data reveal quark-matter cores in ~2 M⊙ neutron stars.
• The authors employ speed-of-sound interpolation within a QCD framework to capture the transition from hadronic to quark matter at extreme densities.
• The findings suggest that observable signatures in neutron star mergers could serve as diagnostics for the presence of quark-matter, advancing our understanding of dense matter.

Analyzing Quark-Matter Cores in Massive Neutron Stars

The paper "Evidence for quark-matter cores in massive neutron stars," authored by Eemeli Annala et al., presents a comprehensive paper of the possible existence of quark-matter cores inside neutron stars (NSs). The researchers utilize a model-independent approach combining astrophysical observations with theoretical quantum chromodynamics (QCD) predictions to explore conditions under which dense baryonic matter may transition to quark matter in these stellar remnants.

The paper primarily focuses on the equation of state (EoS) that governs neutron star matter, critical for understanding the phase of matter within NS cores. Across different models, the transition from hadronic matter to quark matter highlights a significant incompatibility at extreme densities, such as those at the core of massive neutron stars with masses around 2 solar masses ($M_\odot$).

Model-Independent Approach and Key Findings

The authors employ an ensemble of interpolated EoSs using different methodological frameworks, notably the "speed-of-sound interpolation," which maintains the flexibility required for capturing complex transitions in the EoS. They constrain their models using robust astrophysical observations, such as the tidal deformability limits from gravitational-wave events and maximum observed neutron star masses.

A critical component of their analysis is the behavior of the speed of sound squared ($c_s^2$) in strongly interacting matter, which provides insights into whether the EoS reflects a transition to quark matter. Their findings suggest that if the conformal bound of $c_s^2 \leq 1/3$ is not strictly violated, massive neutron stars are predicted to have sizable quark-matter cores.

Implications and Theoretical Insights

The presence of quark-matter cores implies significant departures from purely hadronic models, particularly where traditional nuclear models predict excessive central pressures and significant deviations in the polytropic index $\gamma$. For massive NSs, the transition from a hadronic phase to a quark phase is marked by observable changes in physical properties, supporting the hypothesis of quark cores.

The implications of these findings extend beyond static measurements. In dynamic astrophysical events like neutron star mergers, the predicted presence of quark cores may influence the generation of shock waves and the emission of gravitational waves, offering an additional diagnostic for indirectly examining the internal composition of neutron stars.

Future Directions and Observational Prospects

The ongoing observations of neutron star mergers and mass-radius constraints are expected to refine our understanding of the NS EoS. The existence of quark cores lays a compelling framework for interpreting gravitational wave signals and neutron star phenomenology. Additionally, detecting neutron stars with larger masses will further validate or challenge the quark-matter core hypothesis.

Theoretical advancements, such as improved ab-initio calculations and more precise EoS modeling within QCD, alongside empirical observations, are poised to drive progress in elucidating the true nature of matter at supra-nuclear densities.

In conclusion, this paper provides substantial evidence supporting the presence of quark-matter cores in massive neutron stars, marking a significant contribution to the field of astrophysics and nuclear physics. As observational capabilities enhance, so too will our comprehension of these enigmatic stellar remnants.