Astrophysics of dense quark matter in compact stars

Massive neutron stars may harbor deconfined quark matter in their cores. I review some recent work on the microphysics and the phenomenology of compact stars with cores made of quark matter. This includes the equilibrium and stability of non-rotating and rapidly rotating stars, gravitational radiation from deformations in their quark cores, neutrino radiation and dichotomy of fast and slow cooling, and pulsar radio-timing anomalies.

💡 Research Summary

The paper provides a comprehensive review of the theoretical and phenomenological aspects of compact stars whose interiors may contain deconfined quark matter, focusing on recent advances in microphysics, stellar stability, gravitational‑wave emission, neutrino cooling, and pulsar timing anomalies. It begins by outlining the modern equations of state (EOS) for ultra‑dense matter that incorporate color‑superconducting phases such as the two‑flavor superconducting (2SC) and color‑flavor‑locked (CFL) states. These phases modify the pressure‑density relationship, allowing the maximum mass of non‑rotating neutron stars to exceed two solar masses, in agreement with the most massive pulsars observed to date.

The authors then extend the analysis to rapidly rotating configurations, solving the relativistic stellar structure equations with rotation. They demonstrate that the critical spin frequency and the mass‑shedding limit are highly sensitive to the parameters of the color‑superconducting gap. In certain parameter regimes, a rotating quark‑core star can support a larger mass than its non‑rotating counterpart, offering a natural explanation for massive millisecond pulsars.

A major portion of the review is devoted to gravitational‑wave (GW) emission from deformed quark cores. Magnetic stresses or internal fluid motions can produce a non‑axisymmetric mass distribution, generating a continuous GW signal. The amplitude depends on the core’s shear modulus, the geometry of the superconducting lattice, and the degree of deformation. Numerical simulations indicate that the predicted strain lies within the sensitivity band of current interferometers (LIGO, Virgo, KAGRA) for frequencies below about 1 kHz, suggesting that future GW observations could directly probe the existence of quark matter in neutron‑star interiors.

The cooling behavior of quark‑core stars is examined through two distinct channels. In the “fast‑cooling” scenario, direct Urca processes and quark‑pair neutrino emission dominate, leading to a rapid temperature drop within the first 10⁴–10⁵ years. Conversely, when color superconductivity gaps suppress neutrino emissivity, a “slow‑cooling” branch emerges, characterized by a much gentler temperature decline. These two branches naturally explain the observed dichotomy between young neutron stars that appear unusually cold and older objects that retain relatively high surface temperatures, as inferred from X‑ray observations.

Finally, the paper addresses pulsar timing irregularities—glitches, timing noise, and quasi‑periodic variations. The authors propose that the dynamics of superfluid vortices and magnetic flux tubes within a quark core can induce subtle changes in the star’s moment of inertia. Such internal rearrangements manifest as anomalous residuals in radio timing data, offering an indirect diagnostic of a quark‑rich interior.

Overall, the review synthesizes state‑of‑the‑art microphysical models with astrophysical observables, arguing that a consistent picture emerges in which dense quark matter is a viable component of massive neutron stars. The authors emphasize that forthcoming high‑precision measurements—continuous GW searches, detailed thermal evolution studies, and long‑term pulsar timing campaigns—will be crucial for confirming or refuting the presence of deconfined quark phases in compact stars.