Gravitational radiation from crystalline color-superconducting hybrid stars

TL;DR

This paper investigates gravitational wave emissions from crystalline color-superconducting quark cores in neutron stars, predicting detectable signals and constraining the properties of exotic matter inside dense stellar remnants.

Contribution

It models the internal structure of hybrid stars with CCS quark cores, calculates their gravitational wave emission, and compares predictions with observational upper limits to constrain matter properties.

Findings

CCS quark cores can support nonaxisymmetric deformations.

Predicted gravitational wave strains are consistent with LIGO and GEO 600 limits.

Constraints on breaking strain and pairing gaps of CCS matter are derived.

Abstract

The interiors of high mass compact (neutron) stars may contain deconfined quark matter in a crystalline color superconducting (CCS) state. On a basis of microscopic nuclear and quark matter equations of states we explore the internal structure of such stars in general relativity. We find that their stable sequence harbors CCS quark cores with masses M_core \le (0.78-0.82)M_{sun} and radii R_core \le 7 km. The CCS quark matter can support nonaxisymmetric deformations, because of its finite shear modulus, and can generate gravitational radiation at twice the rotation frequency of the star. Assuming that the CCS core is maximally strained we compute the maximal quadrupole moment it can sustain. The characteristic strain of gravitational wave emission $h_0$ predicted by our models are compared to the upper limits obtained by the LIGO and GEO 600 detectors. The upper limits are consistent with the breaking strain of CCS matter \sigma \le 10^{-4} and large pairing gaps \Delta \sim 50 MeV, or, alternatively, with \sigma \sim 10^{-3} and small pairing gaps \Delta \sim 15 MeV. An observationally determined value of the characteristic strain h_0 can pin down the product \sigma\Delta^2. On the theoretical side a better understanding of the breaking strain of CCS matter will be needed to predict reliably the level of the deformation of CCS quark core from first principles.