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Observational constraints on neutron star matter equation of state

Authors: Malik, Tuhin; Cartaxo, João; Providência, Constança

Ref.: J.Subatomic Part.Cosmol. 4, 100086 (2025)

Abstract: The equation of state (EOS) of ultra-dense matter is a critical link between astrophysical observations and nuclear physics, essential for understanding neutron star (NS) interiors. This review synthesizes current observational constraints on the dense matter EOS from multiple independent sources. We examine how multi-messenger astronomy — combining gravitational wave detections, X-ray timing observations, radio pulsar measurements, and electromagnetic counterparts — provides complementary constraints on NS properties including mass, radius, and tidal deformability. We analyze several relativistic mean-field (RMF) models within a Bayesian framework, comparing nucleonic-only models with those incorporating exotic degrees of freedom, such as hyperons. Our analysis demonstrates that models that explicitly include chiral symmetry such as CMF and eNJL provide the best agreement with observational constraints, particularly in the mass range of 1.4–1.8 M⊙, while suggesting a preferred radius range of 11–13 km for canonical 1.4 M⊙ NSs. We find that the tidal deformability of canonical NSs falls within the range Λ1.4∼300–700, consistent with the constraints of GW170817. The integration of constraints from chiral effective field theory (χEFT) at low densities and perturbative quantum chromodynamics (pQCD) at high densities has significantly narrowed viable EOS models while highlighting density regions (2–5 ρ0) where uncertainty remains greatest and exotic phase transitions could occur. These findings have profound implications for understanding fundamental aspects of nuclear physics and the potential existence of exotic states of matter in NS interiors.

DOI: 10.1016/j.jspc.2025.100086

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