How Neutron Star Observations Point Towards Exotic Matter: Existing Explanations and a Prospective Proposal

Multi-messenger astronomical observations of neutron stars, together with more precise calculations and constraints coming from dense matter microphysics, are generating tension with regard to equations of state models used to describe neutron star cores. Assuming an abrupt first-order phase transition with a slow conversion speed between phases, we propose different slow stable hybrid star configurations aiming to reconcile all current constraints simultaneously; within this framework, we also introduce a novel non-CSS parametrization to the quark matter equation of state and discuss its strengths and limitations. We analyze our model results in conjunction with a review of other relevant theoretical possibilities existing in the literature. We found that modern neutron star observations seem to favor the existence of some type of exotic matter in the neutron star cores; in particular, our slow stable hybrid star scenario remains a proposal capable of satisfying these constraints. However, due both to the existing skepticism regarding some of the adopted hypotheses in most extreme neutron star measurements and to the precise adjustment needed for the equation-of-state parameters, significant tension and open questions remain.

💡 Research Summary

The paper addresses the growing tension between modern multi‑messenger observations of neutron stars (mass measurements of ≳2 M⊙ pulsars, NICER radius estimates, GW170817 tidal‑deformability constraints) and theoretical equations of state (EOS) derived from nuclear and particle physics (χEFT at low density, perturbative QCD at very high density). In addition to these “standard” constraints, the authors highlight outlier objects such as the ultra‑low‑mass HESS J1731‑347 and the ultra‑compact XTE J1814‑338, which exacerbate the incompatibility of many conventional EOS models.

To reconcile all data, the authors adopt a hybrid star framework in which a first‑order hadron‑to‑quark phase transition occurs at a pressure Pₜ with an energy‑density jump Δε. Crucially, they assume that the conversion from hadronic to quark matter proceeds on a timescale much longer than the fundamental radial oscillation period. In this “slow conversion” regime, the usual stability criterion (∂M/∂ε_c > 0) no longer applies; instead a new branch of “slow stable hybrid stars” (SSHS) can exist even when the mass decreases with central density. This concept, originally demonstrated in Ref.