Strong model-agnostic constraints for twin-star solutions

TL;DR

This paper conducts a Bayesian, model-agnostic analysis of neutron star equations of state, identifying two classes of twin-star solutions consistent with current constraints, and ruling out the standard deconfinement scenario.

Contribution

It introduces a systematic, model-agnostic Bayesian framework to analyze twin-star solutions, considering explicit phase transitions and current astrophysical constraints.

Findings

Only two classes of twin stars remain viable under current constraints.

Standard deconfinement-based twin-star scenarios are ruled out.

Remaining solutions are disfavored and likely to be excluded with future data.

Abstract

We perform a model-agnostic Bayesian analysis of the neutron-star-matter equation of state (EoS), using known ab-initio constraints and astrophysical observations to limit its behavior at intermediate densities. Permitting explicit first-order phase transitions allows us to systematically search for twin-star solutions, i.e. the existence of stars degenerate in mass but differing in radius. We find that current observational constraints exclude all but two classes of twin stars. The first is characterized by a first-order transition occurring at a very low density, where the material properties of the system either stay largely intact or move away from the conformal limit. In the second, more interesting class, the discontinuity in the mass-radius curve emerges after a rapid crossover transition at a significantly higher density, with the speed of sound exhibiting two sharp peaks at distinct densities. Since neither class shows clear conformalization upon entering the second branch, the standard twin-star scenario linking the mass-radius discontinuity to deconfinement can be firmly ruled out, while even the remaining solutions -- disfavored by per-mille Bayes factors and in tension with theoretical bounds -- are likely to be excluded in the future.