Ridges in rotating neutron-star properties due to first order phase transitions

TL;DR

This paper explores how first order phase transitions in neutron star matter can be detected through specific observable features in rotating neutron stars, using the Hartle-Thorne model to identify signatures like ridges in ellipticity and angular momentum.

Contribution

It introduces a method to identify potential phase transitions in neutron stars by analyzing observable combinations and their nonanalytic features, extending previous static star analyses to rotating stars.

Findings

First order phase transitions produce distinct ridges in ellipticity and angular momentum observables.

Static neutron stars in GR are generally not compact enough to have an external light ring, except for extreme EoS.

Rotating stars may form an outer light ring, which can constrain the EoS or gravity theories.

Abstract

We identify combinations of observables for rotating neutron stars that can one day bear on the question of whether there can be first order phase transitions in the neutron matter therein. We employ the Hartle-Thorne theory for stationary, rotating neutron stars at conventional angular velocities (in the pulsar and millisecond pulsar ranges) and extract three-dimensional sections of the ellipticity or the dynamical angular momentum as function of the star's mass and angular velocity. An eventual first order phase transition in the equation of state (EoS) leaves a clear ridge (nonanalyticity) in these observables, akin to the sudden kink in popular mass-radius diagrams for static stars. Finally, we observe that static neutron stars in General Relativity (GR) will fail to be compact enough for the light ring's position at r=3M to be outside the star, except for the most extreme equations of state. The outer light ring of a rotating star might however be formed unless the EoS softens too much, and its eventual detection can then be used to constrain the EoS (or the gravity theory).