Rapidly rotating neutron stars: Universal relations and EOS inference

TL;DR

This paper develops universal relations to estimate key properties of rotating neutron stars from non-rotating models, enabling efficient EOS inference with high-precision observational data.

Contribution

It introduces accurate universal relations for rotating neutron stars that improve parameter estimation and incorporate rotation effects into Bayesian EOS inference frameworks.

Findings

Universal relations enable estimation of I and T/W from non-rotating models.

The relations facilitate computationally cheap EOS inference including rotation.

Inference results achieve around percent level precision with simulated data.

Abstract

We provide accurate universal relations that allow to estimate the moment of inertia $I$ and the ratio of kinetic to gravitational binding energy $T/W$ of uniformly rotating neutron stars from the knowledge of mass, radius, and moment of inertia of an associated non-rotating neutron star. Based on these, several other fluid quantities can be estimated as well. Astrophysical neutron stars rotate to varying degrees, and, although rotational effects may be neglected in some cases, not modeling them will inevitably introduce bias when performing parameter estimation. This is especially important for future, high-precision measurements coming from electromagnetic and gravitational wave observations. The proposed universal relations facilitate computationally cheap EOS inference codes that permit the inclusion of observations of rotating neutron stars. To demonstrate this, we deploy them into a recent Bayesian framework for equation of state parameter estimation that is now valid for arbitrary, uniform rotation. Our inference results are robust up to around percent level precision for the generated neutron star observations, consisting of the mass, equatorial radius, rotation rate, as well as co- and counter-rotating $f$-mode frequencies, that enter the framework as data.