On the last stable orbit around rapidly rotating neutron stars

TL;DR

This paper derives accurate analytic formulas for the last stable orbit around rotating neutron stars, aiding astrophysical modeling by comparing these results with Kerr black hole metrics.

Contribution

It provides simple, accurate formulas for the binding energy and angular momentum at the last stable orbit around rotating neutron stars, validated across multiple equations of state.

Findings

Formulas fit numerical results within high accuracy

Results are consistent across different EOS models

Comparison with Kerr metric highlights differences in neutron star orbits

Abstract

We compute the binding energy and angular momentum of a test-particle at the last stable circular orbit (LSO) on the equatorial plane around a general relativistic, rotating neutron star (NS). We present simple, analytic, but accurate formulas for these quantities that fit the numerical results and which can be used in several astrophysical applications. We demonstrate the accuracy of these formulas for three different equations of state (EOS) based on nuclear relativistic mean-field theory models and argue that they should remain still valid for any NS EOS that satisfy current astrophysical constraints. We compare and contrast our numerical results with the corresponding ones for the Kerr metric characterized by the same mass and angular momentum.