# Dynamically stable ergostars exist: General relativistic models and simulations

## TL;DR

This paper presents the first demonstration of dynamically stable ergostars, equilibrium neutron stars with ergoregions, through full general relativistic simulations, revealing their potential astrophysical significance.

## Contribution

It introduces the first stable ergostar models with a causal equation of state and confirms their stability via long-term dynamical evolution in full general relativity.

## Key findings

- Stable ergostars can exist for over 30 rotational periods.
- They are highly differentially rotating hypermassive neutron stars.
- Ergostars may form after binary neutron star mergers and power gamma-ray bursts.

## Abstract

We construct the first dynamically stable ergostars (equilibrium neutron stars that contain an ergoregion) for a compressible, causal equation of state. We demonstrate their stability by evolving both strict and perturbed equilibrium configurations in full general relativity for over a hundred dynamical timescales ($\gtrsim 30$ rotational periods) and observing their stationary behavior. This stability is in contrast to earlier models which prove radially unstable to collapse. Our solutions are highly differentially rotating hypermassive neutron stars with a corresponding spherical compaction of $C=0.3$. Such ergostars can provide new insights into the geometry of spacetimes around highly compact, rotating objects and on the equation of state at supranuclear densities. Ergostars may form as remnants of extreme binary neutron star mergers and possibly provide another mechanism for powering short gamma-ray bursts.