# Optically thin outbursts of rotating neutron stars can not be spherical

## TL;DR

This paper studies how rotation and radiation forces affect matter ejected from neutron stars, revealing that instead of forming spherical shells, matter collapses into equatorial rings due to combined effects.

## Contribution

It demonstrates that rotation breaks spherical symmetry in neutron star outflows, leading to equatorial ring formation instead of spherical shells, which was not previously understood.

## Key findings

- Spherical shells cannot be sustained in rotating neutron star outflows.
- Radiation drag forces push matter towards the equatorial plane.
- Ejected matter collapses into stable equatorial rings.

## Abstract

We investigate three-dimensional relativistic trajectories of test particles in the spacetime of a slowly rotating compact star, under the combined influence of gravity and a strong, near-Eddington radiation field. While in the static case a spherically symmetric shell of matter suspended above the stellar surface can be formed at the location of radial equilibrium of effective forces, the same is not true for a rotating star. In the latter case the symmetry is broken by the interplay between motion in the non-static spacetime and the influence of strong radiation drag forces, pushing particles towards the equatorial plane. As a result an expanding spherical shell of matter ejected from the neutron star surface collapses on a short timescale into a single stable equatorial ring supported by radiation. These findings have implications for the geometry of optically thin outflows during luminous neutron star bursts.