Levitating atmospheres of Eddington-luminosity neutron stars

TL;DR

This paper models relativistic, levitating atmospheres around luminous neutron stars, revealing their properties and potential relevance to X-ray burst phenomena.

Contribution

It introduces a fully general relativistic model of neutron star atmospheres supported by radiation flux, including temperature-dependent scattering opacity and anisotropic radiation treatment.

Findings

Atmospheres can be optically thick and levitate above the star surface.

Photospheric radius expands with increasing luminosity.

Relativistic effects cause gas pressure and density inversion.

Abstract

We construct models of static, spherically symmetric shells supported by the radiation flux of a luminous neutron star in the Schwarzschild metric. The atmospheres are disconnected from the star and levitate above its surface. Gas pressure and density inversion appear in the inner region of these atmospheres, which is a purely relativistic phenomenon. We account for the scattering opacity dependence on temperature and utilize the relativistic M1 closure scheme for the radiation tensor, hence allowing for a fully GR-consistent treatment of the photon flux and radiation tensor anisotropy. In this way we are able to address atmospheres of both large and moderate/low optical depths with the same set of equations. We discuss properties of the levitating atmospheres and find that they may indeed be optically thick, with the distance between star surface and the photosphere expanding as luminosity increases. These results may be relevant for the photosphereric radius expansion X-ray bursts.