Radiative GRMHD simulations of accretion and outflow in non-magnetized neutron stars and ultraluminous X-ray sources

TL;DR

This study uses GRRMHD simulations to compare accretion and outflows around non-magnetized neutron stars and black holes, revealing differences in luminosity, outflow properties, and potential pulsation obscuration in ULXs.

Contribution

First GRRMHD simulations of super-Eddington accretion on non-magnetized neutron stars, highlighting unique flow structures and observational signatures.

Findings

Neutron star simulations show a transition layer with higher outflow rates.

Neutron star luminosity appears Eddington-limited from all viewing angles.

Outflows are optically thick, potentially obscuring pulsations in ULXs.

Abstract

We run two GRRMHD simulations of super-Eddington accretion disks around a black hole and a non-magnetized, non-rotating neutron star. The neutron star was modeled using a reflective inner boundary condition. We observe the formation of a transition layer in the inner region of the disk in the neutron star simulation which leads to a larger mass outflow rate and a lower radiative luminosity over the black hole case. Sphereization of the flow leads to an observable luminosity at infinity around the Eddington value when viewed from all directions for the neutron star case, contrasting to the black hole case where collimation of the emission leads to observable luminosities about an order of magnitude higher when observed along the disk axis. We find the outflow to be optically thick to scattering, which would lead to the obscuring of any neutron star pulsations observed in corresponding ULXs.