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

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.

Beamed emission from a neutron-star ULX in a GRRMHD simulation

“…It is also evident that the polar region is largely devoid of gas. The gas is confined by the magnetic field to mid-latitudes, strongly contrasting to what was observed in Abarca et al (2018), where the absence of a stellar magnetic field allowed outflowing gas to fill the whole domain.…”

“…In order to model a neutron star accreting at super-Eddington accretion rates, it is necessary to run general-relativistic radiative magnetohydrodynamic (GRRMHD) simulations. As of writing this letter, there is only one such global simulation (Takahashi & Ohsuga 2017) which includes a stellar magnetic field, and two which do not (Takahashi et al 2018;Abarca et al 2018).…”

No magnetars in ULXs

“…Numerical simulations, although very impressive, cannot be of much help (even if they discuss the "spherization radius"). Instead of an external thin disc, they have a torus (Takahashi & Ohsuga 2017;Takahashi, Mineshige & Ohsuga 2018;Abarca, Kluźniak & Sadowski 2018). The critical radii are uncertain by no more than factors ∼ 1, so we expect (as here) the results of KLK17 will remain robust in comparison with both observations and future numerical simulations.…”

Super-Eddington accretion discs with advection and outflows around magnetized neutron stars

“…While most of this theoretical work on supercritical accretion was devoted to discs around black holes, the discovery of ULXPs has drawn attention to magnetized NSs accreting at high rates. Recently, simulations of supercritical accretion onto a nonmagnetized NS were performed by Abarca et al (2018). It is important to note that even in highly supercritical ULXPs like M82 X-2 and NGC 7793 P13, the accretion disc outside the magnetosphere may remain in a sub-critical regime.…”