Two-dimensional Radiation-hydrodynamic Model for Supercritical Disk Accretion Flows onto Neutron Stars

TL;DR

This study uses two-dimensional radiation hydrodynamic simulations to explore supercritical accretion flows onto neutron stars, revealing shell-shaped high-density regions, strong outflows, and implications for SS433 jets.

Contribution

First detailed 2D simulations of supercritical accretion onto neutron stars, highlighting differences from black hole accretion and explaining jet properties.

Findings

Mass-accretion rate exceeds critical Eddington rate but is lower than that onto black holes.

Outflows reach velocities of 0.2-0.3c with high mass-outflow rates.

Supercritical accretion onto neutron stars can explain SS433 jets, with some differences in collimation angle.

Abstract

We performed two-dimensional radiation hydrodynamic simulations of supercritical accretion flows around neutron stars (NSs). In contrast with the accretion flows onto black holes (BHs), we find that the shell-shaped high-density regions form around the NSs, since the radiation force is enhanced in the innermost regions. The enhanced radiation force drives strong outflows above and below the disk. The mass-accretion rate onto the NS exceeds the critical rate, $L_{\rm E}/c^2$, with $L_{\rm E}$ being the Eddington luminosity. However it is about $20-30%$ of that onto the BH, under the condition that we employ the same mass-input rate, $\dot{M}_{\rm input}$, which is mass injected from the outer disk boundary per unit time. The mass-outflow rate is a few-times larger in flows around NSs than in flows around BHs. The supercritical NS accretion flows mainly release the accretion energy as the kinetic energy of the outflows, though the disk luminosity is predominant over the kinetic energy output rate in the BH accretion flows. The resulting velocity and mass-outflow rate of the outflows are $0.2-0.3c$ and $150-700L_{\rm E}/c^2$, respectively, for the mass-input rate of $3\times 10^2\lsim \dot{M}_{\rm input}/(L_{\rm E}/c^2)\lsim 3\times 10^3$. This implies that the SS433 jets can be roughly explained by the supercritical accretion onto a NS. However, the collimation angle of the outflows in our simulations ($\sim 20^\circ$) is larger than that of the SS433 jets (a few degrees).