Radiation hydrodynamical simulations of super-Eddington mass transfer and black hole growth in close binaries

TL;DR

This study uses advanced radiation-hydrodynamical simulations to explore how super-Eddington mass transfer in close binaries leads to radiation-driven outflows, affecting black hole growth and binary evolution.

Contribution

It presents the first multi-dimensional simulations of super-Eddington accretion in close binaries, revealing outflow mechanisms and proposing a new formula for mass transfer regulation.

Findings

Rapid mass transfer causes significant outflows that regulate accretion rates.

Outflows are anisotropic, occurring mainly toward specific binary points.

A new formula links outflows to binary evolution and black hole growth.

Abstract

Radiation-driven outflows play a crucial role in extracting mass and angular momentum from binary systems undergoing rapid mass transfer at super-Eddington rates. To study the mass transfer process from a massive donor star to a stellar-mass black hole (BH), we perform multi-dimensional radiation-hydrodynamical simulations that follow accretion flows from the first Lagrange point down to about a hundred times the Schwarzschild radius of the accreting BH. Our simulations reveal that rapid mass transfer occurring at over a thousand times the Eddington rate leads to significant mass loss from the accretion disk via radiation-driven outflows. Consequently, the inflow rates at the innermost radius are regulated by two orders of magnitude smaller than the transfer rates. We find that convective motions within the accretion disk drive outward energy and momentum transport, enhancing the radiation pressure in the outskirts of the disk and ultimately generating large-scale outflows with sufficient energy to leave the binary. Furthermore, we observe strong anisotropy in the outflows, which occur preferentially toward both the closest and furthest points from the donor star. However, when averaged over all directions, the specific angular momentum of the outflows is nearly comparable to the value predicted in the isotropic emission case. Based on our simulation results, we propose a formula that quantifies the mass growth rates on BHs and the mass loss rates from binaries due to radiation-driven outflows. This formula provides important implications for the binary evolution and the formation of merging binary BHs.