Radiative cooling effects on black hole hot accretion flows around the sub-Bondi radius

TL;DR

This study uses 2D magnetohydrodynamic simulations to explore how radiative cooling influences wind production and accretion flow stability around black holes near the sub-Bondi radius, revealing cooling's suppressive effects.

Contribution

It provides new insights into the impact of radiative cooling on wind strength and accretion flow stability at the sub-Bondi radius, an area less explored in previous simulations.

Findings

Radiative cooling reduces the thickness of accretion disks as the mass accretion rate increases.

Winds significantly decrease the inward mass flow in weak cooling scenarios.

Strong radiative cooling suppresses winds and shifts the dominant inward mass flow mechanism to turbulence.

Abstract

It is difficult to implement numerical simulations on a region extending from the vicinity of a black hole to the Bondi radius. Most previous numerical simulations have primarily concentrated on the region close to the black hole. They found that strong winds can be generated in the hot accretion flows near the black hole, and that radiative cooling significantly affects the strength of these winds. However, the effects of radiative cooling on the production and properties of winds around the Bondi radius remain unclear. In this paper, we perform two-dimensional magnetohydrodynamic simulations to study the impact of radiative cooling on the dynamics and wind production in hot accretion flows around the sub-Bondi radius. As the increase of mass accretion rate, radiative cooling gradually becomes strong, resulting in a reduction in the thickness of the accretion disk (defined as the accretion flows within the density scale height). Based on the H{\o}iland criterion, we find that within the accretion disk, the region of convective stability accounts for $\sim$ 55 - 62 %, and therefore the accretion flows are marginally stable in convective stability. In the runs with weak radiative cooling, the winds play a significant role in the inward decrease of the mass inflow rate. In the runs with strong radiative cooling, the mass outflow rate of winds is significantly reduced and then the inward decrease of the mass inflow rate is mainly attributed to turbulence driven by magnetorotational instability and convection. Radiative cooling has the potential to suppress accretion processes and reduce the power of winds.