Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD

TL;DR

This study uses advanced 3D GRMHD simulations to explore how magnetic fields influence black hole accretion, revealing suppressed accretion rates and energy feedback mechanisms across scales, aligning with observed phenomena.

Contribution

It introduces a multi-scale GRMHD simulation method that captures magnetic effects on SMBH accretion and feedback, providing new insights into low accretion rates and energy transport.

Findings

Magnetic fields saturate near the SMBH, altering density profiles.

Accretion rate is suppressed by over 2 orders of magnitude.

Energy feedback occurs at about 1% of accretion energy.

Abstract

Fueling and feedback couple supermassive black holes (SMBHs) to their host galaxies across many orders of magnitude in spatial and temporal scales, making this problem notoriously challenging to simulate. We use a multi-zone computational method based on the general relativistic magneto-hydrodynamic (GRMHD) code KHARMA that allows us to span $7$ orders of magnitude in spatial scale, to simulate accretion onto a non-spinning SMBH from an external medium with Bondi radius $R_B\approx 2\times 10^5 \,GM_\bullet/c^2$, where $M_\bullet$ is the SMBH mass. For the classic idealized Bondi problem, spherical gas accretion without magnetic fields, our simulation results agree very well with the general relativistic analytic solution. Meanwhile, when the accreting gas is magnetized, the SMBH magnetosphere becomes saturated with a strong magnetic field. The density profile varies as $\sim r^{-1}$ rather than $r^{-3/2}$ and the accretion rate $\dot{M}$ is consequently suppressed by over 2 orders of magnitude below the Bondi rate $\dot{M}_B$. We find continuous energy feedback from the accretion flow to the external medium at a level of $\sim10^{-2}\dot{M}c^2 \sim 5\times 10^{-5} \dot{M}_B c^2$. Energy transport across these widely disparate scales occurs via turbulent convection triggered by magnetic field reconnection near the SMBH. Thus, strong magnetic fields that accumulate on horizon scales transform the flow dynamics far from the SMBH and naturally explain observed extremely low accretion rates compared to the Bondi rate, as well as at least part of the energy feedback.