Magnetically Modified Spherical Accretion in GRMHD: Reconnection-Driven Convection and Jet Propagation

TL;DR

This paper uses 3D GRMHD simulations to explore how magnetic reconnection influences accretion flows and jet formation around rotating black holes, revealing turbulence-driven jets with fluctuating directions and efficiencies up to 200%.

Contribution

It demonstrates that turbulence and magnetic reconnection can produce powerful, fluctuating jets without requiring initial net vertical magnetic flux, advancing understanding of low luminosity black hole accretion.

Findings

Jets reach several hundred gravitational radii before dissipating.

Jet directions fluctuate up to 30 degrees, independent of initial magnetic field tilt.

Peak jet power occurs with initial magnetic field tilted by 40-80 degrees.

Abstract

We present 3D general relativistic magnetohydrodynamic(GRMHD) simulations of zero angular momentum accretion around a rapidly rotating black hole, modified by the presence of initially uniform magnetic fields. We consider serveral angles between the magnetic field direction and the black hole spin. In the resulting flows, the midplane dynamics are governed by magnetic reconnection-driven turbulence in a magnetically arrested (or a nearly arrested) state. Electromagnetic jets with outflow efficiencies ~10-200% occupy the polar regions, reaching several hundred gravitational radii before they dissipate due to the kink instability. The jet directions fluctuate in time and can be tilted by as much as ~30 degrees with respect to black hole spin, but this tilt does not depend strongly on the tilt of the initial magnetic field. A jet forms even when there is no initial net vertical magnetic flux since turbulent, horizon-scale fluctuations can generate a net vertical field locally. Peak jet power is obtained for an initial magnetic field tilted by 40-80 degrees with respect to the black hole spin because this maximizes the amount of magnetic flux that can reach the black hole. These simulations may be a reasonable model for low luminosity black hole accretion flows such as Sgr A* or M87.