Magnetically modified spherical accretion in GRMHD: reconnection-driven convection and jet propagation

We present 3D general relativistic magnetohydrodynamic simulations of zero angular momentum accretion around a rapidly rotating black hole, modified by the presence of initially uniform magnetic fields. We consider several angles between the magnetic field direction and the black hole spin. In the resulting flows, the mid-plane dynamics are governed by magnetic reconnectiondriven turbulence in a magnetically arrested (or a nearly arrested) state. Electromagnetic jets with outflow efficiencies similar to 10-200 per cent 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 similar to 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 degrees-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.

Automated summary

The 3D general relativistic magnetohydrodynamic simulations show that electromagnetic jets with outflow efficiencies can reach several hundred gravitational radii before dissipating. The direction of the jets fluctuates in time and can be tilted by as much as 30 degrees with respect to black hole spin. The simulations suggest that a tilted initial magnetic field can optimize the amount of magnetic flux reaching the black hole.