Formation of Precessing Jets by Tilted Black-hole Discs in 3D General Relativistic MHD Simulations

TL;DR

This study uses advanced 3D GRMHD simulations to show that tilted accretion discs around spinning black holes can launch jets aligned with the disc's rotation axis, and these jets can serve as probes of disc precession.

Contribution

First high-resolution 3D GRMHD simulations demonstrating jet formation along tilted disc axes and their evolution towards alignment and precession.

Findings

Jets are launched along the tilted disc's rotation axis.

Strong magnetic flux can align jets with black hole spin.

Jets can be used to probe disc precession.

Abstract

Gas falling into a black hole (BH) from large distances is unaware of BH spin direction, and misalignment between the accretion disc and BH spin is expected to be common. However, the physics of tilted discs (e.g., angular momentum transport and jet formation) is poorly understood. Using our new GPU-accelerated code H-AMR, we performed 3D general relativistic magnetohydrodynamic simulations of tilted thick accretion discs around rapidly spinning BHs, at the highest resolution to date. We explored the limit where disc thermal pressure dominates magnetic pressure, and showed for the first time that, for different magnetic field strengths on the BH, these flows launch magnetized relativistic jets propagating along the rotation axis of the tilted disc (rather than of the BH). If strong large-scale magnetic flux reaches the BH, it bends the inner few gravitational radii of the disc and jets into partial alignment with the BH spin. On longer time scales, the simulated disc-jet system as a whole undergoes Lense-Thirring precession and approaches alignment, demonstrating for the first time that jets can be used as probes of disc precession. When the disc turbulence is well-resolved, our isolated discs spread out, causing both the alignment and precession to slow down.