Formation of Precessing Jets by Tilted Black-hole Discs in 3D General Relativistic MHD Simulations (1707.06619v4)

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.

• The paper shows that tilted black hole accretion disks produce precessing jets that align with the disk’s rotation rather than the black hole’s spin.
• It reveals that magnetic field strength critically influences jet alignment and can boost jet speeds to Lorentz factors as high as ten.
• The study emphasizes that high numerical resolution is essential to accurately capture magnetorotational instability and angular momentum transport.

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

The paper presented in the paper explores the dynamics and jet formation processes of tilted black hole (BH) accretion disks using three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations. The authors leverage their novel GPU-accelerated H-AMR code to achieve unprecedented levels of resolution and simulation duration, providing new insights into the behavior of these complex systems.

The paper's primary focus is on understanding the influence of tilted accretion disks on jet formation and precession. Tilted accretion disks, often observed in systems such as X-ray binaries and AGN, are challenging to model due to the misalignment of angular momentum vectors between the disk and the black hole spin. This misalignment can lead to phenomena such as Lense-Thirring precession, which has been implicated in the mechanisms behind quasi-periodic oscillations in black hole systems.

The authors conducted highly resolved simulations of accretion flows around rapidly spinning black holes. These simulations revealed that jets from tilted disks can indeed align with the disk's rotational axis rather than the black hole's spin axis—a critical finding that was not firmly established in lower-resolution studies. Furthermore, the jets and disks were observed to undergo simultaneous precession, providing a potential method for using jets as probes of disk precession.

Notably, the paper demonstrates that the jets are capable of achieving relativistic speeds, reaching Lorentz factors as high as ten within relatively small spatial extents. The strength of the magnetic field plays a significant role in whether the jets align with the black hole's spin. In scenarios with strong magnetic flux (e.g., magnetically arrested disk or MAD state), the inward magnetic pressure partially aligns the inner disk and jets with the black hole spin at closer radii, contrasting with scenarios of weaker magnetic fields.

The authors report that numerical resolution plays a vital role in accurately simulating these processes. Properly resolving magnetorotational instability (MRI) is especially critical for modeling the angular momentum transport and expansion of the accretion disk. Such resolutions influence the rate of precession and alignment with the black hole spin, indicating the necessity for high-resolution simulations to capture the subtle dynamics of these astrophysical systems.

One implication of the paper is its potential to link jet dynamics with observational data on X-ray oscillations and jet precession in black hole binaries and AGNs. The research provides a theoretical framework for interpreting Type-C quasi-periodic oscillations and AGN jet precessions, which are linked to the physical characteristics of the accretion flow and jet dynamics. The results might offer explanations for phenomena such as variability in jet orientations and challenges in solving the cooling flow problem in galactic clusters.

Overall, the findings of this paper advance our understanding of accretion disk physics and jet dynamics in astrophysical environments. Future research may extend these simulations to explore more parameters, such as different BH spins, accretion rates, and magnetic field configurations, as well as incorporating realistic feeding from large-scale accretion environments. This will enhance our ability to simulate and predict the complex behaviors of accreting black holes and their associated relativistic jets.