Bardeen-Petterson Alignment, Jets and Magnetic Truncation in GRMHD Simulations of Tilted Thin Accretion Discs (1810.00883v2)

Abstract: Prevalent around luminous accreting black holes, thin discs are challenging to resolve in numerical simulations. When the disc and black hole angular momentum vectors are misaligned, the challenge becomes extreme, requiring adaptive meshes to follow the disc proper as it moves through the computational grid. With our new high-performance general relativistic magnetohydrodynamic (GRMHD) code H-AMR we have simulated the thinnest accretion disc to date, of aspect ratio H/R~0.03, around a rapidly spinning (a=0.9375) black hole, using a cooling function. Initially tilted at 10 degrees, the disc warps inside r~5 r_g into alignment with the black hole, where r_g is the gravitational radius. This is the first demonstration of Bardeen-Petterson alignment in MHD with viscosity self-consistently generated by magnetized turbulence. The disc develops a low-density high-viscosity (alpha_eff ~ 1.0) magnetic-pressure--dominated inner region at r<25 r_g that rapidly empties itself into the black hole. This inner region may in reality, due to thermal decoupling of ions and electrons, evaporate into a radiatively inefficient accretion flow if, as we propose, the cooling time exceeds the accretion time set by the order unity effective viscosity. We furthermore find the unexpected result that even our very thin disc can sustain large-scale vertical magnetic flux on the black hole, which launches powerful relativistic jets that carry 20-50% of the accretion power along the angular momentum vector of the outer tilted disc, providing a potential explanation for the origin of jets in radio-loud quasars.

• The paper demonstrates BP alignment by aligning the inner disc with the black hole’s equatorial plane up to approximately 5r_g.
• The paper shows that extremely thin discs can maintain vertical magnetic flux to launch powerful relativistic jets with 20-50% energy conversion efficiency.
• The paper reveals that magnetic pressure truncates the accretion disc, forming a low-density inner region around 25r_g that may lead to disc evaporation.

An Overview of Accreting Systems: Bardeen-Petterson Alignment, Jets, and Magnetic Dynamics

The paper explores the complex dynamics of accretion discs around rapidly spinning black holes (BHs), with a focus on the Bardeen-Petterson (BP) alignment mechanism, jet formation, and magnetic field influences within thin, tilted accretion discs. The paper harnesses high-resolution general relativistic magnetohydrodynamics (GRMHD) simulations that challenge existing paradigms and provide quantitative insights into these astrophysical phenomena.

Simulation Approach

Utilizing the advanced GRMHD code H-AMR, the researchers simulate an incredibly thin accretion disc (aspect ratio $H/R \approx 0.03$) around a Kerr black hole with a spin parameter $a = 0.9375$. The system features a misalignment between the accretion disc and the BH's spin axis, imposing significant computational demands and necessitating adaptive mesh refinement (AMR) to capture the associated complex disc dynamics accurately.

Key Findings

1. Bardeen-Petterson Alignment: For the first time, the simulations demonstrate the BP alignment within magnetized, turbulent accretion discs, showcasing alignment of the inner disc with the BH's equatorial plane up to a radius of approximately $5r_g$. This finding supports theoretical predictions and provides a more nuanced understanding of the angular momentum dynamics in thin accretion discs.
2. Jet Formation and Dynamics: Contrary to traditional expectations, the paper reveals that even extremely thin discs can sustain significant vertical magnetic flux, which in turn drives the emergence of powerful relativistic jets. These jets boast an efficiency of converting accretion power into outflow power in the range of 20-50%.
3. Magnetic Truncation and Disc Structure: The inner disc develops into a low-density, high-viscosity region with effective viscosity $\alpha_{\rm eff} \sim 1.0$, suggesting a magnetic-pressure-dominated environment. The outer high-density region transitions into this low-density inner region around $r \approx 25r_g$. This structure may potentially lead to disc evaporation into a radiatively inefficient accretion flow under specific thermal dynamics.

Implications and Future Directions

The findings challenge existing models of BH accretion, particularly those excluding jet formation from thin discs. The demonstrated BP alignment supports mechanisms for BH spin evolution that could influence supermassive BH growth models. In practical terms, understanding how jets and accretion discs interact provides insights into feedback mechanisms in galaxy evolution and quasar activity.

Additionally, the results imply that the presence of large-scale poloidal magnetic fields is a crucial factor for maintaining jet activity, urging further exploration of these dynamics in transitioning accretion states such as those seen in X-ray binaries during state changes.

Future research avenues may include extending these simulations to encompass a more comprehensive range of disc parameters, thereby correlating specific dynamical states to observed astrophysical phenomena. Expanding the exploration of different magnetic field initial conditions and their effects on angular momentum transport and outflow efficiency could yield deeper insights into disc-jet symbiosis in astrophysical contexts.

In essence, this paper significantly advances the field’s understanding of thin accretion disc behavior within the complex relativistic environments of spinning BHs, opening pathways for future research and theoretical exploration.