Super-Eddington Accretion Disks around Supermassive black Holes (1709.02845v1)

Abstract: We use global three dimensional radiation magneto-hydrodynamical simulations to study accretion disks onto a $5\times 10^8M_{\odot}$ black hole with accretion rates varying from $\sim 250L_{Edd}/c^2$ to $1500 L_{Edd}/c^2$. We form the disks with torus centered at $50-80$ gravitational radii with self-consistent turbulence initially generated by the magneto-rotational instability. We study cases with and without net vertical magnetic flux. The inner regions of all disks have radiation pressure $\sim 10^4-10^6$ times the gas pressure. Non-axisymmetric density waves that steepen into spiral shocks form as gas flows towards the black hole. In simulations without net vertical magnetic flux, Reynolds stress generated by the spiral shocks are the dominant mechanism to transfer angular momentum. Maxwell stress from MRI turbulence can be larger than the Reynolds stress only when net vertical magnetic flux is sufficiently large. Outflows are formed with speed $\sim 0.1-0.4c$. When the accretion rate is smaller than $\sim 500 L_{Edd}/c^2$, outflows start around $10$ gravitational radii and the radiative efficiency is $\sim 5\%-7\%$ with both magnetic field configurations. With accretion rate reaching $1500 L_{Edd}/c^2$, most of the funnel region close to the rotation axis becomes optically thick and the outflow only develops beyond $50$ gravitational radii. The radiative efficiency is reduced to $1\%$. We always find the kinetic energy luminosity associated with the outflow is only $\sim 15\%-30\%$ of the radiative luminosity. The mass flux lost in the outflow is $\sim 15\%-50\%$ of the net mass accretion rates. We discuss implications of our simulation results on the observational properties of these disks.

• The paper demonstrates that high-resolution 3D RMHD simulations reveal radiation pressure dominance driving complex disk dynamics around SMBHs.
• The paper shows that angular momentum transfer shifts from spiral shock-induced Reynolds stress to MRI-driven Maxwell stress with net vertical magnetic flux.
• The paper finds that outflow speeds range from 0.1c to 0.4c while radiative efficiency drops from 5%-7% to 1% as the accretion rate increases.

Super-Eddington Accretion Disks around Supermassive Black Holes: A Numerical Study

The paper examines the intricate dynamics of super-Eddington accretion disks around supermassive black holes (SMBHs) through detailed numerical simulations. Utilizing three-dimensional radiation magnetohydrodynamical (RMHD) simulations, the paper explores accretion flows onto a black hole with a mass of $5 \times 10^8 M_\odot$ and varying accretion rates from $250 \dot{M}_{\rm Edd}$ to $1500 \dot{M}_{\rm Edd}$. A significant focus is placed on the influence of different magnetic field configurations, both with and without net vertical magnetic flux.

Key Findings

• Radiation Pressure Dominance: The inner regions of the disks are characterized by radiation pressures that range from $10^4$ to $10^6$ times the gas pressure. This results in significant radiation-driven dynamics across the accretion flow.
• Angular Momentum Transfer: In the absence of net vertical magnetic flux, angular momentum is primarily transferred through Reynolds stress generated by spiral shocks. When sufficient net vertical magnetic flux exists, the Maxwell stress from magneto-rotational instability (MRI) turbulence can surpass Reynolds stress, enhancing angular momentum transport.
• Outflows and Radiative Efficiency: Outflows emerge with speeds between $0.1c$ to $0.4c$. The onset of outflows and their extent depends on the rate of accretion. When the accretion rate is below $500 \dot{M}_{\rm Edd}$, the outflows initiate around $10$ gravitational radii with a radiative efficiency of $5\%-7\%$. At the higher accretion rate of $1500 \dot{M}_{\rm Edd}$, the radiative efficiency drops to $1\%$, and outflows develop beyond $50$ gravitational radii.
• Energy Flux Comparisons: The kinetic energy luminosity of the outflows is only a fraction ($15\%-30\%$) of the radiative luminosity. The mass loss in the form of outflows varies significantly and constitutes $15\%-50\%$ of the net mass accretion rate.

Implications and Future Directions

The results have substantial implications for understanding the observational characteristics of AGNs possessing super-Eddington accretion disks. The documented ranges of radiative and kinetic energy outflows, alongside variable radiative efficiencies, provide important guidance for interpreting AGN feedback mechanisms in host galaxies. The paper hints at broader implications for the evolution and energy regulation of galaxies influenced by active galactic nuclei, particularly when the central SMBH undergoes periods of super-Eddington accretion.

Future work could greatly benefit by exploring the transition from sub-Eddington to super-Eddington regimes in AGN disks with expanded numerical models. Additionally, incorporating relativistic effects would be crucial for simulations involving spinning black holes and studying the potential development of relativistic jets. It would also be advantageous to include frequency-dependent radiation transport to better capture the spectral properties of AGN emissions, potentially aligning simulation outcomes more closely with observations. Lastly, assessing the impact of line-driven winds on AGN disk dynamics could provide deeper insights into mass ejection processes associated with luminous quasar activity.

This paper underscores the complexities of super-Eddington accretion disks and demonstrates the critical role of advanced RMHD simulations in enhancing our theoretical understanding of AGN phenomena.