A Global Three Dimensional Radiation Magneto-hydrodynamic Simulation of Super-Eddington Accretion Disks (1410.0678v1)

Abstract: We study super-Eddington accretion flows onto black holes using a global three dimensional radiation magneto-hydrodynamical simulation. We solve the time dependent radiative transfer equation for the specific intensities to accurately calculate the angular distribution of the emitted radiation. Turbulence generated by the magneto-rotational instability provides self-consistent angular momentum transfer. The simulation reaches inflow equilibrium with an accretion rate ~220L_edd/c^2 and forms a radiation driven outflow along the rotation axis. The mechanical energy flux carried by the outflow is ~20% of the radiative energy flux. The total mass flux lost in the outflow is about 29% of the net accretion rate. The radiative luminosity of this flow is ~10L_edd. This yields a radiative efficiency ~4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in most models, such as the slim disk model, which neglect vertical advection. Our inferred radiative efficiencies also exceed published results from previous global numerical simulations, which did not attribute a significant role to vertical advection. We briefly discuss the implications for the growth of supermassive black holes in the early universe and describe how these results provided a basis for explaining the spectrum and population statistics of ultraluminous X-ray sources.

• The paper demonstrates that 3D RMHD simulations achieve an inflow equilibrium with an accretion rate of ~220/c² and induce radiation-driven outflows losing ~29% of the net mass flux.
• It reveals vertical advection, propelled by magnetic buoyancy, allows photons to escape more efficiently, boosting radiative efficiency to about 4.5% compared to slim disk models.
• Results imply that efficient super-Eddington accretion could drive rapid supermassive black hole growth in the early universe and explain the luminosity of ultraluminous X-ray sources.

Overview of Three-Dimensional Radiation Magneto-Hydrodynamic Simulation of Super-Eddington Accretion Disks

This paper presents a comprehensive examination of super-Eddington accretion flows onto black holes using three-dimensional radiation magneto-hydrodynamic (RMHD) simulations. These simulations incorporate a sophisticated treatment of radiative transfer, enabling a detailed analysis of the angular distribution of emitted radiation and the dynamics of the accretion disk.

Key Insights and Numerical Results

The simulation reaches an inflow equilibrium state with an accretion rate of approximately $220/c^2$. A significant outcome of the paper is the formation of a radiation-driven outflow along the rotation axis, with the mechanical energy flux in the outflow reaching about 20% of the radiative energy flux. The total mass flux lost in the outflow is approximately 29% of the net accretion rate, emphasizing the dynamic nature of the accretion process and the potential for substantial mass and energy loss through outflows.

The radiative luminosity of the flow achieves a value of approximately 10 times the Eddington luminosity. This corresponds to a radiative efficiency of about 4.5%, which aligns closely with predictions from the standard thin disk model. This offers a clear contrast to the slim disk models and previous global numerical simulations, both of which typically infer lower radiative efficiencies due to neglecting vertical advection as a cooling mechanism.

Vertical Advection and Energy Transport

A pivotal finding of this research is the role of vertical advection in energy transport within the accretion disk. The magnetic buoyancy instigates vertical advection that outpaces photon diffusion, allowing photons to escape the disk before being advected into the black hole. This mechanism leads to higher effective radiative efficiencies than those predicted by traditional slim disk models, in which energy advection remains primarily radial. These results underscore the importance of considering multi-dimensional energy transport processes in theoretical models of super-Eddington accretion.

Implications and Future Work

The simulation's findings have significant implications for our understanding of the growth of supermassive black holes in the early universe, suggesting that efficient, super-Eddington accretion could enable rapid black hole growth even at high redshifts. Moreover, these insights contribute to explanations of the spectral and population characteristics of ultraluminous X-ray sources (ULXs), possibly reconciling the high luminosities and efficiencies observed without invoking unrealistically large beaming factors.

Future work is anticipated to extend these simulations to include the effects of general relativity, black hole spin, and Compton scattering. Such work could provide a more comprehensive understanding of the complex interplay between magnetic fields, radiation, and gravity in the context of super-Eddington accretion flows. Expanding the simulation domain to capture a broader radial range could also yield detailed insights into the radial profiles of quantifiable observables such as radiation flux.

Overall, this paper advances our understanding of accretion physics and offers critical insights that challenge existing models while paving the way for further exploration and refinement of high-energy astrophysical phenomena.