Super-Eddington Accretion Disks around Supermassive Black Holes

Abstract

We use global three dimensional radiation magneto-hydrodynamical simulations to study accretion disks onto a 5 × 10 8 M black hole with accretion rates varying from ∼ 250L Edd /c 2 to 1500L 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 ∼ 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 ∼ 0.1 − 0.4c. When the accretion rate is smaller than ∼ 500L Edd /c 2 , outflows start around 10 gravitational radii and the radiative efficiency is ∼ 5% − 7% with both magnetic field configurations. With accretion rate reaching 1500L 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 ∼ 15% − 30% of the radiative luminosity. The mass flux lost in the outflow is ∼ 15% − 50% of the net mass accretion rates. We discuss implications of our simulation results on the observational properties of these disks.

MHD turbulence

“…The length unit is two gravitational radii of the black hole. Image reproduced with permission from Jiang et al (2019); copyright by AAS In the solar system, we can make in-situ measurements in the solar wind, the flow of tenuous magnetized plasma emitted from the Sun at speeds 400-800 km/s and propagating outwards to the boundaries of the solar system. Such direct measurements of the solar wind parameters and fluctuations in different regions from 0.3 to 5 AU distance to the Sun are especially valuable because they convey much more precise information about turbulent fluctuations compared to astrophysical observations of ISM and ICM mired by limited resolution and projection effects.…”

Global 3D radiation magnetohydrodynamic simulations for FU Ori’s accretion disc and observational signatures of magnetic fields

“…However, for a fluid that is moving at v, additional correction terms on the order of (v/c) and (v/c) 2 need to be added (Jiang et al 2014a). Jiang et al (2019a) has adopted a mixed frame approach to solve the radiative transfer equation for moving fluid consistently. After integrating the radiative transfer equation over frequency, the equation becomes 1 c ∂I ∂t + n · ∇I = S(I, n) .…”

“…Although the radiative transfer scheme has been tested extensively (e.g. Jiang et al 2014aJiang et al , 2019a, we still need to test if the scheme is applicable to our particular FU Ori disk setup. Thus, we set up a 1-D plane-parallel atmosphere with a density profile of…”

The Nature of Soft Excess in ESO 362-G18 Revealed by XMM-Newton and NuSTAR Spectroscopy

“…In this scenario, most of the gravitational accretion power is proposed to be released into the warm corona rather than in the mid-plane of the accretion disk (Petrucci et al 2020). In addition, some general-relativistic magnetohydrodynamic (GRMHD) simulations indicate that the energy could be transported vertically through the atmosphere within the magnetically dominated accretion disk (e.g., Beckwith et al 2009;Begelman et al 2015;Jiang et al 2019b). The warm corona explanation is supported by the observed correlation between the optical/UV and the soft X-ray emission (Mehdipour et al 2011;Petrucci et al 2018).…”