[Paper Review] Accreting Black Holes

Mitchell C. Begelman|arXiv (Cornell University)|Oct 29, 2014

Astrophysical Phenomena and Observations2 references17 citations

TL;DR

This paper reviews the theory of black hole accretion, emphasizing the interplay between black hole spacetime and complex plasma, radiation, and magnetohydrodynamical processes. It argues that global magnetohydrodynamic simulations incorporating microphysics and full radiation hydrodynamics are essential to resolve key mysteries in accretion, such as jet formation and energy efficiency in X-ray binaries, active galactic nuclei, tidal disruption events, and gamma-ray bursts.

ABSTRACT

I outline the theory of accretion onto black holes, and its application to observed phenomena such as X-ray binaries, active galactic nuclei, tidal disruption events, and gamma-ray bursts. The dynamics as well as radiative signatures of black hole accretion depend on interactions between the relatively simple black-hole spacetime and complex radiation, plasma and magnetohydrodynamical processes in the surrounding gas. I will show how transient accretion processes could provide clues to these interactions. Larger global magnetohydrodynamic simulations as well as simulations incorporating plasma microphysics and full radiation hydrodynamics will be needed to unravel some of the current mysteries of black hole accretion.

Motivation & Objective

• To synthesize current theoretical understanding of accretion onto black holes, particularly focusing on the role of spacetime geometry and plasma dynamics.
• To identify key unresolved problems in black hole accretion, including energy release efficiency, jet formation, and radiation feedback.
• To argue for the necessity of large-scale, high-dynamic-range MHD simulations incorporating radiation and microphysics to resolve outstanding questions.
• To explore how transient accretion processes in X-ray binaries, tidal disruption events, and gamma-ray bursts can reveal insights into accretion physics.
• To examine the role of magnetic fields, radiation pressure, and turbulent reconnection in regulating accretion flows and powering relativistic jets.

Proposed method

• Uses general relativistic magnetohydrodynamics (GRMHD) to model accretion flows, with emphasis on the innermost stable circular orbit (ISCO) and marginally bound orbits as key radiative and dynamical boundaries.
• Applies the α-viscosity model and MRI (magnetorotational instability) to describe angular momentum transport in accretion disks, linking it to observed luminosity and variability.
• Analyzes the role of poloidal magnetic flux and magnetic pressure in regulating accretion, particularly in magnetically arrested disks (MADs), using numerical simulations and theoretical constraints.
• Evaluates radiative efficiency and energy dissipation mechanisms, including radiation pressure, viscous heating, and Poynting flux, in radiation-dominated and super-Eddington regimes.
• Considers jet acceleration mechanisms via magnetic fields and radiation drag, incorporating relativistic aberration and self-shielding effects in optically thick outflows.
• Proposes that future simulations must include full radiation hydrodynamics, collisional and collisionless microphysics (e.g., resistivity, heat conduction), and large-scale current sheet dynamics to model turbulent reconnection and energy conversion.

Experimental results

Research questions

• RQ1 How do magnetic fields and radiation pressure regulate accretion efficiency and angular momentum transport in black hole accretion flows?
• RQ2 What mechanisms allow super-Eddington accretion flows to launch and sustain relativistic jets, particularly in tidal disruption events and gamma-ray bursts?
• RQ3 Why do observed jets in AGN and X-ray binaries have Lorentz factors limited to a few to tens, despite the potential for higher speeds via magnetic energy conversion?
• RQ4 To what extent can chaotic magnetic fields in radiation-dominated regions drive jet acceleration, and how do they compare to coherent large-scale fields?
• RQ5 How do microphysical processes such as collisionless reconnection and resistivity affect MRI-driven turbulence and energy dissipation in accretion disks?

Key findings

• The innermost stable circular orbit (ISCO) for a Kerr black hole ranges from 6M (Schwarzschild) to M (extreme Kerr), increasing accretion efficiency from ~6% to ~42% of rest-mass energy.
• Gas pressure and magnetic forces can allow accretion from orbits closer than ISCO, reducing binding energy and lowering accretion efficiency, especially near the marginally bound orbit at 4M.
• Poloidal magnetic pressure as low as 0.1% of gas pressure can significantly enhance angular momentum transport via MRI, indicating strong sensitivity to magnetic flux in the inner disk.
• Radiation pressure can drive outflows and inflate accretion flows into nearly spherical configurations when pressure support dominates rotation, leading to complex stability and dynamics.
• Jets in gamma-ray bursts and tidal disruption events may be powered by turbulent, chaotic magnetic fields that decay via reconnection into radiation, with acceleration limited by radiation drag and relativistic aberration.
• Self-shielding in marginally optically thick jets can enable acceleration to Lorentz factors proportional to a fractional power (~1/4) of the Eddington ratio, explaining the high speeds of GRB jets despite radiation drag.