General Overview of Black Hole Accretion Theory

TL;DR

This paper provides a comprehensive overview of black hole accretion theories, focusing on the alpha stress model, its physical basis, and the properties of different accretion flow regimes, connecting theoretical models with observations.

Contribution

It synthesizes current understanding of accretion flow models, emphasizing the role of MRI-driven turbulence and discussing stability and global flow structures.

Findings

Alpha stress remains the primary model for accretion turbulence.

MRI is the plausible physical mechanism behind alpha stress.

Different accretion regimes exhibit distinct stability and observational properties.

Abstract

I provide a broad overview of the basic theoretical paradigms of black hole accretion flows. Models that make contact with observations continue to be mostly based on the four decade old alpha stress prescription of Shakura & Sunyaev (1973), and I discuss the properties of both radiatively efficient and inefficient models, including their local properties, their expected stability to secular perturbations, and how they might be tied together in global flow geometries. The alpha stress is a prescription for turbulence, for which the only existing plausible candidate is that which develops from the magnetorotational instability (MRI). I therefore also review what is currently known about the local properties of such turbulence, and the physical issues that have been elucidated and that remain uncertain that are relevant for the various alpha-based black hole accretion flow models.