# Observational tests of the picture of disk accretion

## TL;DR

This paper reviews observational evidence supporting the classical model of disk accretion, focusing on black holes and other systems, and discusses how various phenomena align with theoretical predictions.

## Contribution

It provides a comprehensive summary of observational tests validating the basic structures and behaviors of accretion disks, including recent quasar microlensing constraints.

## Key findings

- Evidence supports the formation of accretion disks in various systems.
- The thermal-viscous ionization instability model matches observed outbursts.
- Variations in mass transfer rates explain some complex outburst behaviors.

## Abstract

In this chapter, I present a summary of observational tests of the basic picture of disk accretion. An emphasis is placed on tests relevant to black holes, but many of the fundamental results are drawn from studies of other classes of systems. Evidence is discussed for the basic structures of accretion flows. The cases of systems with and without accretion disks are discussed, as is the evidence that disks actually form. Also discussed are the hot spots where accretion streams impact the disks, and the boundary layers in the inner parts of systems where the accretors are not black holes. The nature of slow, large amplitude variability is discussed. It is shown that some of the key predictions of the classical thermal-viscous ionization instability model for producing outbursts are in excellent agreement with observational results. It is also show that there are systems whose outbursts are extremely difficult to explain without invoking variations in the rate of mass transfer from the donor star into the outer accretion disk, or tidally induced variations in the mass transfer rates. Finally, I briefly discuss recent quasar microlensing measurements which give truly independent constraints on the inner accretion geometry around black holes.