Draft:Accretion disk corona

In high-energy astrophysics, an accretion disk corona (ADC) is a region of hot plasma surrounding an accretion disk, analogous to a stellar corona. Accretion disk coronae are particularly relevant to the study of relativistic reflection of black hole and neutron star spectra.

Formation

Some models suggest that ADC may be formed from material evaporated off of the surface of the accretion disk.

Mechanism

ADC are regions of optically-thin, hot plasma with a temperature greater than 10⁸K that emit primarily in hard X-rays. This has a distinct spectral contrast with the accretion disk, which emits primarily in soft X-rays and may have a temperature that is lower by two or more orders of magnitude, resulting in the observation of two distinct spectral components.

When an accretion disk radiates, that radiation is Compton-scattered by hot electrons in the corona, producing X-ray and gamma-ray power-law radiation. About half of that radiation hits the accretion disk and is eventually re-emitted as blackbody radiation.

Geometry

The corona is thought to exist in a compact region near the innermost stable circular orbit (ISCO).

Effects

Effects on disks

When a large proportion of energy released from accretion of matter is dissipated by an ADC, the disk becomes colder, denser, geometrically thinner, and optically thicker.

Partial X-ray eclipses

Sometimes, X-ray sources with coronae dim in the X-ray spectrum due to diffusion of light from the accretion disk by the corona. These eclipses can help astronomers verify that a source has a corona.

Known sources with accretion disk coronae

• 4U 1822–37
• 4U 2129+47