Accretion disk

History

The foundations of disk theory were laid in the mid-20th century. The key 'α-disk' model was proposed by N. I. Shakura and R. A. Sunyaev in 1973. Even earlier, in 1916, Karl Schwarzschild found the solution to Einstein's equations for a non-rotating black hole, and Subrahmanyan Chandrasekhar in the 1930s determined the limiting mass of white dwarfs. In the 1960s, the discovery of quasars showed that accretion onto supermassive black holes explains their enormous luminosity.

How it works

Any cloud of gas has at least a slight intrinsic rotation. Falling toward the center, it conserves angular momentum (rotational impulse) and thus flattens into a disk. Then viscous friction plays a key role: it transfers angular momentum from the inner layers to the outer ones, allowing gas to move inward. The gravitational energy released in the infall is converted into heat and radiation, which we observe.

History

The foundations of disk theory were laid in the mid-20th century. The key 'α-disk' model was proposed by N. I. Shakura and R. A. Sunyaev in 1973. Even earlier, in 1916, Karl Schwarzschild found the solution to Einstein's equations for a non-rotating black hole, and Subrahmanyan Chandrasekhar in the 1930s determined the limiting mass of white dwarfs. In the 1960s, the discovery of quasars showed that accretion onto supermassive black holes explains their enormous luminosity.

How it works

Any cloud of gas has at least a slight intrinsic rotation. Falling toward the center, it conserves angular momentum (rotational impulse) and thus flattens into a disk. Then viscous friction plays a key role: it transfers angular momentum from the inner layers to the outer ones, allowing gas to move inward. The gravitational energy released in the infall is converted into heat and radiation, which we observe.

History

The theory of disk accretion actively developed from the 1960s. The α-disk model, which became a standard, was published by N. I. Shakura and R. A. Sunyaev in 1973. The discovery of quasars (Maarten Schmidt, 1963) pointed to accretion as the source of their incredible luminosity. X-ray binary systems (e.g., Cygnus X-1) became natural laboratories for testing the theory. Modern instruments—Hubble, Chandra, Spektr-RG—continuously refine the picture, and the Event Horizon Telescope in 2019 directly imaged for the first time the disk around the supermassive black hole in the center of M87.

How it works

In a steady state, the disk structure is determined by the balance of gravity, pressure, and rotation. For a Keplerian disk, the angular velocity Ω = √(GM/r³). The radiation flux at radius r: F(r) = (3GMṀ / 8πr³) [1 − √(r_in/r)], where Ṁ is the accretion rate, r_in is the inner radius (ISCO). The total luminosity L = GMṀ / (2r_in). Angular momentum transport is provided by magnetorotational instability (MRI), generating turbulent viscosity. In a strong gravitational field, general relativistic corrections are important: ISCO moves closer to the horizon for rotating holes, increasing energy output. Part of the energy can be extracted from the hole's rotation via the Blandford–Znajek mechanism and focused into relativistic jets.

Open problems & fun facts

Understanding the viscosity mechanism and angular momentum transport (the role of magnetorotational instability and other processes).; Mechanism of formation and acceleration of relativistic jets.; Stability of radiation-dominated disks: thermal and viscous instability.; Super-Eddington accretion and description of thick disks (slim disks).; Interaction of the disk with the magnetosphere of a neutron star or white dwarf.

💡 Matter at the inner edge of a disk around a black hole can move at near light speed. Due to the relativistic Doppler effect, the side of the disk facing us shines many times brighter.