M5 Accretion and Accretion Disks

Topic

The central feature of this model is the accretion disk that forms around a compact object.

The accretion of matter onto black holes, neutron stars, or white dwarfs is the most efficient energy-release mechanism known in the universe; it is precisely what powers active galactic nuclei, quasars, X-ray binaries, and accreting pulsars.

The standard thin-disk model describes the matter within the accretion disk as moving in circular Keplerian orbits; as the matter loses angular momentum due to the disk's own turbulent viscosity, it spirals inward toward the central object. Because the disk's temperature drops sharply with increasing radius, the disk as a whole emits radiation akin to a collection of black bodies at varying temperatures—producing a true multi-colored spectrum.

When the rate of infalling matter exceeds the Eddington limit, the resulting radiation pressure becomes intense enough to drive massive winds and even relativistic jets, completely altering the nature of the flow: the system shifts from a quiescent disk to one dominated by outflows of matter.

When these jets originate in active galactic nuclei, they transport mechanical energy and relativistic particles across scales of millions of light-years—far beyond the host galaxy itself.

Current modeling of these accretion disks and jets combines magnetohydrodynamics with general relativity; it describes how magnetic field lines threading a rotating black hole can directly extract a portion of the black hole's rotational energy to power the jet. Thus, from the accretion disk with which this document begins to the extracted rotational energy with which it concludes—and encompassing the standard thin-disk model and the Eddington limit—accretion disks and their associated jets ensure that the infall of matter onto a compact object releases energy in forms ranging from the light of a quiescent disk to relativistic jets capable of traversing an entire galaxy, depending on the rate of that infall.

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