Making black holes visible: Accretion, radiation, and jets

Summary

With the fundamental stress mechanism of accretion disks identified—correlated MHD turbulence driven by the magneto-rotational instability—it has become possible to make numerical simulations of accretion disk dynamics based on well-understood physics. A sampling of results from both Newtonian 3-d shearing box and general relativistic global disk MHD simulations is reported. Among other things, these simulations have shown that: contrary to long-held assumptions, stress is continuous through the marginally stable and plunging regions around black holes, so that rotating black holes can electromagnetically give substantial amounts of angular momentum to surrounding matter; the upper layers of accretion disks are primarily supported by magnetic pressure, potentially leading to interesting departures from local black-body emitted spectra; and initially local magnetic fields in accretion flows can, in some cases, spontaneously generate large-scale fields that connect rotating black holes to infinity and mediate strong relativistic jets.

Prolog: The classical view of accretion disks

It has been understood for decades that accretion through disks can be an extremely powerful source of energy for the generation of both photons and material outflows. When the central object is a black hole, the gravitational potential at the center of the disk is relativistically deep, so that the amount of energy that might be released per unit rest-mass accreted can be a substantial fraction of unity. If the central black hole spins, an additional store of tappable energy resides in its rotation.