Dynamics of Accretion onto Black Holes

09/12/2008 -
12:30pm to 1:30pm
Julian Krolik
Speaker affiliation: 
Johns Hopkins University

The fundamental problem in deriving energy from accretion onto black
holes is the nature of angular momentum transport.   Strong arguments
link this process to MHD turbulence driven by the magneto-rotational instability.
Using large-scale numerical simulations that include full general relativity, it is now
possible to trace MHD turbulence and the resulting accretion dynamics in
considerable detail.  These simulations have enriched our understanding of
black hole accretion flows and changed long-standing views about them.  Contrary
to the central guess of the Novikov-Thorne model, magnetic stresses persist throughout
the flow, and are particularly strong when the black hole rotates rapidly.  With
toy-model cooling functions, we can now calculate directly the radiation arriving
at infinity; due to these additional stresses, the radiative efficiency can be
greater than the classical prediction.  When the internal magnetic field topology
is appropriate, large-scale magnetic field can be spontaneously generated in a cone
around the rotation axis, creating a relativistic Poynting-dominated jet
whose strength increases sharply with increasing black hole spin.  Lastly,
contrary to a prediction of the Shakura-Sunyaev alpha model, the inner portions
of bright disks around black holes, where radiation pressure dominates, are
thermally stable.

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