PoS - Proceedings of Science
Volume 329 - International Conference on Black Holes as Cosmic Batteries: UHECRs and Multimessenger Astronomy (BHCB2018) - Main session
Black Holes as Cosmic Dynamos
R.D. Blandford
Full text: pdf
Pre-published on: May 13, 2019
Published on: May 21, 2019
An introduction is given to a meeting on the role of massive and stellar black holes in powering non-thermal activity in a rich variety of cosmic sources. Relevant properties of magnetized, spinning, black holes are summarized and their observational expression, in terms of radio loudness and Fanaroff-Riley class, is briefly described. The dependence of the accretion mode on the rate and manner of the mass supply beyond the black hole sphere of influence is also discussed. It is argued that hydromagnetic outflows from accretion disks are generally expected over as many as six decades of radius and that they may be the source of emission line gas. These outflows collimate the relativistic jets which are probably generated in an electromagnetic form but become hydromagnetic as they entrain gas through boundary layers where most of the initial nonthermal emission occurs. It is proposed that the particle acceleration close to the hole emphasizes the proton channel which allows secondary pairs to be created at far higher energies than is possible from direct acceleration. These pairs radiate $\gamma$-rays by the synchrotron process which can escape along the jet because the outflow effectively shields them from pair-producing, soft photons. Jets are subject to helical instabilities which can tangle their magnetic field and may destroy them. The jet should become plasma-dominated as intermittent, ``magnetoluminescent'' untangling of the field accelerates particles which emit nonthermally all along the jet. Powerful jets remain supersonic out to the ``hot spots'' at the extremities of the source; weaker jets become subsonic plumes or bubbles. The prospects for learning much more about the nature and operation of jets over the next decade are excellent.
DOI: https://doi.org/10.22323/1.329.0025
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