Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 494, Issue 3, Pages 4203-4225Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/staa943
Keywords
accretion, accretion discs; black hole physics; magnetic fields; methods: numerical
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Funding
- Ph.D. grant of the Studienstiftung des Deutschen Volkes
- PHAROS COST Action [CA16214]
- GWverse COST Action [CA16104]
- [AYA2015-66899-C2-1-P]
- [PGC2018095984-B-I00]
- [PROMETEO-II-2014-069]
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Black hole - accretion disc systems are the central engines of relativistic jets from stellar to galactic scales. We numerically quantify the unsteady outgoing Poynting flux through the horizon of a rapidly spinning black hole endowed with a rotating accretion disc. The disc supports small-scale, concentric, flux tubes with zero net magnetic flux. Our general relativistic force-free electrodynamics simulations follow the accretion on to the black hole over several hundred dynamical time-scales in 3D. For the case of counter-rotating accretion discs, the average process efficiency reaches up to approximate to 0.43, compared to a stationary energy extraction by the Blandford/Znajek process. The process efficiency depends on the cross-sectional area of the loops, i.e. on the product l x h, where l is the radial loop thickness and It its vertical scale height. We identify a strong correlation between efficient electromagnetic energy extraction and the quasi-stationary setting of ideal conditions for the operation of the Blandford/Znajek process (e.g. optimal field line angular velocity and fulfillment of the so-called Znajek condition). Remarkably, the energy extraction operates intermittently (alternating episodes of high and low efficiency) without imposing any large-scale magnetic field embedding the central object. Scaling our results to supermassive black holes, we estimate that the typical variability time-scale of the system is of the order of days to months. Such time-scales may account for the longest variability scales of TeV emission observed, e.g. in M87.
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