Electromagnetic extraction of rotational energy from disc-fed black holes: the strength of the Blandford-Znajek process

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Abstract
We critically assess the current state of knowledge of the strengths of the magnetic fields Bℋ threading the horizons of accretion disc-fed black holes in quasars and radio galaxies. We show that those values of Bℋ which correspond to the strengths of the magnetic fields expected to be maintained in accretion discs by the action of hydromagnetic dynamos, according to recent numerical simulations, are significantly lower than those which have been suggested previously, and consequently the strength of the Blandford-Znajek (BZ) process for the extraction of the rotational energy of the black hole is lower than imagined previously. This result appears to be rather robust in the sense that it depends neither on the details of the simulation nor on the actual value of the magnetic field generated by the disc dynamo, but only on the strength of the magnetic transport of angular momentum in the disc, the value of which always emerges as roughly constant from the above simulations. Thus, the BZ process does not appear to be adequate for accounting for the strongest double radio sources accreting at far sub-Eddington rates. However, the BZ luminosity (a) still dominates the luminosity due to accretion and (b) easily accounts for the multitude of low and medium strength radio sources, so that the strongest radio sources can be attributed to BZ luminosities corresponding to exceptionally strong Bℋ, while avoiding the conundrum of too many strong sources which a universally strong BZ process would imply.

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