Session 66 - Radio - Loud AGN & Extragalactic Jets.
Oral session, Thursday, January 08
Monroe,

## [66.05] Electromagnetic Extraction of Rotational Energy from Disk-fed Black Holes: the Strength of the Blandford-Znajek Process

P. Ghosh (NASA/GSFC and Tata Inst. of Fundamental Research), M. A. Abramowicz (Göteborg U. and Chalmers Inst. of Tech.)

We critically assess the current state of knowledge of the strengths of the magnetic fields B_\cal H threading the horizons of accretion disk-fed black holes in quasars and radio galaxies. We show that those values of B_\cal H which correspond to the strengths of magnetic fields expected to be maintained in accretion disks by the action of hydromagnetic dynamos according to the 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 black hole's rotational energy 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 disk dynamo, but only on the strength of the magnetic transport of angular momentum in the disk, whose value always emerges to be roughly the same 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 accretion rates. However, the BZ luminosity (a) still dominates that 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 correspnding to exceptionally strong B_\cal H, while avoiding the conundrum of too many strong sources which a universally strong BZ process would imply.