Abstract
Abstract We consider the Blandford–Znajek (BZ) mechanism for extracting black hole spin energy to drive astrophysical jets. Analyses of the BZ mechanism generally take no account of any electric charge on the black hole. But, as noted by Wald and others, if the medium surrounding the black hole is an ionized plasma with mobile charges, then a spinning hole quickly acquires an electric charge. The effect of this charge is to nullify the electric field structures which drive the BZ mechanism. Since jets are now observed in a wide variety of classes of accreting objects, most of which do not contain a central black hole, it seems likely that the jet-driving mechanism in all astrophysical objects uses energy directly from the accretion disk, rather than black hole spin.
Highlights
Maps of double–lobe radio galaxies (e.g. Mitton & Ryle, 1969) showed amorphous blobs of radio emission symmetrically placed each side of the central galaxy. Rees (1971) suggested that an unknown object in the galactic nucleus channels energy to the radio lobes through jets
The process is often cited in papers which concern numerical MHD simulations of jets and outflows produced by magnetic accretion discs
In line with the original ideas of Wald (1974; see Gibbons et al, 2013) that in a realistic space plasma which permits a net flow of charge into the black hole the BZ mechanism cannot tap the spin energy of a black hole continuously, and is not a viable mechanism for powering continuous astrophysical jets
Summary
Maps of double–lobe radio galaxies (e.g. Mitton & Ryle, 1969) showed amorphous blobs of radio emission symmetrically placed each side of the central galaxy. Rees (1971) suggested that an unknown object in the galactic nucleus channels energy to the radio lobes through jets. There is almost universal agreement that the central object in radio galaxies is a supermassive black hole (Rees 1984), and that the high energy activity in the nucleus is powered by accretion (Salpeter, 1964), most likely through an accretion disc (LyndenBell, 1969). Blandford & Znajek (1977, hereafter BZ77) proposed a radical new mechanism in which the jets from galactic nuclei are powered by direct electromagnetic extraction of the spin energy from the central black hole.
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