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Evidence in favour of supermassive black holes lurking at the centre of all active galactic nuclei continues to mount, as does evidence that jets are launched very close to those central engines. While it is not yet possible to make a clear choice between competing models for beam formation, those requiring that a dominant role be played by magnetic fields are growing in sophistication and popularity. Still, none of the models have been developed to the point where clear predictions concerning the structure and emission properties of jets are made, so that confrontations with observations are not as restrictive as we would desire. Further, even if those predictions are firmly made, the observations needed to make clear choices between models will be extremely difficult to obtain and to interpret without bias. Even though some models may not stand the test of time, the possibility that more than one basic mechanism may be involved should not be overlooked.

Related to the possibility of multiple jet formation mechanisms being operative, the hypothesis that most galaxies house SMBHs and go through active phases of different types, depending upon the available fuel supply, is also gaining favour. The interactions of sub-parsec jets with the certainly non-uniform surrounding medium is bound to be extremely complex, and investigations of this subject are really just beginning. Significant advances in this area will probably only emerge from increasingly detailed numerical simulations advancing in tandem with improved theoretical understanding of the relevant physics. Uncertainties in our knowledge of the processes occurring in the region where severely warped space-time is invaded by extraordinarily hot plasmas are likely to remain for quite a long time, and a definitive answer to the question "Where exactly do jets come from?" is likely to remain elusive.

I am grateful to many colleagues for conversations on this subject and for relevant preprints. This work was supported in part by NSF grant AST87-17912 and by a Smithsonian Institution Foreign Currency Research Grant. I thank the Indian Institutes of Science and Astrophysics and the Tata Institute of Fundamental Research for their hospitality while I was beginning work on this chapter.

Symbols used in This Article

Symbol Meaning

a angular momentum parameter for BH
A magnetic vector potential
Å Ångstrom unit
B, B magnetic field
BpH poloidal component of the field intersecting the BH horizon
cs local speed of sound
d distance along a beam or jet
E electric field
f radiation anisotropy factor
F force
Feff radiative flux
geff effective gravitational acceleration
h disc half-thickness
I current
j current density
J, J angular momentum of BH
kB Boltzmann's constant
l specific angular momentum
lKep specific angular momentum of a particle on a Keplerian orbit
L total luminosity
Leff effective luminosity
LE Eddington luminosity
LEM Poynting luminosity
Li luminosity at infinity
L+ - lepton beam luminosity
mdot specific accretion rate, Mdot / MdotE
Mdot accretion rate
MdotE Eddington (critical) accretion rate
Modot solar mass
MBH mass of black hole (BH)
Mdisc mass of accretion disc
Mir irreducible mass of BH
n power law for specific angular momentum
Pdisc pressure in the disc
Pgas gas pressure
Pjet pressure exerted by the jet
Prad radiation pressure
q power law for angular velocity
r cylindrical radial coordinate
rc critical radius (sonic point in wind flow)
ri energy injection radius
rt radiation trapping radius
rin inner edge of accretion disc
rout outer edge of accretion disc
RL electrical resistivity of plasma
Rmb radius of marginally bound orbit
Rms radius of marginally stable orbit
Rs radius of event horizon
SBH entropy of BH
tphir component of shear tensor
Torb orbital period
Tdisc average black body temperature radiated by a disc
Te electron temperature
TBH temperature of BH
Tmax maximum temperature in an accretion disc
Tp proton temperature
u energy density in radiation
v three-velocity
v four-velocity
vphi azimuthal component of velocity
vr radial component of velocity
V voltage drop
z cylindrical axial coordinate
ZH impedance of the BH horizon
alpha* viscosity parameter
beta bulk speed/c
Deltat time scale of variability
epsilon efficiency of conversion of mass to energy
Upsilon angle between spin and magnetic axes for BH
gamma Lorentz factor for random velocities of particles
gammab Lorentz factor for bulk flow of beam
gammaj Lorentz factor for bulk flow of jet
Gamma polytropic (adiabatic) index
eta magnetic diffusivity
kappa opacity
µ charge per unit mass
rho mass density
rhoc space charge density
sigma electrical conductivity
phi opening angle of beam
tau optical depth
Psi magnetic flux function
Omega angular velocity, or solid angle
OmegaF angular velocity of magnetic field lines
OmegaBH angular velocity of BH

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