© CAMBRIDGE UNIVERSITY PRESS 1991

4. CONCLUSIONS

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
	specific accretion rate, / E
	accretion rate
E	Eddington (critical) accretion rate
M	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
tr	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
v	azimuthal component of velocity
vr	radial component of velocity
V	voltage drop
z	cylindrical axial coordinate
ZH	impedance of the BH horizon
*	viscosity parameter
	bulk speed/c
t	time scale of variability
	efficiency of conversion of mass to energy
	angle between spin and magnetic axes for BH
	Lorentz factor for random velocities of particles
b	Lorentz factor for bulk flow of beam
j	Lorentz factor for bulk flow of jet
	polytropic (adiabatic) index
	magnetic diffusivity
	opacity
µ	charge per unit mass
	mass density
c	space charge density
	electrical conductivity
	opening angle of beam
	optical depth
	magnetic flux function
	angular velocity, or solid angle
F	angular velocity of magnetic field lines
BH	angular velocity of BH