and Cosmology

5.5 Family Relations of AGNs
relativistic velocities (β ∼ 0.9) this can already be a considerable
factor, i.e., the radiation from the relativistic
jet may appear highly amplified. Another consequence
of beaming is that if a second jet exists which is moving
away from us (the so-called counter-jet), its radiation
will be weakened by a factor
(
)
1 2+α
D − =
(5.32)
γ(1 + β cos φ)
relative to the stationary source. Obviously, D − can
be obtained from D + by replacing φ by φ + π, since
the counter-jet is moving in the opposite direction. In
particular, the flux ratio of jet and counter-jet is
D +
D −
=
( 1 + β cos φ
1 − β cos φ
) 2+α
, (5.33)
and this factor may easily be a hundred or more
(Fig. 5.31). The large flux ratio (5.33) for relativistic jets
is the canonical explanation for VLBI jets being virtually
always only one-sided. This effect is also denoted
as “Doppler favoritism” – the jet pointing towards us is
observed preferentially because of the beaming effect
and the resulting amplification of its flux.
Beaming and the Blazar Phenomenon. If we observe
a source from a direction very close to the jet axis and
if the jet is relativistic, its radiation can outshine all
other radiation from the AGN because D + can become
very large in this case. Especially if the beamed radiation
extends into the optical/UV part of the spectrum,
the line emission may also become invisible relative
to the jet emission, and the source will appear to us
as a BL Lac object. If the line radiation is not outshined
completely, the source may appear as an OVV.
The synchrotron nature of the optical light is also the
explanation for the optical polarization of blazars since
synchrotron emission can be polarized, in contrast to
thermal emission.
The strong beaming factor also provides an explanation
for the rapid variability of blazars. If the velocity
of the emitting component is close to the speed of light,
β 1, even small changes in the jet velocity or its direction
may noticeably change the Doppler factor D + .
Such small changes in the direction are expected because
there is no reason to expect a smooth outflow of
material along the jet at constant velocity. In addition,
we argued that, very probably, magnetic fields play an
important role in the generation and collimation of jets.
These magnetic fields are toroidally spun-up, and emitting
plasma can, at least partially, follow the field lines
along helical orbits (see Fig. 5.32).
Hence beaming can explain the dominance of radiation
from the jet components if the gas is relativistic,
and also the absence or relative weakness of emission
lines. At the same time, it provides a plausible scenario
for the strong variability of blazars. The relative strength
of the core emission and the extended radio emission
depends heavily on the viewing direction. In blazars,
a dominance of the core emission is expected, which is
exactly what we observe.
211
Fig. 5.31. The logarithm of the flux ratio of jet and counterjet
(5.33) is plotted as a function of the angle φ for different
values of the Lorentz factor γ . Even at relatively small values
of γ , this ratio is large if φ is close to 0, but even at φ ∼ 30 ◦
the ratio is still appreciable. Hence the plot shows the Doppler
favoritism and explains why, in most compact radio AGNs,
one jet is visible but the counter-jet is not
5.5.3 Beaming on Large Scales
A consequence of this model is that the jets on kpc
scales, which are mainly observed by the VLA, also
need to be at least semi-relativistic: kiloparsec-scale
jets are in most cases also one-sided, and they are always
on the same side of the core as the VLBI jet on
pc scales. Thus, if the one-sidedness of the VLBI jet is
caused by beaming and the corresponding Doppler fa-