and Cosmology

5. Active Galactic Nuclei
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5.5.4 Jets at Higher Frequencies
Fig. 5.32. Illustration of the relativistic jet model. The acceleration
of the jet to velocities close to the speed of light
is probably caused by a combination of very strong gravitational
fields in the vicinity of the SMBH and strong magnetic
fields which are rotating rapidly because they are anchored in
the accretion disk. Shock fronts within the jet lead to acceleration
processes of relativistic electrons, which then strongly
radiate and become visible as “blobs” in the jets. By rotation
of the accretion disk in which the magnetic field lines are anchored,
the field lines obtain a characteristic helical shape. It
is supposed that this process is responsible for the focusing
(collimation) of the jet
voritism of an otherwise intrinsically symmetric source,
the one-sidedness of large-scale jets should have the
same explanation, implying relativistic velocities for
them as well. These do not need to be as close to c as
those of the components that show superluminal motion,
but their velocity should also be at least a few
tenths of the speed of light. In addition, it follows that
the kiloparsec-scale jet is moving towards us and is
therefore closer to us than the core of the AGN; for the
counter-jet we have the opposite case. This prediction
can be tested empirically, and it was confirmed in polarization
measurements. Radiation from the counter-jet
crosses the ISM of the host galaxy, where it experiences
additional Faraday rotation (see Sect. 2.3.4). It is in fact
observed that the Faraday rotation of counter-jets is systematically
larger than that of jets. This can be explained
by the fact that the counter-jet is located behind the host
galaxy and we are thus observing it through the gas of
that galaxy.
Optical Jets. In Sect. 5.1.2, we discussed the radio
emission of jets, and Sect. 5.3.3 described how their
relativistic motion is detected from their structural
changes, i.e., superluminal motion. However, jets are
not only observable at radio frequencies; they also
emit at much shorter wavelengths. Indeed, the first two
jets were detected in optical observations, namely in
QSO 3C273 (Fig. 5.33) and in the radio galaxy M87
(Fig. 5.34), as a linear source structure pointing radially
away from the core of the respective galaxy. With
the commissioning of the VLA (Fig. 1.21) as a sensitive
and high-resolution radio interferometer, the discovery
and examination of hundreds of jets at radio frequencies
became possible.
The HST, with its unique angular resolution, has detected
numerous jets in the optical (see also Fig. 5.12).
They are situated on the same side of the corresponding
AGNs as the main radio jet. Optical counterparts of
radio counter-jets have not been detected thus far. Optical
jets are always shorter, narrower, and show more
structure than the corresponding radio jets. The spectrum
of optical jets follows a power law (5.2) similar to
that in the radio domain, with an index α that describes,
in general, a slightly steeper spectrum. In some cases,
linear polarization in the optical jet radiation of ∼ 10%
was also detected. If we also take into account that the
positions of the knots in the optical and in the radio jets
agree very well, we inevitably come to the conclusion
that the optical radiation is also synchrotron emission.
This conclusion is further supported by a nearly constant
flux ratio of radio and optical radiation along
the jets.
As was mentioned in Sect. 5.1.3, the relativistic electrons
that produce the synchrotron radiation lose energy
by emission. In many cases, the cooling time (5.6)
of the electrons responsible for the radio emission is
longer than the time of flow of the material from the
central core along the jet, in particular if the flow is
(semi-)relativistic. It is thus possible that relativistic
electrons are produced or accelerated in the immediate
vicinity of the AGN and are then transported away
by the jet. This is not the case for those electrons producing
the optical synchrotron radiation, however, because
the cooling time for emission at optical wavelengths is
only t cool ∼ 10 3 (B/10 −4 G) yr. Even if the relativistic