and Cosmology

5. Active Galactic Nuclei
214
electrons are transported in a (semi-)relativistic jet, they
cannot travel more than a distance of ∼ 1 kpc before losing
their energy. The observed length of optical jets is
much larger, though. For this reason, the corresponding
electrons cannot be originating in the AGN itself but instead
must be produced locally in the jet. The knots in
the jets, which are probably shock fronts in the outflow,
are thought to represent the location of the acceleration
of relativistic particles. Quantitative estimates of
the cooling time are hampered by the unknown beaming
factor (5.31). Since optical jets are all one-sided,
and in most cases observed in radio sources with a flat
spectrum, a very large beaming factor is generally assumed.
Transforming back into the rest-frame of the
electrons yields a lower frequency and a lower luminosity.
Since the latter is utilized for estimating the strength
of the magnetic fields (by assuming equipartition of
energy, for instance), this also changes the estimated
cooling time.
X-Ray Radiation of Jets. The Chandra satellite discovered
that many of the jets which had been identified
in the radio are also visible in X-ray light (Fig. 5.35).
This came as a real surprise. This discovery and the
strong correlation of the spatial distribution of radio,
optical, and X-ray emission imply that they must all
originate from the same regions in the jets, i.e., that the
origins of the emission must be linked to each other. As
we have discussed, radio and optical radiation originate
from synchrotron emission, the emission by relativistic
electrons moving in a magnetic field. The same electrons
that are responsible for the radio emission can
also produce X-ray photons by inverse Compton scattering.
In this process, low-energy photons are scattered
to much higher energies by collisions with relativistic
electrons – a photon of frequency ν may have a frequency
ν ′ ≈ γ 2 ν after being scattered by an electron
of energy γm e c 2 . Since the characteristic Lorentz factors
of electrons causing the synchrotron radiation of
radio jets may reach values of γ ∼ 10 4 , these electrons
may scatter, by inverse Compton scattering, radio
photons into the X-ray domain of the spectrum. This
effect is also called synchrotron self-Compton radiation.
Alternatively, relativistic electrons can also scatter
optical photons from the AGN, for which less energetic
electrons are required. The omnipresent CMB
may also be considered as a photon source for the inverse
Compton effect, and in many cases the observed
X-ray radiation is probably Compton-scattered CMB
radiation.
The inverse Compton model cannot, however, be
applied to all X-ray jets without serious problems occurring.
For instance, variability in X-ray emission was
observed in the knots of M87, indicating a very short
cooling time for the electrons. Since the electrons must
have a much larger Lorentz factor γ if the radiation, at
Fig. 5.35. X-ray images of AGN jets. Left: a Chandra image of
the jet in the QSO PKS 1127−145, with overlaid contours of
radio emission (1.4 cm, VLA). The direction of the jet and its
substructure are very similar at both wavelengths, suggesting
an interpretation in which the radiation is caused by the same
population of relativistic electrons. Right: a Chandra image of
the active galaxy Centaurus A. Here the jet is visible, as well
as a large number of compact sources interpreted to be X-ray
binaries