and Cosmology

5.3 The Central Engine: A Black Hole
The processes that lead to the formation of jets are
still subject to intensive research. Most likely magnetic
fields play a central role. Such fields may be anchored in
the accretion disk, and then spun up and thereby amplified.
The wound-up field lines may then act as a kind of
spring, accelerating plasma outwards along the rotation
axis of the disk. In addition, it is possible that rotational
energy is extracted from a rotating black hole, a process
in which magnetic fields again play a key role. As is
always the case in astrophysics, detailed predictions in
situations where magnetic fields dominate the dynamics
of a system (like, e.g., in star formation) are extremely
difficult to obtain because the corresponding coupled
equations for the plasma and the magnetic field are very
hard to solve.
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5.3.4 Further Arguments for SMBHs
A black hole is not only the simplest solution of the
equations of Einstein’s General Relativity, it is also the
natural final state of a very compact mass distribution.
The occurrence of SMBHs is thus highly plausible from
a theoretical point of view. The evidence for the existence
of SMBHs in the center of galaxies that has
been detected in recent years (see Sect. 3.5) provides
an additional argument for the presence of SMBHs in
AGNs.
Furthermore, we find that the direction of the jets on
a milliarcsecond scale, as observed by VLBI, is essentially
identical to the direction of jets on much larger
scales and to the direction of the corresponding radio
lobes. These lobes often have a huge distance from the
core, indicating a long lifetime of the source. Hence,
the central engine must have some long-term memory
because the outflow direction is stable over ∼ 10 7 yr.
A rotating SMBH is an ideal gyroscope, with a direction
being defined by its angular momentum vector.
X-ray observations of an iron line of rest energy
h P ν = 6.35 keV in Seyfert galaxies clearly indicate that
the emission must be produced in the inner region of an
accretion disk, within only a few Schwarzschild radii of
a SMBH. An example for this is given in Fig. 5.15. The
shape of the line is caused by a combination of a strong
Doppler effect due to high rotation velocities in the disk
and by the strong gravitational field of the black hole,
as is explained in Fig. 5.16.
Fig. 5.15. The spectral form of the broad iron line in the
Seyfert 1 galaxy MCG-6-30-15 as observed with the ASCA
satellite. If the material emitting the line were at rest we would
observeanarrowlineath P ν = 6.35 keV. We see that the
line is (a) broad, (b) strongly asymmetric, and (c) shifted to
smaller energies. A model for the shape of the line, based on
a disk around a black hole that is emitting in the radius range
r S ≤ r ≤ 20r S ,issketchedinFig.5.16
This iron line is not only detected in individual
AGNs, but also in the average spectrum of an ensemble
of AGNs. In a deep (∼ 7.7 × 10 5 s) XMM-Newton
exposure of the Lockman hole, a region of very low
column density of Galactic hydrogen, a large number
of AGNs were identified and spectroscopically verified.
The X-ray spectrum of these AGNs in the energy ranges
of 0.2 to 3 keV and of 8 to 20 keV (each in the AGN
rest-frame) was modeled by a power law plus intrinsic
absorption. The ratio of the measured spectrum of each
individual AGN and the fitted model spectrum was then
averaged over the AGN population, after transforming
the spectra into the rest-frame of the individual sources.
As shown in Fig. 5.17, this ratio clearly shows the presence
of a strong and broad emission line. The shape of
this average emission line can be very well modeled by
emission from an accretion disk around a black hole
where the radiation originates from a region lying between
∼ 3and∼ 400 Schwarzschild radii. The strength
of the iron line indicates a high metallicity of the gas in
these AGNs.