and Cosmology

5.4 Components of an AGN
5.4 Components of an AGN
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In contrast to stars, which have a simple geometry, we
expect several source components in AGNs with different,
sometimes very complex geometric configurations
to produce the various components of the spectrum;
this is sketched in Fig. 5.19. Accretion disks and jets
in AGNs are clear indicators for a significant deviation
from spherical symmetry in these sources. The relation
between source components and the corresponding
spectral components is not always obvious. However,
combining theoretical arguments with detailed
observations has led to quite satisfactory models.
5.4.1 The IR, Optical, and UV Continuum
In Sect. 5.3.2 we considered an accretion disk with
a characteristic temperature, following from (5.14), of
( ) ṁ 1/4
T(r) ≈ 6.3 × 10 5 K
ṁ edd
( ) −1/4 ( )
M• r
−3/4
×
10 8 . (5.29)
M ⊙ r S
The thermal emission of an accretion disk with this
radial temperature profile produces a broad spectrum
with its maximum in the UV. The continuum spectrum
of QSOs indeed shows an obvious increase towards
UV wavelengths, up to the limit of observable wavelengths,
λ 1000 Å. (This is the observed wavelength;
QSOs at high redshifts can be observed at significantly
shorter wavelengths in the QSO rest-frame.) At wavelengths
λ ≤ 912 Å, photoelectric absorption by neutral
hydrogen in the ISM of the Galaxy sets in, so that
the Milky Way is opaque for this radiation. Only at
considerably higher frequencies, namely in the soft
X-ray band (h P ν 0.2 keV), does the extragalactic sky
become observable again.
If the UV radiation of a QSO originates mainly from
an accretion disk, which can be assumed because of
the observed increase of the spectrum towards the UV,
the question arises whether the thermal emission of the
disk is also visible in the X-ray domain. In this case,
the spectrum in the range hidden from observation, at
13 eV h P ν 0.2 keV, could be interpolated by such
an accretion disk spectrum. This seems indeed to be
the case. The X-ray spectrum of QSOs often shows
Fig. 5.19. Sketch of the characteristic spectral behavior of
a QSO. We distinguish between radio-loud (dashed curve)
and radio-quiet (solid curve) QSOs. Plotted is νS ν (in arbitrary
units), so that flat sections in the spectrum correspond
to equal energy per logarithmic frequency interval. The most
prominent feature is the big blue bump, a broad maximum
in the UV up to the soft X-ray domain of the spectrum. Besides
this maximum, a less prominent secondary maximum is
found in the IR. The spectrum increases towards higher energies
in the X-ray domain of the spectrum – typically ∼ 10%
of the total energy is emitted as X-rays
a very simple spectral shape in the form of a power
law, S ν ∝ ν −α , where α ∼ 0.7 is a characteristic value.
However, the spectrum follows this power law only
at energies down to ∼ 0.5 keV. At lower energies, the
spectral flux is higher than predicted by the extrapolation
of the power-law spectrum observed at higher
energies. One interpretation of this finding is that the
(non-thermal) source of the X-ray emission produces
a simple power law, and the additional flux at lower
X-ray energies is thermal emission from the accretion
disk (see Fig. 5.19).
Presumably, these two spectral properties – the increase
of the spectrum towards the UV and the radiation
excess in the soft X-ray – have the same origin, being
two wings of a broad maximum in the energy distribution,
which itself is located in the spectral range
unobservable for us. This maximum is called the big
blue bump (BBB). A description of the BBB is possible
using detailed models of accretion disks (Fig. 5.20).
For this modeling, however, the assumption of a local