and Cosmology

9.3 Background Radiation at Smaller Wavelengths
the AGNs found by ROSAT from the CXB spectrum
(9.3), one obtains an even harder spectrum, resembling
very closely that of thermal bremsstrahlung. Therefore,
it was supposed for a long time that the CXB is, at
higher energies, produced by a hot intergalactic gas at
temperatures of k B T ∼ 30 keV.
This model was excluded, however, by the precise
measurement of the thermal spectrum of the CMB by
COBE, showing that the CMB has a perfect blackbody
spectrum. If a postulated hot intergalactic gas were able
to produce the CXB, it would cause significant deviations
of the CMB from the Planck spectrum, namely by
the inverse Compton effect (the same effect that causes
the SZ effect in clusters of galaxies – see Sect. 6.3.4).
Thus, the COBE results clearly ruled out this possibility.
By now, the nature of the CXB at higher energies has
also essentially been determined (see Fig. 9.26), mainly
through very deep observations with the Chandra satellite.
An example of a very deep observation, the Chandra
Deep Field South, is shown in Fig. 9.27. From source
counts performed in such fields, about 75% of the CXB
in the energy range of 2 keV ≤ E ≤ 10 keV could be
resolved into discrete sources. Again, most of these
sources are AGNs, but typically with a significantly
harder (i.e., flatter) spectrum than the AGNs that are
producing the low-energy CXB. Such a flat X-ray spectrum
can be produced by photoelectric absorption of an
intrinsically steep power-law spectrum, where photons
closer to the ionization energy are more efficiently absorbed
than those at higher energy. According to the
381
Fig. 9.26. In the left panel, the total intensity of discrete
sources with an individual flux > S in the energy range
2keV≤ E ≤ 10 keV is plotted (thick curve), together with
the uncertainty range (between the two thin curves). Most
of the data are from a 3 × 10 5 s exposure of the Chandra
Deep Field. The dashed lines show different measurements
of the CXB flux in this energy range; depending on which
of these values is the correct one, between 60% and 90%
of the CXB in the Chandra Deep Field at this energy is
resolved into discrete sources. In the right panel, the hardness
ratio HR – specifying the ratio of photons in the energy
range 2 keV ≤ E ≤ 10 keV to those in 0.5keV≤ E ≤ 2keV,
HR = (S >2keV − S 2keV + S