It has been pointed out one a decade ago  that prominent spectral features, and especially the around 6.4 keV iron Ka emission line commonly found in Seyferts
Fig. 22.9 Left: First spectrally resolved Fe K line (MCG-6-30-50), obtained with ASCA . Both, relativistic redshift effects and relativistically Doppler boosting are clearly visible. Right: High quality spectrum obtained of the relativistic Fe K line profile obtained with XMM-Newton (MCG-6-30-50), 
spectra, may lead AGN synthesis models to predict a detectable signature in the spectrum of the cosmic X-ray background (XRB) around a few keV. Despite the increasing number of independent observations of the XRB spectrum below 10 keV, yielding good signal to noise ratio measurements of the extragalactic background, the accuracy reached so far is not such to detect the iron features at the level expected by the model predictions (about 3-7 per cent, [20,23]). An alternative approach, devised to avoid the line smearing due to the large redshift range over which AGN spectra are summed and the present uncertainties in the XRB spectrum, is to search for iron features over appropriate redshift bins. A careful search for iron line emission in stacked X-ray spectra has been carried out among faint X-ray sources identified in the 1Ms Chandra Deep Field South and in the 2Ms Chandra Deep Field North . Individual source spectra were stacked together in seven redshift bins over the range z=0.5—4. With the exception of the highest redshift bin, a significant excess above a power-law continuum is present over the energy range 1+6z4 to 1+6z4. keV, where zmin and zmax are the bin boundaries (see Fig. 22.10). The measured
Fig. 22.10 Residuals of a simple power-law fit to the source spectra in seven different redshift bins as labelled. The vertical line in each panel is at the expected position for the redshifted 6.4 keV Fe Ka line while the shaded region encompasses the bin width defined as AE = j+z4--1 64. keV
EW are in agreement with those expected by simple pre-Chandra estimates based on X-ray background synthesis models in a scenario in which their intensity does not change significantly with redshift (and/or luminosity). Although there might be hints for the presence of gravitationally redshifted broad line components, we caution that their intensity and profile significantly depends on the modelling of the underlying continuum. The average rest frame equivalent width of the iron line does not show significant changes with redshift and the iron line emission is an ubiquitous property of X-ray sources up to z ~ 3. (; (c.f. Fig. 22.10)).
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