Properties of the integrated spectrum of serendipitous 2XMM catalogue
  sources
  Our analysis is aimed at characterizing the properties of the integrated
spectrum of active galactic nuclei (AGNs) such as the ubiquity of the Fe
K{\alpha} emission in AGNs and the dependence of the spectral parameters on the
X-ray luminosity and redshift. We selected 2646 point sources from the 2XMM
catalogue at high galactic latitude (|BII| > 25 degrees) and with the sum of
EPIC-PN and EPIC-MOS 0.2-12 keV counts greater than 1000. Redshifts were
obtained for 916 sources from the NED. The final sample consists of 507 AGN.
Individual source spectra have been summed in the observed frame to compute the
integrated spectra in different redshift and luminosity bins over the range
0<z<5. Detailed analysis of these spectra has been performed. We find that the
narrow Fe K{\alpha} line at 6.4 keV is significantly detected up to z=1. The
line equivalent width decreases with increasing X-ray luminosity in the 2-10
keV band (''IT effect''). The anti-correlation is characterized by the relation
log(EWFe) = (1.66 +/- 0.09) + (-0.43 +/- 0.07) log(LX,44), where EWFe is the
rest frame equivalent width of the neutral iron K{\alpha} line in eV and LX,44
is the 2-10 keV X-ray luminosity in units of 10^{44} erg s^{-1}. The equivalent
width is nearly independent of redshift up to z ~ 0.8 with an average value of
101+/-40 (rms dispersion) eV in the luminosity range 43.5<= logLX <= 44.5. Our
analysis also confirmed the hardening of the spectral indices at low
luminosities implying a dependence of obscuration on luminosity. We confirm
that the neutral narrow Fe K{\alpha} line is an almost ubiquitous feature of
AGNs. We find compelling evidence for the ''IT effect'' over a redshift
interval larger than probed in any previous study. We detect no evolution of
the average rest frame equivalent width of the Fe K{\alpha} line with redshift.
