Properties of the integrated spectrum of serendipitous 2XMM catalog sources

Aims. 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. Methods. We selected 2646 point sources from the 2XMM catalog at high galactic latitude (vertical bar BII vertical bar > 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 Nasa's Extragalactic Database. We excluded sources classified as HII regions, groups/clusters, star-forming/starburst galaxies. The final sample consists of 507 AGN. Individual source spectra were 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 used appropriately normalized background spectra and exposure time-weighted response and ancillary files. Results. We find that the narrow Fe Ka 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 (Iwasawa-Taniguchi effect). The anticorrelation is characterized by the relation log(EWFe) = (1.66 +/- 0.09) + (-0.43 +/- 0.07) log(L-X,L-44), where EWFe is the rest frame equivalent width of the neutral iron K alpha line in eV and L-X,L-44 is the 2-10 keV X-ray luminosity in units of 1044 erg s(-1). The equivalent width is nearly independent of redshift up to z similar to 0.8 with an average value of 101 +/- 40 (rms dispersion) eV in the luminosity range 43.5 <= log L-X <= 44.5. Our analysis also confirms the hardening of the spectral indices at low luminosities, implying a dependence of obscuration on luminosity. Conclusions. We confirm that the neutral narrow Fe K alpha line is an almost ubiquitous feature of AGNs. We find compelling evidence supporting the Iwasawa-Taniguchi 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.