  One of the most important parameters in the XRB (X-ray background) synthesis
models is the average efficiency of accretion onto SMBH (super-massive black
holes). This can be inferred from the shape of broad relativistic Fe lines seen
in X-ray spectra of AGN (active galactic nuclei). Several studies have tried to
measure the mean Fe emission properties of AGN at different depths with very
different results. We compute the mean Fe emission from a large and
representative sample of AGN X-ray spectra up to redshift ~ 3.5. We developed a
method of computing the rest-frame X-ray average spectrum and applied it to a
large sample (more than 600 objects) of type 1 AGN from two complementary
medium sensitivity surveys based on XMM-Newton data, the AXIS and XWAS samples.
This method makes use of medium-to-low quality spectra without needing to fit
complex models to the individual spectra but with computing a mean spectrum for
the whole sample. Extensive quality tests were performed by comparing real to
simulated data, and a significance for the detection of any feature over an
underlying continuum was derived. We detect with a 99.9% significance an
unresolved Fe K-alpha emission line around 6.4 keV with an EW ~ 90 eV, but we
find no compelling evidence of any significant broad relativistic emission line
in the final average spectrum. Deviations from a power law around the narrow
line are best represented by a reflection component arising from cold or
low-ionization material. We estimate an upper limit for the EW of any
relativistic line of 400 eV at a 3 sigma confidence level. We also marginally
detect the so-called Iwasawa-Taniguchi effect on the EW for the unresolved
emission line, which appears weaker for higher luminosity AGN.
