We report on the results of a detailed analysis of the X-ray spectral properties of a large sample of sources detected serendipitously with the XMM-Newton observatory in 25 selected fields , for which optical identification is in progress .
The survey covers a total solid angle of \sim 3.5 \mathrm { deg } ^ { 2 } and contains 1137 sources with \sim 10 ^ { -15 } < { \it S } _ { 0.5 - 10 } < 10 ^ { -12 } { erg cm } ^ { -2 } { s } ^ { -1 } with good enough spectral quality as to perform a detailed X-ray spectral analysis of each individual object .
We find evidence for hardening of the average X-ray spectra of the sources towards fainter fluxes , and we interpret this as indicating a higher degree of photoelectric absorption amongst the fainter population .
Absorption is detected at 95 % confidence in 20 % of the sources , but it could certainly be present in many other sources below our detection capabilities .
For Broad Line AGNs ( BLAGNs ) , we detect absorption in \sim 10 \% of the sources with column densities in the range 10 ^ { 21 } -10 ^ { 22 } { cm } ^ { -2 } .
The fraction of absorbed Narrow Emission Line galaxies ( NELGs , most with intrinsic X-ray luminosities > 10 ^ { 43 } { erg s ^ { -1 } } , and therefore classified as type 2 AGNs ) is significantly higher ( 40 % ) , with a hint of moderately higher columns .
After correcting for absorption , we do not find evidence for a redshift evolution of the underlying power law index of BLAGNs , which stays roughly constant at \Gamma \sim 1.9 , with intrinsic dispersion of 0.4 .
A small fraction ( \sim 7 \% ) of BLAGNs and NELGs require the presence of a soft excess , that we model as a black body with temperature ranging from 0.1 to 0.3 keV .
Comparing our results on absorption to popular X-ray background synthesis models , we find absorption in only \sim 40 \% of the sources expected .
This is due to a deficiency of heavily absorbed sources ( with { N _ { H } } \sim 10 ^ { 22 } -10 ^ { 24 } { cm } ^ { -2 } ) in our sample in comparison with the models .
We therefore conclude that the synthesis models require some revision in their specific parameters .