X-ray spectra of XMM-Newton serendipitous medium flux sources

¹ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
² Department of Physics and Astronomy, University of Leicester, LE1 7RH, UK e-mail: sm279@star.le.ac.uk
³ Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Mullard Space Science Laboratory, UCL, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK

Received: 21 May 2004
Accepted: 14 December 2004

Abstract

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 ~3.5  and contains 1137 sources with ~ 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 ~ of the sources with column densities in the range . The fraction of absorbed Narrow Emission Line galaxies (NELGs, most with intrinsic X-ray luminosities >, 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 , with intrinsic dispersion of 0.4. A small fraction (~) 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 ~ of the sources expected. This is due to a deficiency of heavily absorbed sources (with ) in our sample in comparison with the models. We therefore conclude that the synthesis models require some revision in their specific parameters.