A serendipitous XMM survey of the SDSS: the evolution of the colour-magnitude diagram of X-ray AGN from z= 0.8 to 0.1

A new serendipitous XMM survey in the area of the Sloan Digital Sky Survey is described (XMM/SDSS), which includes features such as merging of overlapping fields to increase the sensitivity to faint sources, use of a new parametrization of the XMM point spread function for the source detection and photometry and an accurate estimation of the survey sensitivity. About 40 000 X-ray point sources are detected over a total area of 122 deg². A subsample of 209 sources detected in the 2-8 keV spectral band with SDSS spectroscopic redshifts in the range of 0.03 < z < 0.2, optical magnitudes r < 17.77 mag and ? (erg s^-1) is selected to explore their distribution on the colour-magnitude diagram. This is compared with the colour-magnitude diagram of X-ray AGN in the AEGIS field at z≈ 0.8. We find no evidence for evolution of the rest-frame colours of X-ray AGN hosts from z= 0.8 to 0.1. This suggests that the dominant accretion mode of the AGN population, which is expected to imprint on the properties of their host galaxies, does not change since z= 0.8. This argues against scenarios that attribute the rapid decline of the accretion power of the Universe with time (1 dex since z= 0.8) to changes in the AGN fuelling/triggering mode.