The clustering of XMM-Newton hard X-ray sources

This Letter presents the clustering properties of hard (2-8 keV) X-ray-selected sources detected in a wide-field (approximate to2 deg(2)), shallow [f(X) (2-8 keV) approximate to 10(-14) ergs cm(-2) s(-1)], and contiguous XMM- Newton survey. We perform an angular correlation function analysis using a total of 171 sources to the above flux limit. We detect an similar to4 sigma correlation signal out to 300" with w(theta < 300 '') similar or equal to 0.13 +/- 0.03. Modeling the two- point correlation function as a power law of the form w(theta) = (theta(0)/theta)(gamma-1), we find theta 0 = 48.9(-24.5)(+15.8) arcsec and gamma = 2.2 +/- 0.30. Fixing the correlation function slope to gamma = 1.8, we obtain theta0 = 22.2(-8.6)(+9.4) arcsec. Using Limber's integral equation and a variety of possible luminosity functions of the hard X- ray population, we find a relatively large correlation length, ranging from r(0) similar to 9 to 19 h(-1) Mpc (for gamma = 1.8 and the concordance cosmological model), with this range reflecting also different evolutionary models for the source luminosities and clustering characteristics. The relatively large correlation length is comparable to that of extremely red objects and luminous radio sources.