The Clustering of XMM-Newton Hard X-ray Sources
  We present the clustering properties of hard (2-8 keV) X-ray selected sources detected in a wide field (~2 deg^{2}) shallow [f(2-8 keV)~ 10^{-14} erg 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 a ~ 4\sigma correlation signal out to 300 arcsec with w(theta < 300^{''}) ~ 0.13 +- 0.03. Modeling the two point correlation function as a power law of the usual form we find: theta_o=48.9^{+15.8}_{-24.5} arcsec and gamma=2.2 +- 0.30. Fixing the correlation function slope to gamma=1.8 we obtain theta_o=22.2^{+9.4}_{-8.6} 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_o ~ 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. 
M. Plionis, S. Basilakos, A. Georgakakis, I. Georgantopoulos