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 (≈2 deg²), shallow [f_X(2-8 keV) ≈ 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 ~4 σ correlation signal out to 300'' with w(θ < 300'') ≃ 0.13 ± 0.03. Modeling the two-point correlation function as a power law of the form w(θ) = (θ₀/θ)^γ-1, we find θ₀ = 48.9 arcsec and γ = 2.2 ± 0.30. Fixing the correlation function slope to γ = 1.8, we obtain θ₀ = 22.2 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 r₀ ~ 9 to 19 h^-1 Mpc (for γ = 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.