The XMM-Newton/2dF Survey -- VI. Clustering and bias of the soft X-ray point sources

We study the clustering properties of X-ray sources detected in the wide area (~ 2deg²) bright, contiguous XMM-{ Newton}/2dF survey. We detect 432 objects to a flux limit of 5× 10⁻¹⁵ ergcm⁻²s⁻¹ in the soft 0.5-2 keV band. Performing the standard angular correlation function analysis, a ~ 3σ correlation signal between 0 and 150 arcsec is detected: w(θ<150'') 0.114± 0.037. If the angular correlation function is modeled as a power law, w(θ)=(θ∘/θ)γ-1, then for its nominal slope of γ=1.8 we estimate, after correcting for the integral constraint and the amplification bias, that θ∘ 10.4 ± 1.9 arcsec. Using a clustering evolution model which is constant in comoving coordinates (ε=-1.2), a luminosity dependent density evolution model for the X-ray luminosity function and the concordance cosmological model (Ωₘ=1-ΩΛ=0.3) we obtain, by inverting Limber's integral equation, a spatial correlation length of r∘~ 16 h⁻¹ Mpc. This value is larger than that of previous { ROSAT} surveys as well as of the optical two-degree quasar redshift survey. Only in models where the clustering remains constant in physical coordinates (ε=-3), do we obtain an r∘ value (~ 7.5 h⁻¹ Mpc) which is consistent with the above surveys.