The XMM-{\em Newton}/2dF Survey - VI: Clustering and Bias of the Soft X-ray Point Sources

Abstract

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