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: $w(\theta <150^{{}^{\prime \prime }})\simeq 0.114\pm 0.037$. 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 (${\Omega }_{\mathrm{m}}=1-{\Omega }_{\Lambda }=0.3$) 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.

11 pages, 6 figures, MNRAS accepted

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