We study the clustering properties of X-ray sources detected in the wide area ( \sim 2 deg ^ { 2 } ) bright , contiguous XMM- 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 .
Very similar results are obtained for the 462 sources detected in the total 0.5-8 keV band ( \theta _ { \circ } \simeq 10.8 \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 _ { 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 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 .

Finally , comparing the measured angular correlation function with the predictions of the concordance cosmological model , we find for two different bias evolution models that the soft X-ray sources at the present time should be biased with respect to the underline matter fluctuation field with bias values in the range ( which depends on the biasing model used ) : 1.9 \raise - 3.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 4.0 pt \hbox { $ < $ } } b _ { % \circ } \raise - 3.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 4.0 pt \hbox { $ < $ } } % 2.7 for \epsilon = -1.2 or 1 \raise - 3.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 4.0 pt \hbox { $ < $ } } b _ { % \circ } \raise - 3.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 4.0 pt \hbox { $ < $ } } % 1.6 for \epsilon = -3 .

Keywords : galaxies : clusters : general - cosmology : theory - large-scale structure of universe