The ROSAT-ESO Flux Limited X-ray (REFLEX) Galaxy Cluster Survey II: The   Spatial Correlation Function
  We report the results of the spatial two-point correlation function xi(r) for the new X-ray galaxy cluster survey REFLEX, which comprises of 452 X-ray selected clusters (449 with redshifts) detected by the ROSAT satellite during the ROSAT All-Sky-Survey. The REFLEX cluster sample is flux limited to 3x10^{-12} erg s^{-1} cm^{-2} in the ROSAT energy band (0.1-2.4 keV) and spans 3 decades in X-ray luminosity (10^{42} - 10^{45} h^{-2} erg s^{-1}), containing galaxy groups and rich clusters out to a redshift z~0.3. Covering a contiguous area of 4.24 sr REFLEX is the largest X-ray cluster sample to date for which spatial clustering has been analysed. Correlation studies using clusters selected on the basis of their X-ray emission are particularly interesting as they are largely free from the projection biases inherent to optical studies. For the entire flux-limited sample we find that the correlation length (the scale at which the correlation amplitude passes through unity) r_0~20 h^{-1}Mpc. For example, if a power-law fit is made to xi(r) over the range 4-40 h^{-1} Mpc then r_0=18.8+-0.9. An indication of the robustness of this result comes from the high degree of isotropy seen in the clustering pattern on scales close to the correlation length. On larger scales xi(r) deviates from a power-law, crossing zero at ~45h^{-1} Mpc. From an examination of 5 volume-limited cluster sub-samples we find no significant trend of r_0 with limiting X-ray luminosity. A comparison with recent model predictions for the clustering properties of X-ray flux-limited samples, indicates that Cold Dark Matter models with the matter density Omega_m=1 fail to produce sufficient clustering to account for the data, while Omega_m~0.3 models provide an excellent fit. 
C.A. Collins, L. Guzzo, H. Boehringer, P. Schuecker, G. Chincarini, R.   Cruddace, S. De Grandi, H.T. MacGillivray, D.M. Neu...