We report the results of the spatial two-point correlation function \xi _ { cc } ( 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 ( RASS ) .
The REFLEX cluster sample is flux limited to 3 \times 10 ^ { -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 \leq 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 } \simeq 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 \pm 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 _ { cc } ( r ) deviates from a power-law , crossing zero at \simeq 45 h ^ { -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 } \simeq 0.3 models provide an excellent fit .