We report the discovery of the galaxy cluster ClJ1226.9+3332 in the Wide Angle ROSAT Pointed Survey ( WARPS ) .
At z = 0.888 and L _ { X } = 1.1 \times 10 ^ { 45 } h _ { 50 } ^ { -2 } erg s ^ { -1 } ( 0.5 - 2.0 keV ) ClJ1226.9+3332 is the most distant X-ray luminous cluster currently known .
The mere existence of this system represents a huge problem for \Omega _ { 0 } = 1 world models .

At the modest ( off-axis ) resolution of the ROSAT PSPC observation in which the system was detected , ClJ1226.9+3332 appears relaxed ; an off-axis HRI observation confirms this impression and rules out significant contamination from point sources .
However , in moderately deep optical images ( R and I band ) the cluster exhibits signs of substructure in its apparent galaxy distribution .
A first crude estimate of the velocity dispersion of the cluster galaxies based on six redshifts yields a high value of 1650 km s ^ { -1 } , indicative of a very massive cluster and/or the presence of substructure along the line of sight .
While a more accurate assessment of the dynamical state of this system requires much better data at both optical and X-ray wavelengths , the high mass of the cluster has already been unambiguously confirmed by a very strong detection of the Sunyaev-Zel ’ dovich effect in its direction ( Joy et al .
2001 ) .

Using ClJ1226.9+3332 and ClJ0152.7–1357 ( z = 0.835 ) , the second-most distant X-ray luminous cluster currently known and also a WARPS discovery , we obtain a first estimate of the cluster X-ray luminosity function at 0.8 < z < 1.4 and L _ { X } > 5 \times 10 ^ { 44 } h _ { 50 } ^ { -2 } erg s ^ { -1 } ( 0.5 - 2.0 keV ) .
Using the best currently available data , we find the comoving space density of very distant , massive clusters to be in excellent agreement with the value measured locally ( z < 0.3 ) , and conclude that negative evolution is not required at these luminosities out to z \sim 1 .
Our findings are in conflict with earlier claims of highly significant ( > 3 \sigma ) negative evolution already at 0.3 < z < 0.6 based on the cluster samples of the EMSS and the CfA 160 degree survey .
Our results agree , however , with the lack of significant evolution of very X-ray luminous clusters out to z \sim 0.4 reported by the MACS team .
Our findings are also consistent with the abundance of very X-ray luminous clusters at z \sim 0.8 inferred from the EMSS cluster sample , provided ClJ0152.7–1357 ( which was missed by the EMSS ) is added in .