The WARPS ( Wide Angle ROSAT Pointed Survey ) team reviews the properties and history of discovery of ClJ0152.7–1357 , an X-ray luminous , rich cluster of galaxies at a redshift of z = 0.833 .
At L _ { X } = 8 \times 10 ^ { 44 } h _ { 50 } ^ { -2 } erg s ^ { -1 } ( 0.5 - 2.0 keV ) ClJ0152.7–1357 is the most X-ray luminous cluster known at redshifts z > 0.55 .
The high X-ray luminosity of the system suggests that massive clusters may begin to form at redshifts considerably greater than unity .
This scenario is supported by the high degree of optical and X-ray substructure in ClJ0152.7–1357 , which is similarly complex as that of other X-ray selected clusters at comparable redshift and consistent with the hypothesized picture of cluster formation by mass infall along large-scale filaments .

X-ray emission from ClJ0152.7–1357 was detected already in 1980 with the EINSTEIN IPC .
However , because the complex morphology of the emission caused its significance to be underestimated , the corresponding source was not included in the cluster sample of the EINSTEIN Extended Medium Sensitivity Survey ( EMSS ) and hence not previously identified .
Simulations of the EMSS source detection and selection procedure performed by us suggest a general , mild bias of the EMSS cluster sample against X-ray luminous clusters with pronounced substructure .

If highly unrelaxed , merging clusters are common at intermediate to high redshift ( as is suggested by the current data ) they could create a bias in some samples as the morphological complexity of mergers may cause them to fall below the flux limit of surveys that make the implicit or explicit assumption of a unimodal spatial source geometry .
Conversely , the enhanced X-ray luminosity of mergers might cause them to , temporarily , rise above the flux limit .
Either effect could lead to erroneous conclusions about the evolution of the comoving cluster space density .
A high fraction of morphologically complex clusters at high redshift would also call into question the validity of evolutionary studies ( and , specifically , cosmological conclusions ) which implicitly or explicitly assume that the systems under investigation are virialized .