We present measurements of the galaxy cluster X-ray Luminosity Function ( XLF ) from the Wide Angle ROSAT Pointed Survey ( WARPS ) and quantify its evolution .
WARPS is a serendipitous survey of the central region of ROSAT pointed observations and was carried out in two phases ( WARPS-I and WARPS-II ) .
The results here are based on a final sample of 124 clusters , complete above a flux limit of 6.5 \times 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } , with members out to redshift z \sim 1.05 , and a sky coverage of 70.9 deg ^ { 2 } .
We find significant evidence for negative evolution of the XLF , which complements the majority of X-ray cluster surveys .
To quantify the suggested evolution , we perform a maximum likelihood analysis and conclude that the evolution is driven by a decreasing number density of high luminosity clusters with redshift , while the bulk of the cluster population remains nearly unchanged out to redshift z \approx 1.1 , as expected in a low density Universe .
The results are found to be insensitive to a variety of sources of systematic uncertainty that affect the measurement of the XLF and determination of the survey selection function .
We perform a Bayesian analysis of the XLF to fully account for uncertainties in the local XLF on the measured evolution , and find that the detected evolution remains significant at the 95 \% level .
We observe a significant excess of clusters in the WARPS at 0.1 < z < 0.3 and L _ { X } \approx 2 \times 10 ^ { 43 } erg s ^ { -1 } compared with the reference low-redshift XLF , or our Bayesian fit to the WARPS data .
We find that the excess can not be explained by sample variance , or Eddington bias , and is unlikely to be due to problems with the survey selection function .