XMM Cluster Survey analysis of the SDSS DR8 redMaPPer catalogue: implications for scatter, selection bias, and isotropy in cluster scaling relations.

In this paper, we present the X-ray analysis of Sloan Digital Sky Survey DR8 redMaPPer (SDSSRM) clusters using data products from the XMM Cluster Survey (XCS). In total, 1189 SDSSRM clusters fall within the XMM – Newton footprint. This has yielded 456 confirmed detections accompanied by X-ray luminosity (L_X) measurements. Of these clusters, 381 have an associated X-ray temperature measurement (T_X). This represents one of the largest samples of coherently derived cluster T_X values to date. Our analysis of the X-ray observable to richness scaling relations has demonstrated that scatter in the T_X –λ relation is roughly a third of that in the L_X –λ relation, and that the L_X –λ scatter is intrinsic, i.e. will not be significantly reduced with larger sample sizes. Analysis of the scaling relation between L_X and T_X has shown that the fits are sensitive to the selection method of the sample, i.e. whether the sample is made up of clusters detected 'serendipitously' compared to those deliberately targeted by XMM. These differences are also seen in the L_X –λ relation and, to a lesser extent, in the T_X –λ relation. Exclusion of the emission from the cluster core does not make a significant impact on the findings. A combination of selection biases is a likely, but yet unproven, reason for these differences. Finally, we have also used our data to probe recent claims of anisotropy in the L_X – T_X relation across the sky. We find no evidence of anistropy, but stress this may be masked in our analysis by the incomplete declination coverage of the SDSS. [ABSTRACT FROM AUTHOR]