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Determining the scaling relations between galaxy cluster observables requires large samples of uniformly observed clusters. We measure the mean X-ray luminosity--optical richness (LX--N₂00) relation for an approximately volume-limited sample of more than 17, 000 optically-selected clusters from the maxBCG catalog spanning the redshift range 0. 1<z<0. 3. By stacking the X-ray emission from many clusters using ROSAT All-Sky Survey data, we are able to measure mean X-ray luminosities to ~10% (including systematic errors) for clusters in nine independent optical richness bins. In addition, we are able to crudely measure individual X-ray emission from ~800 of the richest clusters. Assuming a log-normal form for the scatter in the LX--N₂00 relation, we measure L=0. 86+/-0. 03 at fixed N₂00. This scatter is large enough to significantly bias the mean stacked relation. The corrected median relation can be parameterized by LX = (e^) (N₂00/40) ^ 10⁴2 h^-2 ergs/s, where = 3. 57+/-0. 08 and = 1. 82+/-0. 05. We find that X-ray selected clusters are significantly brighter than optically-selected clusters at a given optical richness. This selection bias explains the apparently X-ray underluminous nature of optically-selected cluster catalogs.
Rykoff et al. (Fri,) studied this question.