We present a joint weak-lensing/X-ray study of galaxy cluster mass-observable scaling relations motivated by the critical importance of accurate calibration of mass proxies for future X-ray missions, including eROSITA. We use a sample of 12 clusters at z 0.2 that we have observed with Subaru and XMM-Newton to construct relationships between the weak-lensing mass (M) and three X-ray observables, gas temperature (T), gas mass (M gas), and quasi-integrated gas pressure (Y X), at overdensities of Δ = 2500, 1000, and 500 with respect to the critical density. We find that M gas at Δ ≤ 1000 appears to be the most promising mass proxy of the three because it has the lowest intrinsic scatter in mass at a fixed observable, σln M 0.1, independent of the cluster dynamical state. The scatter in mass at fixed T and Y X is a factor of ~2-3 larger than at fixed M gas, which are indicative of the structural segregation that we find in the M-T and M-Y X relationships. Undisturbed clusters are found to be ~40% and ~20% more massive than disturbed clusters at fixed T and Y X, respectively, at ~2σ significance. In particular, A?1914—a well-known merging cluster—significantly increases the scatter and lowers the normalization of the relation for disturbed clusters. We also investigated the covariance between the intrinsic scatter in M-M gas and M-T relations, finding that they are positively correlated. This contradicts the adaptive mesh refinement simulations that motivated the idea that Y X may be a low-scatter mass proxy, and agrees with more recent smoothed particle hydrodynamic simulations based on the Millennium Simulation. We also propose a method to identify a robust mass proxy based on principal component analysis. The statistical precision of our results is limited by the small sample size and the presence of the extreme merging cluster in our sample. We therefore look forward to studying a larger, more complete sample in the future.
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