  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\simeq0.2 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 (Mgas), and quasi-integrated gas
pressure (Yx) at overdensities of \Delta=2500, 1000, and 500 with respect to
the critical density. We find that Mgas at \Delta\le1000 appears to be the most
promising mass proxy of the three, because it has the lowest intrinsic scatter
in mass at fixed observable: \sigma_lnM\simeq0.1, independent of cluster
dynamical state. The scatter in mass at fixed T and Yx is a factor of \sim2-3
larger than at fixed Mgas, which are indicative of the structural segregation
that we find in the M-T and M-Yx relationships. Undisturbed clusters are found
to be \sim40% and \sim20% more massive than disturbed clusters at fixed T and
Yx respectively at \sim2\sigma significance. In particular, A1914 - a
well-known merging cluster - significantly increases the scatter and lowers the
the normalization of the relation for disturbed clusters. We also investigated
the covariance between intrinsic scatter in M-Mgas and M-T relations, finding
that they are positively correlated. This contradicts the adaptive mesh
refinement simulations that motivated the idea that Yx 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 are limited by the small sample size and
the presence of the extreme merging cluster in our sample.
