Mass variance from archival X-ray properties of Dark Energy Survey Year-1 galaxy clusters

Using archival X-ray observations and a lognormal population model, we estimate constraints on the intrinsic scatter in halo mass at fixed optical richness for a galaxy cluster sample identified in Dark Energy Survey Year-One (DES-Y1) data with the redMaPPer algorithm. We examine the scaling behaviour of X-ray temperatures, T_X, with optical richness, λ_RM, for clusters in the redshift range 0.2 < z < 0.7. X-ray temperatures are obtained from Chandra and XMM observations for 58 and 110 redMaPPer systems, respectively. Despite non-uniform sky coverage, the T_X measurements are > 50 per cent complete for clusters with λ_RM > 130. Regression analysis on the two samples produces consistent posterior scaling parameters, from which we derive a combined constraint on the residual scatter, σ_ln T | λ = 0.275 ± 0.019. Joined with constraints for T_X scaling with halo mass from the Weighing the Giants program and richness-temperature covariance estimates from the LoCuSS sample, we derive the richness-conditioned scatter in mass, σ_ln M | λ = 0.30 ± 0.04 _(stat) ± 0.09 _(sys), at an optical richness of approximately 100. Uncertainties in external parameters, particularly the slope and variance of the T_X-mass relation and the covariance of T_X and λ_RM at fixed mass, dominate the systematic error. The 95 per cent confidence region from joint sample analysis is relatively broad, σ_ln M | _λ ∈ [0.14, 0.55], or a factor 10 in variance.