Mass Variance from Archival X-ray Properties of Dark Energy Survey
  Year-1 Galaxy Clusters
  Using archival X-ray observations and a log-normal 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 behavior of
X-ray temperatures, $T_X$, with optical richness, $\lambda_{RM}$, 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 $> 50\%$ complete for
clusters with $\lambda_{RM} > 130$. Regression analysis on the two samples
produces consistent posterior scaling parameters, from which we derive a
combined constraint on the residual scatter, $\sigma_{\ln Tx | \lambda} = 0.275
\pm 0.019$. 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,
$\sigma_{\ln M | \lambda} = 0.30 \pm 0.04\, _{({\rm stat})} \pm 0.09\, _{({\rm
sys})}$, at an optical richness of approximately 70. Uncertainties in external
parameters, particularly the slope and variance of the $T_X$--mass relation and
the covariance of $T_X$ and $\lambda_{RM}$ at fixed mass, dominate the
systematic error. The $95\%$ confidence region from joint sample analysis is
relatively broad, $\sigma_{\ln M | \lambda} \in [0.14, \, 0.55]$, or a factor
ten in variance.
