  We use gravitational lens models and X-ray spectral analysis of ten X-ray
luminous galaxy clusters at z~0.2 to study the impact of cluster substructure
on attempts to normalize the matter power spectrum. We estimate that unrelaxed
clusters are 30% hotter than relaxed clusters causing sigma8 to be
over-estimated by 20% if the cluster selection function is not accounted for
correctly. This helps to explain the wide range in sigma8 derived from
different techniques, sigma8~0.6-1, and offers a physically motivated
explanation for some of the discrepancy. We also identify two further
systematics in our analysis: (i) extrapolation of small field-of-view mass
measurements to the cluster virial radius and (ii) projection of 3-dimensional
masses contained in numerical simulations to the 2-dimensional information that
is available from observations. We combine quantitative estimates of these two
effects with our model fitting to estimate from the current data that
sigma8=0.75+/-0.05(statistical)+/-0.15(systematic), where the systematic error
reflects the extrapolation and projection uncertainties. All three systematics
(substructure, extrapolation and projection) are fundamental to future
cluster-based measurements of sigma8 regardless of the techniques employed.
However, we identify gravitational lensing as the tool of choice for such
studies, because a combination of strong- and weak-lensing offers the most
direct route to control the systematics and thus achieve an unbiased comparison
between observation and theory.
