  Comparison of the high resolution X-ray ROSAT HRI image of A2218 with the
optical HST image shows several interesting correlations. The X-ray emission
within a 1' radius core is resolved into several components; the central
dominant galaxy does not coincide with either of them or the emission centroid.
The major X-ray peak is an elongated feature which coincides with optical arcs
at r=20'' from the cD. We speculate that this may be lensed X-ray emission, for
example, of the same object lensed in the optical. Alternatively, this feature
may be a merger shock, or a gas trail of an infalling subgroup. Two other X-ray
enhancements are close to the two major mass concentrations known from the
lensing analysis. Both lensing and a merger are likely.
  Previous X-ray derivations of the A2218 mass used the data with angular
resolution that blurred the features mentioned above into a broad constant
core. As the HRI data show, such a core does not exist. Because of this, the
hydrostatic estimate of the projected mass within the lensing radius can in
principle be increased by a factor of 1.4 (and the mass within a sphere by a
factor of 2.6) compared to previous analyses. However, for a merging cluster,
the hydrostatic analysis is generally inapplicable. Most other lensing clusters
are more distant than A2218 and obtaining adequate X-ray data for them is even
more difficult. Together with the likely overestimation of mass by the lensing
analysis (as in the simulations), oversimplification of the gas model resulting
from inadequate resolution may account for the lensing/X-ray mass discrepancy
as suggested for A2218. (ABRIDGED)
