  Long arcs in clusters of galaxies, produced by gravitational lensing, can be
used to estimate the mass interior to the arcs and therefore, constrain the
cluster mass distribution. The radial density distribution of the intracluster
gas (ICM) can be extracted from the X-ray surface brightness observations. If
the gas temperature is also known, it is then possible to probe the dynamical
state of the gas and test whether the ICM is in hydro- static equilibrium
within the gravitational potential of the cluster as a result of thermal
pressure support. We analyze three clusters that exhibit large arcs, whose
X-ray surface brightness profiles have been observed, and whose gas
temperatures have been determined. In two of the clusters, A2218 and A1689, the
central mass implied by lensing is a factor of $2$--$2.5$ too large for the gas
at the observed temperature to be in hydrostatic equilibrium solely due to
thermal pressure support. In other words, if we accept the mass estimate
derived from the lensing analysis and demand that the X-ray surface brightness
profile be consistent with the observations, the required gas temperature is a
factor of $2$--$2.5$ higher than observed. The results for the third cluster,
A2163 (the most luminous and the hottest cluster known), are more ambiguous.
The discrepancy between the X-ray and the lensing mass estimates arise because
the presence of arcs imply a highly concentrated cluster mass distribution
whereas the observed X-ray profiles imply a more extended mass distribution.
The large X-ray core radii are not the result of the limited resolution of the
X-ray detectors. We consider various possibilities that could account for the
discrepancy.
