  We present spatially-resolved X-ray spectroscopy of the X-ray luminous
cluster of galaxies, PKS0745-191, using observations made with the ASCA and
ROSAT satellites. The X-ray data measure the density, temperature, and
metallicity profiles of the cluster gas and constrain the distribution of mass
in the cluster to a radius of \sim 1.5 Mpc. We report the discovery of a bright
gravitationally-lensed arc at a redshift, z_{\rm arc} = 0.433, in the cluster.
This result identifies PKS0745 as the lowest-redshift cluster (z_{\rm clus} =
0.1028) known to exhibit gravitational lensing. The properties of the lensed
source are consistent with an S_{\rm ab} galaxy with ongoing star formation.
The projected cluster mass determined from lensing arguments is in excellent
agreement with the value measured from a multiphase analysis of the X-ray data.
This contrasts with previously-reported results for the clusters Abell 1689 and
Abell 2218, for which discrepancies of \sim a factor 2 in the lensing and X-ray
masses are observed. At both X-ray and optical wavelengths PKS0745 appears
regular and dynamically relaxed. Abell 1689 and Abell 2218 exhibit more complex
dynamical states, indicative of ongoing merger events. We suggest that merging
activity has lead to line-of-sight mass enhancements in these systems. The
X-ray spectra and images presented here confirm that PKS0745 contains one of
the largest known cooling flows, with a mass deposition rate of \sim 1000
\Msunpyr.

Astrophysics