  We present a multi-wavelength study of RX J0848+4456, a cluster of galaxies
discovered through X-ray emission in the ROSAT Deep Cluster Survey. Our
observations consist of WFPC2 imaging, optical spectra, and X-ray data
collected with the Chandra observatory. We find that RX J0848+4456 consists of
an X-ray emitting cluster of galaxies at a redshift of z=0.570 and a group at
slightly lower redshift, z=0.543, with little X-ray emission. This lower
redshift system, however, is a gravitational lens, with the lensed galaxy an
unusual AGN or star-forming system at z=3.356.
  The cluster has an X-ray temperature of kT = 3.6 +/- 0.4 keV, a bolometric
luminosity of 1.0e44 +/- 0.3e44 erg/s and a velocity dispersion of 670 +/- 50
km/s. These values all agree with the low redshift correlations for clusters of
galaxies, implying a relaxed system with the ICM in equilibrium with the dark
matter potential. The lower redshift group of galaxies at z=0.543 has, at most
1/5, more likely 1/10, of the X-ray luminosity of RX J0848+4456. Despite being
a gravitational lens, this is a low mass system, with an X-ray temperature of
kT = 2.3 +0.5 -0.4 keV and a velocity dispersion of only 430 +/- 20 km/s. Our
observations show the importance of detailed studies of clusters of galaxies
when using them as probes of cosmological mass functions.
