  We present the results of a multiwavelength investigation of the very X-ray
luminous galaxy cluster MACSJ0553.4-3342 ($z = 0.4270$; hereafter MACSJ0553).
Combining high-resolution data obtained with the Hubble Space Telescope and the
Chandra X-ray Observatory with ground-based galaxy spectroscopy, our analysis
establishes the system unambiguously as a binary, post-collision merger of
massive clusters. Key characteristics include perfect alignment of luminous and
dark matter for one component, a separation of almost 650 kpc (in projection)
between the dark-matter peak of the other subcluster and the second X-ray peak,
extremely hot gas (k$T > 15$ keV) at either end of the merger axis, a potential
cold front in the east, an unusually low gas mass fraction of approximately
0.075 for the western component, a velocity dispersion of $1490_{-130}^{+104}$
km s$^{-1}$, and no indication of significant substructure along the line of
sight. We propose that the MACSJ0553 merger proceeds not in the plane of the
sky, but at a large inclination angle, is observed very close to turnaround,
and that the eastern X-ray peak is the cool core of the slightly less massive
western component that was fully stripped and captured by the eastern
subcluster during the collision. If correct, this hypothesis would make
MACSJ0553 a superb target for a competitive study of ram-pressure stripping and
the collisional behaviour of luminous and dark matter during cluster formation.
