  Using CHANDRA, we investigate the spatial temperature distribution of the
intracluster medium (ICM) within 700 kpc of the center of the massive merging
cluster MACSJ0717.5+3745 at z=0.55. Combining the X-ray evidence with
information about the distribution and velocities of the cluster galaxies near
the core provides us with a snapshot of the three-dimensional geometry and
dynamics of one of the most complex cluster studied to date. We find
MACSJ0717.5+3745 to be an active triple merger with ICM temperatures exceeding
20 keV. Although radial velocity information and X-ray/optical offsets indicate
that all three mergers proceed along distinctly different directions, the
partial alignment of the merger axes points to a common origin in the
large-scale filament south-east of the cluster core. Clear decrements in the
ICM temperature observed near two of these subclusters identify the respective
X-ray surface brightness peaks as remnants of cool cores; the compactness and
low temperature of 5.7 keV of one of these features suggest that the respective
merger, a high-velocity collision at 3,000 km/s, is still in its very early
stages. Looking beyond the triple merger, we find the large-scale filament to
not only provide a spatial as well as temporal arrow for the interpretation of
the dynamics of the merger events near the cluster core; we also find
tantalizing, if circumstantial, evidence for direct, large-scale heating of the
ICM by contiguous infall of low-density gas from the filament.
