  We report the discovery of highly distorted X-ray emission associated with
the nearby cluster Zw1718.1-0108, one of whose dominant members is the powerful
radio galaxy 3C353. This cluster has been missed by previous X-ray cluster
surveys because of its low Galactic latitude (b = 19.5deg), despite its
brightness in the hard X-ray band (2-10 keV flux of 1.2E-11 erg/s/cm2). Our
optical CCD image of the central part of the cluster reveals many member
galaxies which are dimmed substantially by heavy Galactic extinction. We have
measured redshifts of three bright galaxies near the X-ray emission peak and
they are all found to be around z=0.028. The ASCA GIS and ROSAT HRI images show
three aligned X-ray clumps embedded in low surface-brightness X-ray emission
extended by about 30 arcmin. The averaged temperature measured with ASCA is
kT=4.3+/-0.2 keV, which appears to be hot for the bolometric luminosity 1.1E+44
erg/s, when compared to the temperature-luminosity correlation of galaxy
clusters. The irregular X-ray morphology and evidence for a non-uniform
temperature distribution suggest that the system is undergoing a merger of
substructures. Since the sizes and luminosities of the individual clumps are
consistent with those of galaxy groups, Zw1718.1-0108 is interpreted as an
on-going merger of galaxy groups in a dark matter halo forming a cluster of
galaxies and thus is in a transition phase of cluster formation.
