  We present the first results of a pilot X-ray study of 37 rich galaxy
clusters at $0.1<z<1.1$ in the Hyper Suprime-Cam Subaru Strategic Program
(HSC-SSP) field. Diffuse X-ray emissions from these clusters were
serendipitously detected in the XMM-Newton fields of view. We systematically
analyze X-ray images of 37 clusters and emission spectra of a subsample of 17
clusters with high photon statistics by using the XMM-Newton archive data. The
frequency distribution of the offset between the X-ray centroid or peak and the
position of the brightest cluster galaxy was derived for the optical cluster
sample. The fraction of relaxed clusters estimated from the X-ray peak offsets
in 17 clusters is $29\pm11(\pm13)$\%, which is smaller than that of the X-ray
cluster samples such as HIFLUGCS. Since the optical cluster search is immune to
the physical state of X-ray-emitting gas, it is likely to cover a larger range
of the cluster morphology. We also derived the luminosity-temperature relation
and found that the slope is marginally shallower than those of X-ray-selected
samples and consistent with the self-similar model prediction of 2.
Accordingly, our results show that the X-ray properties of the optical clusters
are marginally different from those observed in the X-ray samples. The
implication of the results and future prospects are briefly discussed.
