X-ray selected galaxy clusters in the Pan-STARRS Medium-Deep Survey
  [abridged]
  We present the results of a pilot study for the extended MACS survey (eMACS),
a comprehensive search for distant, X-ray luminous galaxy clusters at z>0.5.
Our pilot study applies the eMACS concept to the 71 deg^2 area extended by the
ten fields of the Pan-STARRS1 (PS1) Medium Deep Survey (MDS). Candidate
clusters are identified by visual inspection of PS1 images in the g,r, i, and z
bands in a 5x5 arcmin^2 region around X-ray sources detected in the ROSAT
All-Sky Survey (RASS). To test and optimize the eMACS X-ray selection criteria,
our pilot study uses the largest possible RASS database, i.e., all RASS sources
listed in the Bright and Faint Source Catalogs (BSC and FSC) that fall within
the MDS footprint. Scrutiny of PS1/MDS images for 41 BSC and 200 FSC sources
combined with dedicated spectroscopic follow-up observations results in a
sample of 11 clusters with estimated or spectroscopic redshifts of z>0.3. X-ray
follow-up observations will be crucial in order to establish robust cluster
luminosities for eMACS clusters.
  Although the small number of distant X-ray luminous clusters in the MDS does
not allow us to make firm predictions for the over 20,000 deg^2 of
extragalactic sky covered by eMACS, the identification of two extremely
promising eMACS cluster candidates at z>0.6 (both yet to be observed with
Chandra) in such a small solid angle is encouraging. Representing a tremendous
gain over the presently known two dozen such systems from X-ray, optical, and
SZ cluster surveys combined, the sample of over 100 extremely massive clusters
at z>0.5 expected from eMACS would be invaluable for the identification of the
most powerful gravitational lenses in the Universe, as well as for in-depth and
statistical studies of the physical properties of the most massive galaxy
clusters out to z~1.
