The XMM-BCS galaxy cluster survey: I. The X-ray selected cluster catalog
  from the initial 6 deg$^2$
  The XMM-Newton - Blanco Cosmology Survey project (XMM-BCS) is a coordinated
X-ray, optical and mid-infrared cluster survey in a field also covered by
Sunyaev-Zel'dovich effect surveys by the South Pole Telescope and the Atacama
Cosmology Telescope. The aim of the project is to study the cluster population
in a 14 deg$^2$ field.
  In this work, we present a catalog of 46 X-ray selected clusters from the
initial 6 deg$^2$ survey core. We describe the XMM-BCS source detection
pipeline and derive physical properties of the clusters. We provide photometric
redshift estimates derived from the BCS imaging data and spectroscopic redshift
measurements for a low redshift subset of the clusters. We derive the cluster
log N - log S relation using an approximation to the survey selection function
and find it in good agreement with previous studies. We carry out an initial
comparison between X-ray luminosity derived masses and masses from optical
estimators from the Southern Cosmology Survey for a subset of the cluster
sample. Optical masses based on cluster richness and total optical luminosity
are found to be significantly higher than the X-ray values.
  (abridged)
