The Swift X-ray Telescope Cluster Survey: data reduction and cluster
  catalog for the GRB fields
  (abridged) We present a new sample of X-ray selected galaxy groups and
clusters serendipitously observed with Swift and the X-ray Telescope (XRT). We
searched the XRT archive for extended sources among 336 GRB fields with
galactic latitude |b|>20{\deg}. Our selection algorithm yields a flux-limited
sample of 72 X-ray groups and clusters with a well defined selection function
and negligible contamination. The sky coverage of the survey goes from the
total 40 deg^2 to 1 deg^2 at a flux limit of 10^-14 erg/s/cm^-2 (0.5-2 keV).
Here we describe the XRT data processing, the statistical calibration of the
survey, and the catalog of detected cluster candidates. All the X-ray sources
are detected in the Swift-XRT soft (0.5-2 keV) band. A size parameter defined
as the half power radius (HPR) measured inside a box of 45x45 arcsec, is
assigned to each source. We select extended sources by applying a threshold on
the Half Power Radius and we calibrate its dependence on the measured net
counts and on the image background with extensive simulations in order to
identify all the sources with ~99% probability of being extended. We compute
the logN-logS of our sample, finding very good agreement with previous deep
cluster surveys. A cross correlation with published X-ray catalogs shows that
only 9 sources were already detected, none of them as extended. Therefore, ~90%
of our sources are new X-ray detections. We also cross correlated our sources
with optical catalogs, finding 20 previously identified clusters. Overall,
about ~65% of our sources are new detections. The XRT follow-up observation of
GRBs is providing an excellent serendipitous survey for groups and clusters of
galaxies, mainly thanks to the low background of XRT and its constant angular
resolution across the field of view. About 33% of the sample has spectroscopic
or photometric redshifts from public optical surveys.
