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°. 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.