The Swift X-ray Telescope Cluster Survey: data reduction and cluster catalog for the GRB fields

Aims. We present a new sample of X-ray selected galaxy groups and clusters serendipitously observed with the X-ray Telescope (XRT) on board of the Swift satellite. Using the XRT archive as of April 2010, we searched for extended sources among 336 gamma-ray burst (GRB) fields with galactic latitude vertical bar b vertical bar > 20 degrees. Our selection algorithm provides us with a flux-limited sample of 72 X-ray groups and clusters with a well defined selection function and an expected 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(-1) cm(-2) (0.5-2 keV). This paper provides a description of the XRT data processing, the statistical calibration of the survey, and the catalog of detected cluster candidates. Methods. All the X-ray sources are detected in the Swift/XRT soft (0.5-2 keV) images with the algorithm wavdetect. A size parameter defined as the half power radius (HPR) measured inside a box of 45 x 45 arcsec, is assigned to each source. We select extended sources by applying a threshold on the HPR. Thanks to extensive simulations, we are able to calibrate the threshold value, which depends on the measured net counts inside the box and on the image background, in order to identify all the sources with a probability similar or equal to 99% of being extended. The net counts associated to each extended source are then computed by simple aperture photometry. Results. We compute the logN-log S of our sample, finding very good agreement with previous deep cluster surveys. We did not find any correlation between the cluster and the GRB positions. A cross correlation with published X-ray catalogs shows that only 9 sources were already detected, none of them as extended. Therefore, similar to 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 similar to 65% of our sources are new detections, both as X-ray sources and as clusters of galaxies. Conclusions. 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. A significant fraction of the sample (similar to 33%) has spectroscopic or photometric redshift thanks to a cross-correlation with public optical surveys.