The Swift X-ray Cluster Survey (SXCS) is an ongoing project aimed at finding serendipitously galaxy clusters in the Swift X-ray Telescope (XRT) archive. The Swift mission, launched in 2004, is dedicated to the study of Gamma-ray bursts (GRBs), which are detected and localized by the Burst Alert Telescope (BAT) and then followed-up by the XRT. The archive of GRB follow-up images obtained in this way constitutes a random survey of the X-ray sky where extended X-ray sources can be identified and studies, also thanks to the low XRT background, its constant PSF across the Field of View, and its effective area spanning the 0.5-7 keV energy range. In addition to the GRB follow-up fields (shown in the Figure), several targeted fields, unrelated to clusters, can be used to search for groups and clusters of galaxies. In the following table, XRT properties are compared to those of other X-ray telescopes. In particular, we emphasize the low background (thanks to the low-Earth orbit) and the constant PSF across the FOV.
| FOV | Eff Area @1.5keV | PSF HEW @1.5keV | Orbit Altitude | |
|---|---|---|---|---|
| ROSAT | 2° diameter | ~200 cm2 | ~25" on-axis | ~580 km |
| XMM-Newton MOS | 33'×33'×7ccd | ~1000 cm2 | 16.8" on-axis | 40,000 ~ 114,000 km |
| Chandra ACIS-I | 16'×16'×4ccd | 600 cm2 | < 0.5" on-axis | 16,000 ~ 133,000 km |
| Swift XRT | 23.6'×23.6' | 110 cm2 | 18" constant | 600 km |
The first data release is based only to the GRB follow-up fields. The properties of the survey and the first Catalog are presented in Tundo et al. 2012. The sources of Catalog I are characterized in Tozzi et al. 2014 thanks to a detailed spectral analysis. We are currently working to a second data release, based on a larger set of XRT fields, and on the use of a Voronoi-based algorithm "EXSdetect" tailored to the XRT data.
Acknowledgment
This project receives support from the "Exchange of Researcher" program for scientific and technological cooperation between Italy and People' Republic of China for the years 2013-2015 (code CN13MO5).