SWXCS fields Distribution

	The Swift X-ray Cluster Survey (SWXCS) 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
Here SWXCS is compared with other X-ray cluster surveys based on Chandra or XMM data (please refer to Tundo et al. 2012 for detailed discussion).
Paper I Table 1

	The first data release is based only on 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.

	These work demonstrated the efficiency of an X-ray telescope as small as XRT for finding and characterizing X-ray extended sources. To extend the survey, we explore the entire XRT archive as of November 2012, which includes more than 10,000 fields, as opposed to the ~300 previously-used GRB follow-up fields. In addition, we apply for the first time to the XRT data the "EXSdetect" software, which was developed on the basis of Voronoi tessellation and friend-of-friend methods and tailored to the XRT data (Liu et al. 2013), to achieve better accuracy and sensitivity.

	Eventually, we present a large catalog of 263 candidates of galaxy clusters (Liu et al. 2015), with well-controlled sky coverage and selection function. X-ray spectral analysis and optical follow-up observations of the extended catalog are ongoing.

Optical follow-up observations

We have observed more than 40 of our sources using the Telescopio Nazionale Galileo (3.58m) and the du Pont Telescope (2.5m). Here are the images of a few sources in six bands (U, B, V, R, I, z) observed with the du Pont.

J032836-2140.4
J032836-2140.4
J222443-0220.5
J222443-0220.5
J014650-215605
J014650-215605
J100026-0735.5
J100026-0735.5
J203723-4401.7
J203723-4401.7
J000324-5253.8
J000324-5253.8
J091257-1152.2
J091257-1152.2
J151313-0905.4
J151313-0905.4

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).