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SWXCSCAT - Swift X-Ray Telescope Cluster Survey Catalog

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Overview

This table contains the Swift X-ray Cluster Survey (SWXCS) catalog obtained using archival data from the X-ray telescope (XRT) on board the Swift satellite acquired from 2005 February to 2012 November, extending the first release of the SWXCS. The catalog provides positions and soft X-ray fluxes for a flux-limited sample of X-ray group and cluster candidates. In Table 3 of the reference paper (available at the HEASARC as the linked table SWXCSOXID), when possible, optical counterparts are given for these candidates. The authors consider the fields with Galactic latitude |b| > 20 degrees so as to avoid regions of high H I column density. They discard all of the observations targeted at groups or clusters of galaxies, as well as particular extragalactic fields not suitable for searching for faint extended sources. The authors finally select ~ 3000 useful fields covering a total solid angle of ~ 400 deg2. They identify extended source candidates in the soft-band (0.5-2 keV) images of these fields using the software EXSdetect, which is specifically calibrated for the XRT data. Extensive simulations are used to evaluate contamination and completeness as a function of the source signal, allowing the authors to minimize the number of spurious detections and to robustly assess the selection function. The final catalog includes 263 candidate galaxy clusters and groups down to a flux limit of 7 x 10-15 erg/cm2/s in the soft band (0.5 - 2.0 keV), and the log N - log S is in very good agreement with previous deep X-ray surveys. In the reference paper, the final list of sources is cross-correlated with published optical, X-ray, and Sunyaev-Zel'dovich catalogs of clusters. The authors find that 137 sources have been previously identified as clusters in the literature in independent surveys, while 126 are new detections. Currently, they have collected redshift information for 158 sources (60% of the entire sample).

From the entire Swift XRT archive in the period 2005 February-2012 November, the authors have selected all the fields that can be used to build an unbiased, serendipitous X-ray cluster catalog.


Catalog Bibcode

2015ApJS..216...28L

References

The Swift X-ray telescope Cluster Survey.
III. Cluster catalog from 2005-2012 archival data.
    Liu T., Tozzi P., Tundo E., Moretti A., Rosati P., Wang J.-X.,
    Tagliaferri G., Campana S., Giavalisco M.
   <Astrophys. J. Suppl. Ser., 216, 28 (2015)>
   =2015ApJS..216...28L    (SIMBAD/NED BibCode)
The Swift X-ray Telescope Cluster Survey.
II. X-ray spectral analysis.
    Tozzi P., Moretti A., Tundo E., Liu T., Rosati P., Borgani S.,
    Tagliaferri G., Campana S., Fugazza D., D'Avanzo P.
   <Astron. Astrophys., 567, A89-89 (2014)>
   =2014A&A...567A..89T
The Swift X-ray Telescope Cluster Survey:
data reduction and cluster catalog for the GRB fields.
    Tundo E., Moretti A., Tozzi P., Teng L., Rosati P., Tagliaferri G.,
    Campana S.
   <Astron. Astrophys. 547, A57 (2012)>
   =2012A&A...547A..57T

Provenance

This table was created by the HEASARC in March 2015 based on an electronic version of Table 2 from the reference paper which was obtained from the CDS as their catalog J/ApJS/216/28 file table2.dat.

Parameters

Name
The Swift X-ray Cluster Survey (SWXCS) galaxy cluster candidate source name, being a J2000.0 position-based source designation registered with the CDS Dictionary of Nomenclature of Celestial Objects, viz., 'SWXCS JHHMMSS+DDMM.m'. This format is different from that used in Paper I (Tundo et al. 2012), but has already been used in Paper II (Tozzi et al. 2014). Note that the sources presented in Paper I keep the same names as used in Paper II, although the new positions typically differ by ~ 5".

Source_Flag
This flag parameter is set to 'I' to indicate that a source was included in the first release of the SWXCS (Tundo et al. 2012, A&A, 547, A57).

RA
The Right Ascension of the centroid of the galaxy cluster candidate in the selected equinox. This was given in J2000 decimal degrees to a precision of 10-6 degrees in the original table. The centroid is defined as the weighted median position of the 27 brightest pixels in the source region (each pixel is weighted by its density which equals to the pixel value divided by the Voronoi cell area).

Dec
The Declination of the centroid of the galaxy cluster candidate in the selected equinox. This was given in J2000 decimal degrees to a precision of 10-6 degrees in the original table. The centroid is defined as the weighted median position of the 27 brightest pixels in the source region (each pixel is weighted by its density which equals to the pixel value divided by the Voronoi cell area).

LII
The Galactic Longitude of the galaxy cluster candidate.

BII
The Galactic Latitude of the galaxy cluster candidate.

Exposure
The effective exposure time teff at the position of the source, in seconds, computed as given in Equation (1) of the reference paper, i.e., as the sum over i of ni divided by the sum over i of ni/ti, where i is the index of the filled pixels within the source extraction region, ni is the photon count in the ith pixel, and ti is the corresponding value in the vignetted exposure map.

NH_Gal
The value of the Galactic neutral Hydrogen (H I) column density, in units of H atoms cm-2, as found in the Leiden/Argentine/Bonn radio survey (Kalberla et al. 2005, A&A, 440, 775).

Extraction_Radius
The effective radius Reff, in arcseconds, defined such that pi * Reff2 equals the source extraction area (which has no a priori constraints on its shape).

Counts
The net counts Nnet measured in the soft band (0.5 - 2.0 keV) image by EXSdetect inside the source extraction region found by the VT+FOF method, after background subtraction and removal of unresolved sources. At present, no correction is applied to compensate for the lost diffuse emission in the region of the removed unresolved source.

Counts_Error
The 1-sigma error in the net counts Nnet measured in the soft band (0.5 - 2.0 keV). This is computed as the square root of (Ntot + Nbkg), where Nbkg is the count of background photons and Ntot = Nnet + Nbkg.

SNR
The signal-to-noise ratio (S/N) in the soft band, computed as the net counts divided by the associated error.

Flux
The estimated soft-band flux S in units of erg cm-2 s-1. To compute the flux for each field, the authors estimate the energy conversion factor (ECF) in the 0.5 - 2.0 keV band, taking into account Galactic absorption in this field, assuming a hot diffuse gas emission model with a temperature of 5 keV, a metal abundance of 0.3 solar, and a redshift of z = 0.4. As shown in Paper I, the ECF depends weakly on the spectral parameters. The flux of each source is calculated as S = Nnet * ECF/teff.

Flux_Error
The one-sigma error in the flux is measured by propagation, considering the error in the net counts and a 4% systematic error in the ECF due to the typical uncertainty in the actual spectral shape of each source (see Paper I). More accurate fluxes will be available from the X-ray spectral analysis of these sources, which is postponed until a forthcoming paper (A. Moretti et al., in preparation).


Contact Person

Questions regarding the SWXCSCAT database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 27-Mar-2015