The Swift AGN and Cluster Survey. I. Number counts of AGNs and galaxy clusters

The Swift active galactic nucleus (AGN) and Cluster Survey (SACS) uses 125 deg² of Swift X-ray Telescope serendipitous fields with variable depths surrounding γ-ray bursts to provide a medium depth ($4\times {10}^{-15}$ erg cm⁻² s⁻¹) and area survey filling the gap between deep, narrow Chandra/XMM-Newton surveys and wide, shallow ROSAT surveys. Here, we present a catalog of 22,563 point sources and 442 extended sources and examine the number counts of the AGN and galaxy cluster populations. SACS provides excellent constraints on the AGN number counts at the bright end with negligible uncertainties due to cosmic variance, and these constraints are consistent with previous measurements. We use Wide-field Infrared Survey Explorer mid-infrared (MIR) colors to classify the sources. For AGNs we can roughly separate the point sources into MIR-red and MIR-blue AGNs, finding roughly equal numbers of each type in the soft X-ray band (0.5–2 keV), but fewer MIR-blue sources in the hard X-ray band (2–8 keV). The cluster number counts, with 5% uncertainties from cosmic variance, are also consistent with previous surveys but span a much larger continuous flux range. Deep optical or IR follow-up observations of this cluster sample will significantly increase the number of higher-redshift ($z>0.5$) X-ray-selected clusters.