The SWIFT AGN and Cluster Survey I: Number Counts of AGN and Galaxy Clusters

TL;DR

The paper presents the Swift AGN and Cluster Survey catalog, analyzing the number counts of AGN and galaxy clusters, providing constraints that bridge previous surveys and classifying sources using mid-infrared colors.

Contribution

It introduces a medium-depth, wide-area X-ray survey catalog with over 22,500 point sources and 442 extended sources, offering new constraints on AGN and galaxy cluster counts.

Findings

AGN number counts are consistent with previous measurements.

Fewer MIR-blue AGN sources are detected in the hard X-ray band.

Cluster counts span a larger flux range with low cosmic variance uncertainties.

Abstract

The Swift AGN and Cluster Survey (SACS) uses 125 deg^2 of Swift XRT serendipitous fields with variable depths surrounding gamma-ray bursts to provide a medium depth (4e-15 erg/s/cm^2) 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 Wise mid-infrared (MIR) colors to classify the sources. For AGN we can roughly separate the point sources into MIR-red and MIR-blue AGN, 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.