The Extragalactic Serendipitous Swift Survey (ExSeSS) – I. Survey definition and measurements of the X-ray number counts. Issue 2 (17th January 2023)

• Record Type: Journal Article
• Authors: Delaney, Jack N; Aird, James; Evans, Phil A; Barlow-Hall, Cassandra; Osborne, Julian P; Watson, Michael G
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 521:Issue 2(2023)
• Page Start: 1620
• Page End: 1632
• Publication Date: 2023-01-17
• Subjects: galaxies: active -- X-rays: galaxies; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac3703
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We present the Extragalactic Serendipitous Swift Survey (ExSeSS), providing a new well-defined sample constructed from the observations performed using the Swift X-ray Telescope. The ExSeSS sample consists of 79 342 sources detected in the medium (1–2 keV), hard (2–10 keV), or total (0.3–10 keV) energy bands, covering 2086.6 deg 2 of sky across a flux range of f 0.3–10 keV ∼ 10 −15 − 10 −10 erg s −1 cm −2 . Using the new ExSeSS sample we present measurements of the differential number counts of X-ray sources as a function of 2–10 keV flux that trace the population of Active Galactic Nuclei (AGNs) in a previously unexplored regime. We find that taking the line-of-sight absorption column density into account has an effect on the differential number count measurements and is vital to obtain agreement with previous results. In the hard band, we obtain a good agreement between the ExSeSS measurements and previous, higher energy data from NuSTAR and Swift /BAT when taking into account the varying column density of the ExSeSS sample as well as the X-ray spectral parameters of each of the samples we are comparing to. We also find discrepancies between the ExSeSS measurements and AGN population synthesis models, indicating a change in the properties of the AGN population over this flux range that is not fully described by current models at these energies, hinting at a larger, moderately obscured population at low redshifts ( z ≲ 0.2) that the models are not currently taking into account.