The Extragalactic Serendipitous Swift Survey (ExSeSS) - I. Survey definition and measurements of the X-ray number counts

Jack N. Delaney*, James Aird, Phil A. Evans, Cassandra Barlow-Hall, Julian P. Osborne, Michael G. Watson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

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 deg2 of sky across a flux range of f0.3-10keV ∼ 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 (AGN) 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.
Original languageEnglish
Pages (from-to)1620-1632
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date17 Jan 2023
Publication statusPublished - 1 May 2023

Keywords / Materials (for Non-textual outputs)

  • X-rays: galaxies
  • galaxies: active


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