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

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 f0.3–10 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.