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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Zappacosta, L., Comastri, A., Civano, F., Puccetti, S., Fiore, F., Aird, J., Moro, A. Del, Lansbury, G. B., Lanzuisi, G., Goulding, A., Mullaney, J. R., Stern, D., Ajello, M., Alexander, D. M., Ballantyne, D. R., Bauer, F. E., Brandt, W. N., Chen, C.-T. J., Farrah, D., Harrison, F. A., Gandhi, P., Lanz, L., Masini, A., Marchesi, S., Ricci, C. & Treister, E. (2018). The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample. The Astrophysical Journal 854(1): 33.

Author(s) from Durham


We discuss the spectral analysis of a sample of 63 active galactic nuclei (AGN) detected above a limiting flux
of S 8 24 keV 7 10 = ´ -14 (– ) erg s cm - - 1 2 in the multi-tiered NuSTAR extragalactic survey program. The sources
span a redshift range z = 0 2.1 – (median ázñ = 0.58). The spectral analysis is performed over the broad 0.5–24 keV
energy range, combining NuSTAR with Chandra and/or XMM-Newton data and employing empirical and
physically motivated models. This constitutes the largest sample of AGN selected at >10 keV to be
homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such
as photon index, column density (NH), reflection parameter (R), and 10–40 keV luminosity (LX). Heavily obscured
(log cm 2 NH 3 [ ] -2  ) and Compton-thick (CT; log cm 24 NH [ ] -2  ) AGN constitute ∼25% (15–17 sources) and
∼2–3% (1–2 sources) of the sample, respectively. The observed NH distribution agrees fairly well with predictions
of cosmic X-ray background population-synthesis models (CXBPSM). We estimate the intrinsic fraction of AGN
as a function of NH, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT
AGN relative to log cm 20 2 NH 4 [ ]– -2 = AGN is poorly constrained, formally in the range 2–56% (90% upper
limit of 66%). We derived a fraction ( fabs) of obscured AGN (log cm 22 2 NH 4 [ ]– -2 = ) as a function of LX in
agreement with CXBPSM and previous z < 1 X-ray determinations. Furthermore, fabs at z = 0.1 0.5 – and
log erg s 43.6 44. Lx 3 ( )– -1 » agrees with observational measurements/trends obtained over larger redshift
intervals. We report a significant anti-correlation of R with LX (confirmed by our companion paper on stacked
spectra) with considerable scatter around the median R values.