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. and Treister, E. (2018) The NuSTAR Extragalactic Surveys: X-Ray spectroscopic analysis of the bright hard-band selected sample. Astrophysical Journal, 854 (1), [33]. (doi:10.3847/1538-4357/aaa550).
Abstract
We discuss the spectral analysis of a sample of 63 active galactic nuclei (AGN) detected above a limiting fluxof S (8-24 keV) = 7×10-14 erg s-1 cm-1 in the multi-tiered NuSTAR extragalactic survey program. The sourcesspan a redshift range z = 0-2.1 (median 〈z〈 = 0.58). The spectral analysis is performed over the broad 0.5-24 keVenergy range, combining NuSTAR with Chandra and/or XMM-Newton data and employing empirical andphysically motivated models. This constitutes the largest sample of AGN selected at >10 keV to behomogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters suchas photon index, column density (NH), reflection parameter (R), and 10-40 keV luminosity (LX). Heavily obscured(log NH cm 23[ -2]) and Compton-thick (CT; log N cm 24 H[ -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 predictionsof cosmic X-ray background population-synthesis models (CXBPSM). We estimate the intrinsic fraction of AGNas a function of NH, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CTAGN relative to log NH cm 20 24[ -2] = - AGN is poorly constrained, formally in the range 2-56% (90% upperlimit of 66%). We derived a fraction ( fabs) of obscured AGN (log NH cm 22 24[-2] = -) as a function of LX inagreement with CXBPSM and previous z < 1 X-ray determinations. Furthermore, fabs at z = 0.1-0.5 andlog Lx erg s 43.6 44.3( -1) - agrees with observational measurements/trends obtained over larger redshiftintervals. We report a significant anti-correlation of R with LX (confirmed by our companion paper on stackedspectra) with considerable scatter around the median R values.
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