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The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample

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posted on 2018-08-10, 11:43 authored by L. Zappacosta, A. Comastri, F. Civano, S. Puccetti, F. Fiore, J. Aird, A. Del Moro, G. B. Lansbury, G. Lanzuisi, A. Goulding, J. R. Mullaney, D. Stern, M. Ajello, D. M. Alexander, D. R. Ballantyne, F. E. Bauer, W. N. Brandt, C-T. J. Chen, D. Farrah, F. A. Harrison, P. Gandhi, L. Lanz, A. Masini, S. Marchesi, C. Ricci, E. Treister
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 x 10^-14 erg s^-1 cm^-2 in the multi-tiered NuSTAR extragalactic survey program. The sources span a redshift range z = 0 - 2.1 (median = 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[NH/cm^-2] >= 23) and Compton-thick (CT; log[NH/cm^-2] >= 24 ) 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[NH/cm^-2] = 20-24 AGN is poorly constrained, formally in the range 2–56% (90% upper limit of 66%). We derived a fraction ( fabs) of obscured AGN (log[NH/cm^-2] = 20-24 ) 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(LX/erg s ^-1) ~~ 43.6-44.3 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.


We thank Johannes Buchner and Yoshihiro Ueda for providing machine-readable results on the absorbed fractions. L.Z. thanks R. Gilli, F. Nicastro, E. Piconcelli, R. Valiante, F. Vito, and D. Burlon for useful discussions. L.Z., A.C., F.F., G.L., and A.M. acknowledge financial support under ASI/INAF contract I/037/12/0. A.C. acknowledges the Caltech Kingsley fellowship program. G.L. acknowledges financial support from the CIG grant eEASY No. 321913. F.E.B. acknowledges support from CONICYT-Chile (Basal-CATA PFB-06/2007, FONDECYT Regular 1141218), the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. This work was supported under NASA Contract NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA).



Astrophysical Journal, 2018, 854, pp. 33-33

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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