The NuSTAR Extragalactic Surveys: The Number Counts of Active Galactic Nuclei and the Resolved Fraction of the Cosmic X-Ray Background

Journal
Date Issued
2016
Author(s)
Harrison, F. A.
•
Aird, J.
•
Civano, F.
•
Lansbury, G.
•
Mullaney, J. R.
•
Ballantyne, D. R.
•
Alexander, D. M.
•
Stern, D.
•
Ajello, M.
•
Barret, D.
•
Bauer, F. E.
•
Baloković, M.
•
Brandt, W. N.
•
Brightman, M.
•
Boggs, S. E.
•
Christensen, F. E.
•
•
Craig, W. W.
•
Del Moro, A.
•
Forster, K.
•
Gandhi, P.
•
Giommi, P.
•
Grefenstette, B. W.
•
Hailey, C. J.
•
Hickox, R. C.
•
Hornstrup, A.
•
Kitaguchi, T.
•
Koglin, J.
•
Luo, B.
•
Madsen, K. K.
•
Mao, P. H.
•
Miyasaka, H.
•
Mori, K.
•
•
Pivovaroff, M.
•
Puccetti, S.
•
Rana, V.
•
Treister, E.
•
Walton, D.
•
Westergaard, N. J.
•
Wik, D.
•
•
Zhang, W. W.
•
Zoglauer, A.
DOI
Abstract
We present the 3-8 keV and 8-24 keV number counts of active galactic nuclei (AGNs) identified in the Nuclear Spectroscopic Telescope Array (NuSTAR) extragalactic surveys. NuSTAR has now resolved 33%-39% of the X-ray background in the 8-24 keV band, directly identifying AGNs with obscuring columns up to ∼ {10}25 {{cm}}-2. In the softer 3-8 keV band the number counts are in general agreement with those measured by XMM-Newton and Chandra over the flux range 5× {10}-15 ≲ S(3-8 keV)/{erg} {{{s}}}-1 {{cm}}-2 ≲ {10}-12 probed by NuSTAR. In the hard 8-24 keV band NuSTAR probes fluxes over the range 2× {10}-14 ≲ S(8-24 keV)/{erg} {{{s}}}-1 {{cm}}-2 ≲ {10}-12, a factor ∼100 fainter than previous measurements. The 8-24 keV number counts match predictions from AGN population synthesis models, directly confirming the existence of a population of obscured and/or hard X-ray sources inferred from the shape of the integrated cosmic X-ray background. The measured NuSTAR counts lie significantly above simple extrapolation with a Euclidian slope to low flux of the Swift/BAT 15-55 keV number counts measured at higher fluxes (S(15-55 keV) ≳ 10-11 {erg} {{{s}}}-1 {{cm}}-2), reflecting the evolution of the AGN population between the Swift/BAT local (z\lt 0.1) sample and NuSTAR’s z∼ 1 sample. CXB synthesis models, which account for AGN evolution, lie above the Swift/BAT measurements, suggesting that they do not fully capture the evolution of obscured AGNs at low redshifts.
Volume
831
Issue
2
Start page
185
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2016ApJ...831..185H
Rights
open.access
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