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THE NuSTAR EXTRAGALACTIC SURVEYS: The NUMBER COUNTS of ACTIVE GALACTIC NUCLEI and the RESOLVED FRACTION of the COSMIC X-RAY BACKGROUND

Published version
Peer-reviewed

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Type

Article

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Authors

Harrison, FA 
Civano, F 
Mullaney, JR 

Abstract

© 2016. The American Astronomical Society. All rights reserved. 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 . 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 S(3-8 keV)/ probed by NuSTAR. In the hard 8-24 keV band NuSTAR probes fluxes over the range S(8-24 keV)/, 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 ), reflecting the evolution of the AGN population between the Swift/BAT local () sample and NuSTAR's 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.

Description

Keywords

galaxies: active, galaxies: nuclei, galaxies: Seyfert, surveys, X-rays: diffuse background, X-rays: galaxies

Journal Title

Astrophysical Journal

Conference Name

Journal ISSN

0004-637X
1538-4357

Volume Title

831

Publisher

IOP
Sponsorship
Science and Technology Facilities Council (ST/N004027/1)
This work was supported under NASA Contract No. 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 the National Aeronautics and Space Administration. 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). J.A. acknowledges support from ERC Advanced Grant FEEDBACK at the University of Cambridge and a COFUND Junior Research Fellowship from the Institute of Advanced Study, Durham University.