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Published November 10, 2016 | Published + Submitted
Journal Article Open

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

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 x 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 x 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 < 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.

Additional Information

© 2016 The American Astronomical Society. Received 2015 October 29; revised 2016 August 23; accepted 2016 August 24; published 2016 November 7. 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.

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Published - Harrison_2016_ApJ_831_185.pdf

Submitted - 1511.04183v1.pdf

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Created:
August 19, 2023
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