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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.

Harrison, F. A. and Aird, J. and Civano, F. and Lansbury, G. and Mullaney, J. R. and Ballantyne, D. R. and Alexander, D. M. and Stern, D. and Ajello, M. and Barret, D. and Bauer, F. E. and Baloković, M. and Brandt, W. N. and Brightman, M. and Boggs, S. E. and Christensen, F. E. and Comastri, A. and Craig, W. W. and Moro, A. Del and Forster, K. and Gandhi, P. and Giommi, P. and Grefenstette, B. W. and Hailey, C. J. and Hickox, R. C. and Hornstrup, A. and Kitaguchi, T. and Koglin, J. and Luo, B. and Madsen, K. K. and Mao, P. H. and Miyasaka, H. and Mori, K. and Perri, M. and Pivovaroff, M. and Puccetti, S. and Rana, V. and Treister, E. and Walton, D. and Westergaard, N. J. and Wik, D. and Zappacosta, L. and Zhang, W. W. and Zoglauer, A. (2016) 'The NuSTAR extragalactic surveys : the number counts of active galactic nuclei and the resolved fraction of the cosmic X-ray background.', Astrophysical journal., 831 (2). p. 185.

## 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 $\sim {10}^{25}\,{\mathrm{cm}}^{-2}$. 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 $5\times {10}^{-15}\,\lesssim$ S(3–8 keV)/$\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}\,\lesssim \,{10}^{-12}$ probed by NuSTAR. In the hard 8–24 keV band NuSTAR probes fluxes over the range $2\times {10}^{-14}\,\lesssim$ S(8–24 keV)/$\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}\,\lesssim \,{10}^{-12}$, 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) gsim 10−11 $\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}$), reflecting the evolution of the AGN population between the Swift/BAT local ($z\lt 0.1$) sample and NuSTAR's $z\sim 1$ 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.