Harrison, F. A.F. A.HarrisonAird, J.J.AirdCivano, F.F.CivanoGiommi, PaoloPaoloGiommiPerri, MatteoMatteoPerriPuccetti, SimonettaSimonettaPuccetti2020-09-172020-09-172016-11-01https://hdl.handle.net/20.500.13025/4788The Astrophysical Journal, Volume 831, Issue 2, article id. 185, <NUMPAGES>8</NUMPAGES> pp. (2016).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.galaxies activegalaxies nucleigalaxies SeyfertsurveysX-rays diffuse backgroundX-rays galaxiesjournal article10.3847/0004-637X/831/2/185http://adsabs.harvard.edu/abs/2016ApJ...831..185H2016ApJ...831..185H