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 . 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.