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

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× {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× {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\lt 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.