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

摘要

We present the 3–8 keV and 8–24 keV number counts of active galactic nuclei (AGNs) identified in the extragalactic surveys. has now resolved 33%–39% of the X-raybackground in the 8–24 keV band, directly identifying AGNs with obscuring columns up to ~10 cm. In the softer 3–8 keV band the number counts are in general agreement with those measured by andover the flux range 5 x 10 ≤ S(3–8 keV)/erg s cm ≤10 probed by In the hard 8–24 keV band probes fluxes over the range 2 x 10≤ (8–24 keV)/erg s cm ≤ 10, a factor ∼100 fainter than previous measurements. The 8–24 keV number counts match predictions from AGN populationsynthesis models, directly confirming the existence of a population of obscured and/or hard X-ray sources inferredfrom the shape of the integrated cosmic X-ray background. The measured NuSTAR counts lie significantly abovesimple 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 erg s cm), reflecting the evolution of the AGN population between the local (z 0.1) sample and z ~ 1 sample. CXB synthesis models, which account for AGNevolution, lie above the Swift/BAT measurements, suggesting that they do not fully capture the evolution of obscured AGNs at low redshifts.