The NuSTAR Extragalactic Surveys: The Number Counts of Active Galactic Nuclei and the Resolved Fraction of the Cosmic X-Ray Background
Harrison, F. A.; Hornstrup, A.; Stern, D.; Zhang, W. W.; Alexander, D. M.; Treister, E.; Bauer, F. E.; Brandt, W. N.; Comastri, A.; Ajello, M.; Giommi, P.; Luo, B.; Gandhi, P.; Civano, F.; Walton, D.; Aird, J.; Westergaard, N. J.; Barret, D.; Brightman, M.; Del Moro, A.; Ballantyne, D. R.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Grefenstette, B. W.; Hailey, C. J.; Hickox, R. C.; Madsen, K. K.; Mullaney, J. R.; Perri, M.; Puccetti, S.; Mori, K.; Baloković, M.; Kitaguchi, T.; Zappacosta, L.; Rana, V.; Forster, K.; Wik, D.; Lansbury, G.; Koglin, J.; Mao, P. H.; Miyasaka, H.; Pivovaroff, M.; Zoglauer, A.
United States, United Kingdom, France, Chile, Denmark, Italy, Japan
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 ∼ {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.