The NuSTAR Extragalactic Surveys: The Number Counts of Active Galactic Nuclei and the Resolved Fraction of the Cosmic X-ray Background
astro-ph.HE
/ Authors
F. A. Harrison, J. Aird, F. Civano, G. Lansbury, J. R. Mullaney, D. R. Ballantyne, D. M. Alexander, D. Stern, M. Ajello, D. Barret
and 34 more authors
F. E. Bauer, M. Balokovic, W. N. Brandt, M. Brightman, S. E. Boggs, F. E. Christensen, A. Comastri, W. W. Craig, A. Del Moro, K. Forster, P. Gandhi, P. Giommi, B. W. Grefenstette, C. J. Hailey, R. C. Hickox, A. Hornstrup, T. Kitaguchi, J. Koglin, B. Luo, K. K. Madsen, P. H. Mao, H. Miyasaka, K. Mori, M. Perri
/ Abstract
We present the 3-8 keV and 8-24 keV number counts of active galactic nuclei (AGN) identified in the NuSTAR extragalactic surveys. NuSTAR has now resolved 33-39% of the X-ray background in the 8-24 keV band, directly identifying AGN with obscuring columns up to approximately 1e25 /cm2. 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 5e-15 < S(3 - 8 keV)/(erg/cm2/s) < 1e-12 probed by NuSTAR. In the hard 8-24 keV band NuSTAR probes fluxes over the range 2e-14 < S(8-24 keV)/(erg/cm2/s) < 1e-12, a factor of approximately 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) > 1e-11 erg/cm2/s, 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 AGN at low redshifts.