摘要：The hard X-ray sky at energies above 10 keV has been extensively explored by theSwift/Gehrelsand theNuSTARmissions in the 14−195 keV and the 3−24 keV bands. respectively. The main population of the hard X-ray detected sources are active galactic nuclei (AGN). A discrepancy has been reported between the number count distributions of the two missions in the sense that the extrapolation of theSwift/Gehrelsnumber counts in the flux regime sampled byNuSTARlies significantly below theNuSTARcounts. We explore this aspect anew by comparing the number count distributions derived from theSwift/Gehrels105-month catalogue with those from the serendipitousNuSTARsource catalogue. We use a novel number count distribution estimator that makes use of the C-statistic on the unbinned data. In the 14−195 keV band, theSwift/BAT counts follow a Euclidean slope withα = 1.51 ± 0.10 (90% confidence level). TheNuSTARcounts in the 8−24 keV band present a steeper slope withα = 1.71 ± 0.20 because of an upturn at fluxes below ∼2 × 10−13erg cm−2s−1. The same upturn is observed in the soft (3−8 keV)NuSTARnumber counts, which generally also present a steep slope withα = 1.82 ± 0.15. Only the bright part of theNuSTAR3−8 keV counts agrees with theChandranumber counts in the 2−10 keV band, while the fainter part (below ∼7 × 10−13erg cm−2s−1) of the softNuSTARcounts is in marked disagreement with theChandracounts. Next, we compare the derived number counts in the different bands using our X-ray AGN population synthesis models. The comparison between theChandraand theSwift/BAT number counts shows a very good agreement for the standard AGN spectrum with a power-law slope Γ = 1.9, a high-energy cut-off at ∼130 keV and a 2−10 keV reflection component of 3%. On the other hand, using this standard AGN spectral model, only the bright part of theNuSTAR8−24 keV and 3−8 keV number counts agrees with the model predictions. It is therefore most likely that the disagreement between the X-ray number counts in the different bands is caused by the faintNuSTARnumber counts. We discuss various possibilities for the origin of this disagreement.
