tokens: (Spergel et al. 2003 ). All the errors and upper limits are quoted at a 90% confidence level, unless otherwise specified. 2. Data and Catalogs The NuSTAR extragalactic survey program consists of tiered observations of well-known survey fields: (i) a set of deep tiled pointings covering the E-CDFS (Mullaney et al. 2015 ) and EGS (A. Del Moro et al. 2017, in preparation) fields, with an area of and arcmin 2 , respectively, and a total exposure of 1.49 Ms in each of the two fields, reaching a maximum exposure of ≈220 ks in E-CDFS and ≈280 ks in EGS (at 3–24 keV in each focal plane module (FPM), vignetting corrected); (ii) a medium-depth set of 121 tiled pointings covering ≈1.7 deg 2 of the COSMOS field (Civano et al. 2015 ) for a total exposure of 3.12 Ms, with a maximum depth of ≈90 ks in the central ∼1.2 deg 2 ; (iii) a serendipitous survey consisting of all serendipitous sources detected in any NuSTAR targeted observation (excluding all sources associated with the target; Alexander et al. 2013 ; Lansbury et al. 2017 ). This last tier of the survey spans a wide range of depths (see, e.g., Aird et al. 2015a ; Lansbury et al. 2017 ) and has the largest sky coverage, reaching deg 2 to date (still ongoing). Since in this work we require low-energy ( keV) Chandra data, together with the NuSTAR data, to produce broad X-ray band composite spectra, we limit our analyses to the sources detected in the E-CDFS, EGS, and COSMOS fields, which have good Chandra coverage and redshift completeness (spectroscopic and photometric), while we exclude the serendipitous survey sources owing to the heterogeneity of the available ancillary data. Although XMM-Newton observations are also available for the E-CDFS and COSMOS fields, we do not include these data in our analyses because combining the data from different X-ray instruments to produce the composite spectra can cause significant distortions in the resulting spectra (see Section 3.2.1 ). Our sample consists of 182 AGNs (see Table 1 ), 49 detected in E-CDFS (Mullaney et al. 2015 ), 42 detected in EGS (A. Del Moro et al. 2017, in preparation), and 91 detected in COSMOS (Civano et al. 2015 ). Of these sources, 64 (∼35%) are detected in the NuSTAR HB (8–24 keV). 25 25 Most of the sources that are formally HB undetected (∼65%) are actually detected also above 8 keV, but with lower significance than the false-probability threshold adopted by Civano et al. ( 2015 ), Mullaney et al. ( 2015 ), and A. Del Moro et al. (2017, in preparation). Table 1 NuSTAR Source Sample Summary Notes. a Number of HB (8–24 keV) detected sources. b Including both spectroscopic and photometric redshifts; the number of spectroscopic redshifts is reported in parentheses (also in the fifth and sixth columns). In Civano et al. ( 2015 ) and Mullaney et al. ( 2015 ), the NuSTAR sources have been matched to the Chandra and/or XMM-Newton point-source catalogs available in these fields (Lehmer et al. 2005 ; Brusa et al. 2010 ; Xue et al. 2011 ;
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