The NuSTAR Extragalactic Surveys: unveiling rare, buried AGNs and detecting the contributors to the peak of the Cosmic X-ray Background

We report on the results of active galactic nuclei (AGNs) detection by NuSTAR performed in three extragalactic survey fields (COSMic Evolutionary Survey field (COSMOS), Ultra Deep Survey (UDS), and Extended Chandra Deep Field-South (ECDFS)) in three hard bands, namely H1 (8-16 keV), H2 (16-24 keV), and VH (35-55 keV). The aggregated area of the surveys is similar to 2.7 deg(2). While a large number of sources is detected in the H1 band (72 at the 97% level of reliability), the H2 band directly probing close to the peak of the Cosmic X-ray Background (CXB) returns four significant detections, and two tentative (although not significant) detections are found in the VH-band. All of the sources detected above 16 keV are also detected at lower energies. We compute the integral number counts for sources in such bands, which show broad consistency with population-synthesis models of the CXB. We furthermore identify two Compton-thick AGNs, one in the COSMOS field, associated with a hard and faint Chandra source, and one in the UDS field, never detected in the X-ray band before. Both sources are at the same redshift z similar to 1.25, which shifts their Compton-hump into the H1 band, and were previously missed in the usually employed NuSTAR bands, confirming the potential for using the H1 band to discover obscured AGNs at z > 1 in deep surveys.