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

TL;DR

This study uses NuSTAR to detect and analyze rare, obscured active galactic nuclei in extragalactic fields, revealing their contribution to the Cosmic X-ray Background and demonstrating the effectiveness of the H1 band for discovering high-redshift, obscured AGNs.

Contribution

It presents new detections of Compton-thick AGNs at high redshift using the H1 band, highlighting its potential for uncovering obscured AGNs missed by previous surveys.

Findings

Detected 72 sources in the 8-16 keV band.

Identified two new Compton-thick AGNs at z ~ 1.25.

Number counts are consistent with CXB population models.

Abstract

We report on the results of active galactic nuclei (AGNs) detection by NuSTAR performed in three extragalactic survey fields (COSMOS, UDS, 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 $\sim 2.7$ 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 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 $z \sim 1.25$, which shifts their Compton-hump into the H1 band, and were previously missed in the usually employed NuSTAR bands, confirming the potential of using the H1 band to discover obscured AGNs at $z > 1$ in deep surveys.