The $\textit{Nu}$STAR Serendipitous Survey: The 40-month Catalog and the Properties of the Distant High-energy X-Ray Source Population $\textit{text}$

Abstract

We present the first full catalog and science results for the $\textit{Nuclear Spectroscopic Telescope Array}$ ($\textit{Nu}$STAR) serendipitous survey. The catalog incorporates data taken during the first 40 months of $\textit{Nu}$STAR operation, which provide ≈20 Ms of effective exposure time over 331 fields, with an areal coverage of 13 deg$^{2}$, and 497 sources detected in total over the 3-24 keV energy range. There are 276 sources with spectroscopic redshifts and classifications, largely resulting from our extensive campaign of ground-based spectroscopic follow-up. We characterize the overall sample in terms of the X-ray, optical, and infrared source properties. The sample is primarily composed of active galactic nuclei (AGNs), detected over a large range in redshift from $z$ = 0.002 to 3.4 (median of $\langle z\rangle =0.56$) = 0.56), but also includes 16 spectroscopically confirmed Galactic sources. There is a large range in X-ray flux, from log(f${3-24keV}$/erg s$^{-1}$ cm$^{-2}$) ≈ -14 to -11, and in rest-frame 10-40 keV luminosity, from (L${10-40keV}$/erg s$^{-1}$) ≈ 39 to 46, with a median of 44.1. Approximately 79% of the $\textit{Nu}$STAR sources have lower-energy (<10 keV) X-ray counterparts from $\textit{XMM-Newton, Chandra,}$ and $\textit{Swift}$ XRT. The mid-infrared (MIR) analysis, using $\textit{WISE}$ all-sky survey data, shows that MIR AGN color selections miss a large fraction of the $\textit{Nu}$STAR-selected AGN population, from ≈15% at the highest luminosities (LX > 10$^{44}$ erg s$^{-1}$) to ≈80% at the lowest luminosities (LX < 10$^{43}$ erg s$^{-1}$). Our optical spectroscopic analysis finds that the observed fraction of optically obscured AGNs (i.e., the type 2 fraction) is F${Type 2}$ = 53$^{+15}{−14}$%, for a well-defined subset of the 8-24 keV selected sample. This is higher, albeit at a low significance level, than the type 2 fraction measured for redshift- and luminosity-matched AGNs selected by <10 keV X-ray missions.