We present the first full catalog and science results for the NuSTAR serendipitous survey .
The catalog incorporates data taken during the first 40 months of NuSTAR operation , which provide \approx 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 followup .
We characterize the overall sample in terms of the X-ray , optical , and infrared source properties .
The sample is primarily comprised of active galactic nuclei ( AGNs ) , detected over a large range in redshift from z = 0.002 to 3.4 ( median of \left < z \right > = 0.56 ) , but also includes 16 spectroscopically confirmed Galactic sources .
There is a large range in X-ray flux , from \log ( f _ { 3 - 24 keV } / \mathrm { erg s ^ { -1 } cm ^ { -2 } } ) \approx - 14 to -11 , and in rest-frame 10 – 40 keV luminosity , from \log ( L _ { 10 - 40 keV } / \mathrm { erg s ^ { -1 } } ) \approx 39 to 46 , with a median of 44.1 .
Approximately 79 \% of the NuSTAR sources have lower energy ( < 10 keV ) X-ray counterparts from XMM-Newton , Chandra , and Swift XRT .
The mid-infrared ( MIR ) analysis , using WISE all-sky survey data , shows that MIR AGN color selections miss a large fraction of the NuSTAR -selected AGN population , from \approx 15 \% at the highest luminosities ( L _ { X } > 10 ^ { 44 } erg s ^ { -1 } ) to \approx 80 \% at the lowest luminosities ( L _ { X } < 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 ^ { +14 } _ { -15 } \% , 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 .