The NuSTAR Serendipitous Survey: The 40 month Catalog and the Properties
  of the Distant High Energy X-ray Source Population
  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 ~20Ms of effective exposure time over
331 fields, with an areal coverage of 13 sq deg, 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 - 3.4 (median of <z> = 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 log( L_10-40keV / erg s^-1 ) ~ 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 ~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_Type2 = 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.
