The NuSTAR Extragalactic Surveys: source catalog and the Compton-thick
  fraction in the UDS field
  We present the results and the source catalog of the NuSTAR survey in the
UKIDSS Ultra Deep Survey (UDS) field, bridging the gap in depth and area
between NuSTAR's ECDFS and COSMOS surveys. The survey covers a $\sim 0.6$
deg$^2$ area of the field for a total observing time of $\sim$ 1.75 Ms, to a
half-area depth of $\sim$ 155 ks corrected for vignetting at $3-24$ keV, and
reaching sensitivity limits at half-area in the full ($3-24$ keV), soft ($3-8$
keV) and hard ($8-24$ keV) bands of $2.2 \times 10^{-14}$ erg cm$^{-2}$
s$^{-1}$, $1.0 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, and $2.7 \times
10^{-14}$ erg cm$^{-2}$ s$^{-1}$, respectively. A total of 67 sources are
detected in at least one of the three bands, 56 of which have a robust optical
redshift with a median of $\langle z\rangle \sim 1.1$. Through a broadband
($0.5-24$ keV) spectral analysis of the whole sample combined with the NuSTAR
hardness ratios, we compute the observed Compton-thick (CT; $N_{\rm H} >
10^{24}$ cm$^{-2}$) fraction. Taking into account the uncertainties on each
$N_{\rm H}$ measurement, the final number of CT sources is $6.8\pm1.2$. This
corresponds to an observed CT fraction of $(11.5\pm2.0)\%$, providing a robust
lower limit to the intrinsic fraction of CT AGN and placing constraints on
cosmic X-ray background synthesis models.
