2018ApJS..235...17M

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 ∼0.6 deg² area of the field for a total observing time of ∼1.75 Ms, to a half-area depth of ∼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 x 10^–14 erg cm^–2 s^–1, 1.0 x 10^–14 erg cm^–2 s^–1, and 2.7 x 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 <z>∼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_H > 10²⁴ cm^–2) fraction. Taking into account the uncertainties on each N_H measurement, the final number of CT sources is 6.8 ± 1.2. This corresponds to an observed CT fraction of 11.5% ± 2.0%, providing a robust lower limit to the intrinsic fraction of CT active galactic nuclei and placing constraints on cosmic X-ray background synthesis models.