The NuSTAR Extragalactic Surveys: Overview and Catalog from the COSMOS Field

TL;DR

This paper presents the NuSTAR survey of the COSMOS field, providing a catalog of 91 X-ray sources, including heavily obscured AGN, and introduces a new detection strategy to improve sensitivity in high-energy X-ray observations.

Contribution

The study offers the first comprehensive NuSTAR catalog for the COSMOS field, with a novel detection method and detailed analysis of source properties, especially identifying Compton-thick AGN.

Findings

Detected 91 NuSTAR sources with luminosities 10^42-10^45.5 erg/s.

Identified 13-20% candidate Compton-thick AGN based on hardness ratios.

Discovered a highly obscured, previously unrecognized Compton-thick AGN.

Abstract

To provide the census of the sources contributing to the X-ray background peak above 10 keV, NuSTAR is performing extragalactic surveys using a three-tier "wedding cake" approach. We present the NuSTAR survey of the COSMOS field, the medium sensitivity and medium area tier, covering 1.7 deg2 and overlapping with both Chandra and XMM-Newton data. This survey consists of 121 observations for a total exposure of ~3 Ms. To fully exploit these data, we developed a new detection strategy, carefully tested through extensive simulations. The survey sensitivity at 20% completeness is 5.9, 2.9 and 6.4 x 10^-14 erg/cm2/s in the 3-24 keV, 3-8 keV and 8-24 keV bands, respectively. By combining detections in 3 bands, we have a sample of 91 NuSTAR sources with 10^42 -10^45.5 erg/s luminosities and redshift z=0.04-2.5. Thirty two sources are detected in the 8-24 keV band with fluxes ~100 times fainter than sources detected by Swift-BAT. Of the 91 detections, all but four are associated with a Chandra and/or XMM-Newton point-like counterpart. One source is associated with an extended lower energy X-ray source. We present the X-ray (hardness ratio and luminosity) and optical-to-X-ray properties. The observed fraction of candidate Compton-thick AGN measured from the hardness ratio is between 13-20%. We discuss the spectral properties of NuSTAR J100259+0220.6 (ID 330) at z=0.044, with the highest hardness ratio in the entire sample. The measured column density exceeds 10^24 /cm2, implying the source is Compton-thick. This source was not previously recognized as such without the >10 keV data.