2024A&A...689A..83M

Aims. We present a catalog of optical and infrared (NIR) identifications (ID) of X-ray sources in the AKARI North Ecliptic Pole (NEP) deep field detected with Chandra, covering ∼0.34 deg² and with 0.5-2 keV flux limits ranging between ∼2-20 × 10^–16 erg s^–1 cm^–2.
Methods. The optical/NIR counterparts of the X-ray sources were taken from our Hyper Suprime Cam (HSC)/Subaru and Wide-Field InfraRed Camera (WIRCam)/Canada-France-Hawaii Telescope (CFHT) data because these have much more accurate source positions due to their spatial resolution than those of Chandra and longer wavelength IR data. We concentrate our identifications in the HSC g band and WIRCam K_s band-based catalogs. To select the best counterpart, we utilized a novel extension of the likelihood-ratio (LR) analysis, where we used the X-ray flux as well as g-K_s colors to calculate the likelihood ratio. The spectroscopic and photometric redshifts of the counterparts are summarized in this work. In addition, simple X-ray spectroscopy was carried out on the sources with sufficient source counts.
Results. We present the resulting catalog in an electronic form. The main ID catalog contains 403 X-ray sources and includes X-ray fluxes, luminosities, g and K_s band magnitudes, redshifts and their sources, and optical spectroscopic properties, as well as intrinsic absorption column densities and power-law indices from simple X-ray spectroscopy. The X-ray sources identified in this work include 27 Milky-Way objects, 57 type I AGNs, 131 other AGNs, and 15 galaxies. The catalog serves as a basis for further investigations of the properties of the X-ray and NIR sources in this field.
Conclusions. We present a catalog of optical (g band) and NIR (K_s band) identifications of Chandra X-ray sources in the AKARI NEP Deep field with available optical/NIR spectroscopic features and redshifts as well as the results of simple X-ray spectroscopy. In the process, we developed a novel X-ray flux-dependent likelihood-ratio analysis for selecting the most likely counterparts among candidates.