Chandra Survey in the AKARI deep field at the North Ecliptic Pole

Optical and near-infrared identifications of X-ray sources

¹ Instituto de Astronomía, Universidad Nacional Autónoma de México Campus Ensenada, A.P. 106, Ensenada, BC 22800, Mexico
² Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 70-264, Ciudad de México, CDMX 04510, Mexico
³ Preuniversitario UC, Pontificia Universidad Católica de Chile, Chile
⁴ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ National Institute of Technology, Wakayama College, 77 Noshima, Nada-cho, Gobo, Wakayama 644-0023, Japan
⁶ Japan Space Forum 3-2-1, Kandasurugadai, Chiyoda-ku, Tokyo 101-0062 Japan
⁷ Space Information Center, Hokkaido Information University, Nishi-Nopporo 59-2, Ebetsu, Hokkaido 069-8585, Japan
⁸ Department of Physics, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
⁹ University of California, Los Angeles, Division of Astronomy & Astrophysics, 430 Portola Plaza, Los Angeles, CA 90095-1547, USA
¹⁰ Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
¹¹ SNU Astronomy Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
¹² Iwate University, 3-18-8 Ueda, Morioka, Iwate 020-8550, Japan
¹³ Institute of Astronomy, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
¹⁴ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan, ROC
¹⁵ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 229-8510, Japan
¹⁶ The Graduate University for Advanced Studies, SOKENDAI, Shonan Village, Hayama, Kanagawa 240-0193, Japan

Received: 19 April 2024
Accepted: 17 July 2024

Abstract

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.