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The ROSAT Raster survey in the north ecliptic pole field: X-ray catalogue and optical identifications

Hasinger, G. and Freyberg, M. and Hu, E. M. and Waters, C. Z. and Capak, P. and Moneti, A. and McCracken, H. J. (2021) The ROSAT Raster survey in the north ecliptic pole field: X-ray catalogue and optical identifications. Astronomy and Astrophysics, 645 . Art. No. A95. ISSN 0004-6361. doi:10.1051/0004-6361/202039476.

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The north ecliptic pole (NEP) is an important region for extragalactic surveys. Deep and wide contiguous surveys are being performed by several space observatories, most currently with the eROSITA telescope. Several more are planned for the near future. We analyse all the ROSAT pointed and survey observations in a region of 40 deg² around the NEP, restricting the ROSAT field of view to the inner 30′ radius. We obtain an X-ray catalogue of 805 sources with 0.5−2 keV fluxes > 2.9 × 10⁻¹⁵ erg cm⁻² s⁻¹, about a factor of three deeper than the ROSAT All-Sky Survey in this field. The sensitivity and angular resolution of our data are comparable to the eROSITA All-Sky Survey expectations. The 50% position error radius of the sample of X-ray sources is ∼10″. We use HEROES optical and near-infrared imaging photometry from the Subaru and Canada/France/Hawaii telescopes together with GALEX, SDSS, Pan-STARRS, and WISE catalogues, as well as images from a new deep and wide Spitzer survey in the field to statistically identify the X-ray sources and to calculate photometric redshifts for the candidate counterparts. In particular, we utilize mid-infrared (mid-IR) colours to identify active galactic nucleus (AGN) X-ray counterparts. Despite the relatively large error circles and often faint counterparts, together with confusion issues and systematic errors, we obtain a rather reliable catalogue of 766 high-quality optical counterparts, corresponding redshifts and optical classifications. The quality of the dataset is sufficient to look at ensemble properties of X-ray source classes. In particular we find a new population of luminous absorbed X-ray AGN at large redshifts, identified through their mid-IR colours. This populous group of AGN was not recognized in previous X-ray surveys, but could be identified in our work due to the unique combination of survey solid angle, X-ray sensitivity, and quality of the multi-wavelength photometry. We also use the WISE and Spitzer photometry to identify a sample of 185 AGN selected purely through their mid-IR colours, most of which are not detected by ROSAT. Their redshifts and upper limits to X-ray luminosity and X-ray–to–optical flux ratios are even higher than for the new class of X-ray selected luminous type 2 AGN (AGN2); they are probably a natural extension of this sample. This unique dataset is important as a reference sample for future deep surveys in the NEP region, in particular for eROSITA and also for Euclid and SPHEREX. We predict that most of the absorbed distant AGN should be readily picked up by eROSITA, but they require sensitive mid-IR imaging to be recognized as optical counterparts.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hasinger, G.0000-0002-0797-0646
Capak, P.0000-0003-3578-6843
McCracken, H. J.0000-0002-9489-7765
Additional Information:© 2021 ESO. Received 20 September 2020; Accepted 9 November 2020; Published online 19 January 2021. We would like to thank an anonymous referee for very helpful suggestions to improve the manuscript. GH would like to thank Olivier Ilbert and Mara Salvato for important help with the LePhare photometric redshift code, and Mara as well for very helpful comments on the paper draft. We would like to thank Len Cowie for designing the HEROES survey, and the former IfA REU students Cameron White & Victoria Jones who have accompanied the HEROES observations and analysis, as well as taken a few HYDRA spectra of X-ray sources at the WIYN telescope. E.M.H. would like to acknowledge the NSF grant AST-1716093, partly funding the HEROES activities. We would like to acknowledge the excellent ROSAT data archive provided by the Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany. This research has made use of data obtained from XMMSL2, the Second XMM-Newton Slew Survey Catalogue, produced by members of the XMM SOC, the EPIC consortium, and using work carried out in the context of the EXTraS project (“Exploring the X-ray Transient and variable Sky”, funded from the EU’s Seventh Framework Programme under grant agreement no. 607452). This research has made use of data obtained from the 4XMM XMM-Newton serendipitous source catalogue compiled by the 10 institutes of the XMM-Newton Survey Science Centre selected by ESA. This work is based in part on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA and it also makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by NASA. It is also based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan, as well as on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
European Research Council (ERC)607452
Subject Keywords:surveys – catalogs – galaxies: nuclei – quasars: general – X-rays: galaxies – X-rays: galaxies: clusters
Record Number:CaltechAUTHORS:20210125-101033580
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Official Citation:The ROSAT Raster survey in the north ecliptic pole field - X-ray catalogue and optical identifications. G. Hasinger, M. Freyberg, E. M. Hu, C. Z. Waters, P. Capak, A. Moneti and H. J. McCracken. A&A, 645 (2021) A95 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107707
Deposited By: Tony Diaz
Deposited On:26 Jan 2021 20:10
Last Modified:16 Nov 2021 19:05

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