Counterpart identification and classification for eRASS1 and characterisation of the active galactic nuclei content

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany
² Exzellenzcluster ORIGINS, Boltzmannstr. 2, D-85748 Garching, Germany
³ Max-Planck-Institut für Astronomie, Königstuhl 17., D-69117 Heidelberg, Germany
⁴ Institute for Astronomy and Astrophysics, National Observatory of Athens, V. Paulou and I. Metaxa 11532, Greece
⁵ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁷ Perimeter Institute for Theoretical Physics, 31 Caroline St. North, Waterloo, ON N2L 2Y5, Canada
⁸ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁹ University of Trento, Via Sommarive 14, I-38123 Trento, Italy
¹⁰ Universitá di Firenze, Dipartimento di Fisica e Astronomia, Via G. Sansone 1, 50019 Sesto F.no, Firenze, Italy
¹¹ INAF – Osservatorio Astrofisico di Arcetri, Via Largo E. Fermi 5, 50125 Firenze, Italy
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ Cerro Tololo Inter-American Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
¹⁴ Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
¹⁵ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
¹⁶ NSF National Optical/Infrared Research Laboratory, 950 N. Cherry Ave, Tucson, AZ 85719, USA
¹⁷ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
¹⁸ Department of Physics & Astronomy, University of Wyoming, 1000 E. University, Department 3905, Laramie, WY 8207, USA
¹⁹ Department of Physics, University of Johannesburg, Johannesburg, South Africa
²⁰ Instituto de Astrofísica de Andalucia (IAA-CSIC), Glorieta de la astronomia S/N, 18008 Granada, Spain
²¹ Las Campanas Observatory – Carnegie Institution for Science Av. Raul Bitran 1200, La Serena, Chile
²² Universitá di Padova, Dipartimento di Fisica e Astronomia Galileo Galilei, Via Francesco Marzolo, 8, 35131 Padova, Italy
²³ South African Astronomical Observatory, PO Box 9 Observatory, Cape Town 7935, South Africa
²⁴ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
²⁵ Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
²⁶ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
²⁷ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

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Received: 27 June 2025
Accepted: 3 September 2025

Abstract

Context. Accurately accounting for the Active Galactic Nucleus (AGN) phase in galaxy evolution requires a large, clean AGN sample. This is now possible with SRG/eROSITA, which completed its first all-sky X-ray survey (eRASS1) on June 12, 2020. The public Data Release 1 (DR1, Jan 31, 2024) includes 930,203 sources from the western Galactic hemisphere.

Aims. The data enable the selection of a large AGN sample and the discovery of rare sources. However, scientific return depends on accurate characterisation of the X-ray emitters, requiring high-quality multi-wavelength data. This paper presents the identification and classification of optical and infrared counterparts to eRASS1 sources.

Methods. Counterparts to eRASS1 X-ray point sources were identified using Gaia DR3, CatWISE2020, and Legacy Survey DR10 (LS10) with the Bayesian NWAY algorithm and trained priors. Sources were classified as Galactic or extragalactic via a machine-learning model combining optical/IR and X-ray properties, trained on a reference sample. For extragalactic LS10 sources, photometric redshifts were computed using CIRCLEZ.

Results. Within the LS10 footprint, all 656,614 eROSITA/DR1 sources have at least one possible optical counterpart; ∼570 000 are extragalactic and likely AGN. Half are new detections compared to AllWISE, Gaia, and Quaia AGN catalogues. Gaia and CatWISE2020 counterparts are less reliable, due to the survey’s shallowness and the limited amount of features available to assess the probability of being an X-ray emitter. In the Galactic plane, where the overdensity of stellar sources also increases the chance of associations, using conservative reliability cuts, we identified approximately 18 000 Gaia and 55 000 CatWISE2020 extragalactic sources.

Conclusions. We have released three high-quality counterpart catalogues – plus the training and validation sets – as a benchmark for the field. These datasets have many applications, but in particular, they empower researchers to build AGN samples tailored for completeness and purity, accelerating the hunt for the Universe’s most energetic engines.