The SRG/eROSITA all-sky survey: Hard X-ray selected active galactic nuclei

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
² Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
³ Exzellenzcluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany
⁴ Max-Planck Institut für Astronomie, Königstuhl 17, 69177 Heidelberg, Germany
⁵ Dipartimento di Fisica e Astronomia “Augusto Righi”, Alma Mater Studiorum 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

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 January 2024
Accepted: 13 January 2025

Abstract

Context. The eROSITA instrument on board the Spectrum Roentgen Gamma (SRG) satellite performed its first all-sky survey between December 2019 and June 2020. This paper presents the resulting hard X-ray (2.3–5 keV) sample, the first created from an all-sky imaging survey in this energy range, for sources within the western galactic sky (eROSITA-DE).

Aims. We produced a large uniform sample of hard-X-ray selected active galactic nuclei (AGN), and characterised them with supporting multi-wavelength astrometry, photometry, and spectroscopy. For the 2863 sources within the sky coverage of the DESI imaging Legacy Survey Data Release 10 (LS10; >15 000 deg²), counterparts were identified and classified. We also performed comparisons with the Swift BAT sample and HEAO-1 AGN sample to attempt to better understand the effectiveness and sensitivity of eROSITA in the hard band.

Methods. The 5466 hard X-ray selected sources detected with eROSITA are presented and discussed here. The Bayesian statistics-based code NWAY was used to identify the counterparts for the X-ray sources. These sources were classified based on their multi-wavelength properties, and the literature was searched to identify spectroscopic redshifts, which further inform the source classification. A total of 2547 sources were found to have good-quality counterparts, and 111 of these have been detected only in the hard band. The median redshift of the extragalactic sources is ~0.19.

Results. Compared with other hard X-ray selected surveys, the eROSITA hard sample covers a larger redshift range and probes dimmer sources, providing a complementary and expanded sample as compared to Swift BAT. Examining the column density distribution of missed and detected eROSITA sources present in the follow-up catalogue of Swift BAT 70 month sources, it is demonstrated that eROSITA can detect obscured sources with column densities >10²³ cm⁻² corresponding to ~14% of the full sample, but that the completeness drops rapidly thereafter. A sample of hard-only sources, many of which are likely to be obscured AGN with column densities ~10²³ cm⁻², is also presented and discussed. We caution that a large number of hard-only sources are believed to be spurious, based on simulations, and that additional cuts on counterpart quality or requiring spectroscopic redshifts should be applied to use this sample. X-ray spectral fitting reveals that these sources have extremely faint soft X-ray emission and their optical images suggest that they are found in more edge-on galaxies with lower b/a.

Conclusions. The first eROSITA all-sky survey provided the first imaging survey above 2 keV, and the resulting X-ray catalogue has been demonstrated to be a powerful tool for understanding AGN, in particular the heavily obscured AGN found in the hard-only sample.