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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.