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We consider the population of point X-ray sources from the two-year SRG/eROSITA survey in the 1 DESI spectroscopic survey region in the eastern Galactic hemisphere (eROSITA–1 DESI–East). The data under consideration combine a large survey area (compared to the medium-format Stripe82X and XMM-XXL X-ray surveys) and a record high completeness of spectroscopy for the optical counterparts of X-ray sources. We compare the results of the photometric (SRGz) and spectroscopic/astrometric (DESI EDR, SDSS, HELP, GAIA) measurements of the classes and redshifts of objects in three X-ray-flux-limited eROSITA–1 DESI–East samples: erg s cm (bright), erg s cm (medium), and erg s cm (faint) with a total area of 91.4, 91.4, and 16.62 deg , respectively. We propose a new method of postprocessing the probabilistic photo-z predictions based on a two-temperature correction of the probability density function . This approach allows the calibration of the probabilistic predictions and confidence intervals of the photometric redshifts for eROSITA X-ray sources obtained by SRGz to be improved significantly. [ABSTRACT FROM AUTHOR]

We present the results of our identification of 14 X-ray sources detected in the eastern Galactic sky () in the 4–12 keV energy band on the combined map of the first five all-sky surveys (from December 2019 to March 2022) with the Mikhail Pavlinsky ART-XC telescope onboard the SRG observatory. All 14 sources are reliably detected by the SRG/eROSITA telescope in the 0.2–8 keV energy band. Six of them have been detected in X-rays for the first time, while the remaining ones have already been known previously as X-ray sources, but their nature has remained unknown. We have taken optical spectra for 12 sources with the 1.6-m AZT-33IK telescope at the Sayan Observatory (the Institute of Solar–Terrestrial Physics, the Siberian Branch of the Russian Academy of Sciences). For two more objects we have analyzed the archival spectra taken during the 6dF survey. All objects have turned out to be Seyfert galaxies (one NLSy1, three Sy1, four Sy1.9, and six Sy2) at redshifts –0.238. Based on data from the eROSITA and ART-XC telescopes onboard the SRG observatory, we have obtained X-ray spectra for all objects in the energy range 0.2–12 keV. In four of them the intrinsic absorption exceeds cm at a 90 confidence level, with one of them being probably heavily obscured ( cm with 90 confidence). This paper continues our series of publications on the identification of hard X-ray sources detected during the all-sky survey with the SRG orbital X-ray observatory. [ABSTRACT FROM AUTHOR]

The results of multiwavelength observations of the very massive galaxy cluster SRGe CL2305.2–2248 detected in X-rays during the first SRG/eROSITA all-sky survey are discussed. This galaxy cluster has also been detected previously in the millimeter band in the South Pole Telescope (SPT-CL J2305–2248) and Atacama Cosmology Telescope (ACT-CL J2305.1–2248) surveys through the observation of the Sunyaev–Zeldovich effect. The spectroscopic redshift has been measured at the 6-m BTA telescope, . In addition, deep photometric measurements of galaxies have been performed at the Russian–Turkish 1.5-m telescope (RTT-150). The mass of the cluster is estimated from the eROSITA data to be . We show that this cluster is among several tens of the most massive clusters in the observable Universe and among only a few most massive galaxy clusters at . [ABSTRACT FROM AUTHOR]