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We present the results of our identification of 11 X-ray sources detected on the half of the sky in the 4–12 keV energy band on the combined map of the first five all-sky surveys with the Mikhail Pavlinsky ART-XC telescope onboard the SRG observatory. All these sources were also detected by the SRG/eROSITA telescope in the 0.2–8 keV energy band, whose data have allowed us to improve their positions and to investigate their X-ray spectra. Five of them have been detected in X-rays for the first time, while the remaining ones have already been known previously, but their nature has remained unknown. We have taken optical spectra for nine 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 from SDSS and the 6dF survey. The objects are classified as Seyfert galaxies (seven Sy1, three Sy1.9, and one Sy2) at redshifts . Our analysis of the X-ray spectra has revealed a noticeable intrinsic absorption ( cm ) in two of the four Seyfert 2 galaxies (Sy1.9–2). The spectrum of one more of them (SRGA J000132.9+240237) cannot be described within the model of an absorbed Comptonization continuum, which may point to a strong absorption and a significant contribution of the reflected radiation. However, the available SRG all-sky survey data are not enough to obtain reliable constraints on the absorption column density in this object, which is also interesting in that it is radio loud. Longer X-ray observations are required to refine the physical properties of this active galactic nucleus. [ABSTRACT FROM AUTHOR]

We present the results of the optical identification and spectroscopic redshift measurements of 216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observations were performed in 2020–2023 with the 6-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope at the Sayan Solar Observatory of the Institute of Solar–Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences, and the 1.5-m Russian–Turkish telescope (RTT-150) at the TÜBİTAK Observatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements have been obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in the SRG/eROSITA survey and 22 galaxy clusters are at redshifts , including three at . Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters at . For these observations we chose the most massive clusters and, therefore, most of the galaxy clusters presented here with the spectroscopic redshifts measured by us will most likely enter in future into the cosmological samples of galaxy clusters from the SRG/eROSITA survey. [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]

We present the results of our identification of 17 X-ray sources detected in the 4–12 keV energy range by the Mikhail Pavlinsky ART-XC telescope during the first year of the SRG all-sky survey. Three of them have been discovered by the ART-XC telescopes, while the remaining ones have already been known previously as X-ray sources, but their nature has remained unknown. We took optical spectra for nine sources located in the northern sky () 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) and the 1.5-m Russian–Turkish telescope at the TÜBITAK National Observatory. For the remaining objects we have analyzed the archival optical spectra taken during the 6dF survey. All of the investigated objects have turned out to be Seyfert galaxies (eight of type 1, seven of type 2, and two of intermediate type 1.8) at redshifts up to . Based on data from the eROSITA and ART-XC telescopes onboard the SRG observatory, we have obtained X-ray spectra in the energy range 0.2–20 keV for eight sources. A significant intrinsic absorption ( cm ) has been detected in three of them, with two of them being probably strongly absorbed ( cm ). This paper is a continuation of the series of publications on the optical identification of active galactic nuclei detected by the ART-XC telescope. [ABSTRACT FROM AUTHOR]

We present the first results of our program of optical observations for galaxy clusters from the Lockman Hole X-ray survey with the eROSITA telescope onboard the SRG space observatory. We present the results of spectroscopic redshift measurements for 11 galaxy clusters that have been identified optically among the extended X-ray sources of the SRG/eROSITA survey using data from optical and infrared sky surveys. The spectroscopic observations were carried out in late 2019–early 2020 with the 1.6-m AZT-33IK telescope of the Sayan Observatory and the 6-m SAO RAS telescope (Bolshoi Teleskop Azimutalnyi, BTA). [ABSTRACT FROM AUTHOR]

We present the results of our identification of eight objects from the preliminary catalogue of X-ray sources detected in the 4–12 keV energy band by the Mikhail Pavlinsky ART-XC telescope onboard the SRG observatory during the first all-sky survey. Three of them (SRGA J005751.0+210846, SRGA J , SRGA J232446.8+440756) have been discovered by the ART-XC telescope, while five have already been known previously as X-ray sources, but their nature has remained unestablished. The last five sources have also been detected in soft X-rays by the eROSITA telescope of the SRG observatory. Our optical observations have been carried out at the 1.6-m AZT-33IK telescope of the Sayan Observatory and the 1.5-m Russian–Turkish telescope (RTT-150). All of the investigated objects have turned out to be active galactic nuclei (AGNs) at redshifts from 0.019 to 0.283. Six of them are Seyfert 2 galaxies (including one Seyfert 1.9 galaxy), one (SRGA J005751.0+210846) is a "hidden" AGN (in an edge-on galaxy), and one (SRGA J224125.9+760343) is a narrow-line Seyfert 1 galaxy. The latter object is characterized by a high X-ray luminosity ( erg s in the 4–12 keV band) and, according to our black hole mass estimate (), an accretion rate close to the Eddington limit. All three AGNs discovered by the ART-XC telescope (which are not detected by the eROSITA telescope) are characterized by a high absorption column density ( cm ). The results obtained confirm the expectations that the ART-XC telescope is an efficient instrument in searching for heavily obscured and other interesting AGNs in the nearby () Universe. The SRG sky survey will last for another 3 years or more, which must allow many such objects to be discovered. [ABSTRACT FROM AUTHOR]

We present the results of our spectroscopic redshift measurements for galaxy clusters detected previously from Planck all-sky survey data as well as Sloan Digital Sky Survey and WISE all-sky survey data. The redshifts have been measured for 23 clusters, including four galaxy clusters from the second Planck catalogue of Sunyaev–Zeldovich sources. The main spectroscopic observations were carried out during 2019–early 2020 at the 1.6-m AZT-33IK telescope of the Sayan Observatory and the 1.5-m Russian–Turkish telescope (RTT-150). Some of the data have been obtained previously at the 3.5-m Calar Alto telescope. Out of the 23 clusters in this sample, 14 objects are located in the half of the sky where the rights to the data from the eROSITA sky survey onboard the SRG orbital X-ray observatory belong to the Russian side. All these clusters were detected with the eROSITA telescope in the course of the sky survey during 2020. On the whole, we have measured the spectroscopic redshifts for 220 galaxy clusters within our program of optical identifications of galaxy clusters from the Planck catalogue during several years. Many of them have already been detected in the SRG/eROSITA survey; a significant fraction of these objects are among the most massive galaxy clusters of the eROSITA sky survey and will most likely enter into cosmological samples of clusters from this survey. [ABSTRACT FROM AUTHOR]

We present the results of spectroscopic redshift measurements for galaxy clusters from the catalogue compiled previously from Planck all-sky survey data in combination with Sloan Digital Sky Survey (SDSS) and WISE data in the optical and infrared ranges. Most of these clusters are massive objects that will also be detected in future all-sky surveys, such as the eROSITA all-sky X-ray survey onboard the Spectrum–Roentgen–Gamma (SRG) space observatory. The spectroscopic observations of these galaxy clusters have been carried out with the 1.5-m Russian–Turkish telescope (RTT150), the 1.6-m Sayan Observatory AZT-33IK telescope, and the 6-m SAO RAS telescope (Bolshoi Teleskop Azimutalnyi, BTA). The spectroscopic redshift measurements have been obtained for 67 galaxy clusters, including 12 galaxy clusters from the second Planck catalogue of Sunyaev–Zeldovich sources. [ABSTRACT FROM AUTHOR]