Search for Outbursts in the Narrow 511-keV Line from Compact Sources Based on INTEGRAL Data

We present the results of a systematic search for outbursts in the narrow positron annihilation line on various time scales (5x10^4 - 10^6 s) based on the SPI/INTEGRAL data obtained from 2003 to 2008. We show that no outbursts were detected with a statistical significance higher than ~6 sigma for any of the time scales considered over the entire period of observations. We also show that, given the large number of independent trials, all of the observed spikes could be associated with purely statistical flux fluctuations and, in part, with a small systematic prediction error of the telescope's instrumental background. Based on the exposure achieved in ~6 yr of INTEGRAL operation, we provide conservative upper limits on the rate of outbursts with a given duration and flux in different parts of the sky.Comment: 16 pages, 8 figures. To be published in Astronomy Letters, 2010, Vol. 36, No 4, p. 23

X-ray Observations of Historical Classical Nova Counterparts with eROSITA Telescope Onboard the SRG Orbital Observatory during the All-Sky Survey

Abstract: The X-ray emission from the counterparts of historical classical novae (CNe) in our Galaxy is studied. For this purpose, we use data from three SRG/eROSITA sky surveys in the half of the sky for the processing of which the Russian SRG/eROSITA consortium is responsible. Out of 309 historical CNe, X-ray emission has been detected from 52 sources with 0.3–2.3 keV luminosities (Formula presented.) erg -1. Among them, two sources with supersoft spectra are associated with the post-nova supersoft X-ray emission. X-ray spectroscopy suggests that systems with magnetized white dwarfs (WDs) may account for some fraction in our sample of CN counterparts detected in X-rays. This suggestion will be checked during further SRG/eROSITA sky surveys. The CN counterparts represent a bona fide sample of accreting WDs with non-steady thermonuclear hydrogen burning on their surface, while their X-ray luminosity in quiescence is a good indicator of the accretion rate in a binary system. Using this fact, we have constructed the accretion rate distribution of WDs with non-steady hydrogen burning and compared it with the accretion rate distribution of known steady supersoft X-ray sources in our Galaxy and nearby external galaxies. There is a pronounced dichotomy between these two distributions—the CN counterparts and the steady supersoft sources are in different regions, in accordance with the predictions of the theory of thermonuclear hydrogen burning on the WD surface

Observations of Her X-1 in low states during SRG/eROSITA all-sky survey

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument onboard the Russian-German 'Spectrum-Roentgen-Gamma' (SRG) mission observed the Her X-1/HZ Her binary system in multiple scans over the source during the first and second SRG all-sky surveys. Both observations occurred during a low state of the X-ray source when the outer parts of the accretion disk blocked the neutron star from view. The orbital modulation of the X-ray flux was detected during the low states. We argue that the detected X-ray radiation results from scattering of the emission of the central source by three distinct regions: (a) an optically thin hot corona with temperature ~(2-4) × 10K above the irradiated hemisphere of the optical star; (b) an optically thin hot halo above the accretion disk; and (c) the optically thick cold atmosphere of the optical star. The latter region effectively scatters photons with energies above 5-6 keV

The First Distant X-ray Quasars (z ~ 4) among the Sources Discovered by the eROSITA Telescope of the SRG Orbital Observatory during a Deep Lockman Hole Survey

© 2020, Pleiades Publishing, Inc. Abstract: During a deep extragalactic Lockman Hole sky survey with an area of 18.5 sq. deg, which was conducted when the SRG observatory was flying to the Lagrange point L2, the eROSITA telescope detected ~7000 X-ray sources. These objects were then provisionally identified and classified using the publicly accessible data of optical and infrared sky surveys by the SRGz machine learning system developed for this purpose at the Space Research Institute of the Russian Academy of Sciences. As a result, a number of new candidates for distant quasars (z ~ 4) have been selected. The spectroscopic observations of the first two candidates from this list carried out with the 1.6-m AZT-33IK telescope of the Sayan Solar Observatory have confirmed that these objects are actually distant quasars at redshifts 3.878 and 4.116 and are characterized by a high X-ray luminosity ~1045 erg s-1 (2–10 keV). The results obtained allow one to count on the detection of a large number of distant quasars during a four-year all-sky survey of the SRG observatory begun in December 2019

Spectroscopic Redshift Measurements for Galaxy Clusters from the Planck Survey and Observations of These Clusters in the SRG/eROSITA Survey

Abstract: 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

Optical Identification of Candidates for Active Galactic Nuclei Detected by the Mikhail Pavlinsky ART-XC Telescope Onboard the SRG Observatory during an All-Sky X-ray Survey

Abstract: 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 J014157.0-032915, 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 (~(2-13) × 1044 erg s-1 in the 4–12 keV band) and, according to our black hole mass estimate (~2 × 107Mꙩ), 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 (Formula presented.). 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 (Formula presented.) Universe. The SRG sky survey will last for another 3 years or more, which must allow many such objects to be discovered

Optical Spectroscopy of SRG/eROSITA Objects with 2.5-m Telescope at the Caucasus Mountain Observatory of the SAI MSU

© 2020, Pleiades Publishing, Inc. Based on observations with the new transient double-beam spectrograph (TDS) at the 2.5-m telescope of the Caucasus Mountain Observatory of SAI MSU, we have determined the types and found the redshifts for six new X-ray sources (four quasars and two galaxy clusters) detected by the SRG space observatory during the Lockman Hole observations at the eROSITA performance verification phase. We show that the TDS allows the spectra of objects ~20m to be taken in 2 h of observations with a signal-tonoise ratio higher than 5 and a resolution R ~1500. The types and redshifts of the objects determined from our spectroscopic observations agree well with the predictions based on photometric data using the SRGz automatic classification system

Spectroscopic Redshift Determination for a Sample of Distant Quasars Detected by the SRG Observatory Based on RTT-150 Observations. II

Abstract: Results of the spectroscopic observations at the 1.5-m Russian–Turkish telescope for another group of 12 X-ray sources discovered by the eROSITA telescope onboard the SRG space observatory and identified by the SRGz machine learning system as candidates for distant X-ray quasars are presented. Ten objects have been confirmed as quasars at redshifts z=2.6{-}3.2 and two sources have turned out to be active galactic nuclei (AGNs) at z=0.42 and 1.02

Observation of a Very Massive Galaxy Cluster at z = 0.76 in the SRG/eROSITA All-Sky Survey

Abstract: 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, z=0.7573. 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 (Formula presented.). 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 z>0.6

GRANAT/SIGMA observation of the early afterglow from GRB 920723 in soft gamma-rays

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman