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Discovery of the Most X-ray Luminous Quasar SRGE J170245.3+130104 at Redshift \(\boldsymbol{z\approx 5.5}\)

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Abstract

During the first all-sky survey of the SRG orbital observatory, the X-ray source SRGE J170245.3+130104 was discovered with the eROSITA telescope on March 13–15, 2020. Its optical counterpart was identified by photometric attributes as a candidate for distant quasars at \(z\approx 5.5\). The spectroscopic observations of the object carried out in August and September 2020 at the 6-m BTA telescope with the SCORPIO-II instrument confirmed that SRGE J170245.3+130104 is a quasar at redshift \(z_{\textrm{{spec}}}=5.466\pm 0.003\). According to the eROSITA data obtained during the first sky survey, the X-ray luminosity of the quasar is \(3.6^{+2.1}_{-1.5}\times 10^{46}\) erg s\({}^{-1}\) in the 2–10 keV energy band, while its X-ray spectrum can be approximately fitted by a power law with a slope \(\Gamma=1.8^{+0.9}_{-0.8}\). The quasar was detected again with the eROSITA telescope half a year later (on September 13–14, 2020) during the second sky survey, with its X-ray luminosity, probably, having decreased approximately by a factor of 2 at a confidence level \({\approx}1.9\sigma\). SRGE J170245.3+130104 has turned out to be the most X-ray luminous quasar among all of the known quasars at redshifts \(z>5\). At the same time, it is also one of the radio loudest distant quasars (radio loudness \(R\sim 10^{3}\)), which may imply that this is a blazar. We present a catalog of all the quasars at \(z>5\) detected in X-rays to date.

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Notes

  1. https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual.

  2. https://heasarc.gsfc.nasa.gov/ftools.

  3. https://www.sao.ru/hq/lsfvo/devices/scorpio-2/index.html.

  4. http://iraf.noao.edu.

  5. https://cirada.ca/vlasscatalogueql0.

  6. https://specutils.readthedocs.io/.

  7. https://extinction.readthedocs.io/en/latest/.

  8. https://pysynphot.readthedocs.io.

  9. http://svo2.cab.inta-csic.es/theory/fps/.

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ACKNOWLEDGMENTS

The observations at the SAO RAS telescopes are supported by the Ministry of Science and Higher Education of the Russian Federation.

This study is based on observations with the eROSITA telescope onboard the SRG observatory. The SRG observatory was built by Roskosmos in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI) within the framework of the Russian Federal Space Program, with the participation of the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG/eROSITA X-ray telescope was built by a consortium of German Institutes led by MPE, and supported by DLR. The SRG spacecraft was designed, built, launched, and is operated by the Lavochkin Association and its subcontractors. The science data are downlinked via the Deep Space Network Antennae in Bear Lakes, Ussurijsk, and Baykonur, funded by Roskosmos. The eROSITA data used in this work were processed with the eSASS software system developed by the German eROSITA consortium and the software developed by the Russian eROSITA consortium. The SRGz system was created at the High-Energy Astrophysics Department of the Space Research Institute of the Russian Academy of Sciences (by the working group on the search for and identification of X-ray sources and the production of a catalog based on SRG/eROSITA data).

We used functions from the astropy (Robitaille et al. 2013), pysynphot lim15, and specutilsFootnote 6 libraries to calculate the Galactic Footnote 7 and intergalacticFootnote 8 absorption, distances, and other astrophysical quantities. The information about the transmission curves and other characteristics of the photometric filters was taken from the site of the Spanish Virtual Observatory Filter Profile ServiceFootnote 9 (Rodrigo et al. 2012, 2020). We used catalogs from the VizieR database (Ochsenbein et al. 2000).

Funding

This study was supported by the Russian Science Foundation (project no. 19-12-00396).

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Correspondence to G. A. Khorunzhev.

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Translated by V. Astakhov

APPENDIX

APPENDIX

CATALOG OF X-RAY QUASARS AT \(z>5\)

The generally known and widely used summary catalogs of distant (\(z\gtrsim 5.5\)) X-ray quasars were compiled several years ago (see, e.g., Nanni et al. 2017; Vito et al. 2019). The search for new quasars is continued in all ranges of the electromagnetic spectrum and, therefore, the published catalogs quickly become obsolete. In light of the successful operation of the SRG observatory, we compiled an updated and complete catalog of all the spectroscopically confirmed quasars at \(z>5\) detected in X-rays (Table 5).

As a basis for the catalog we took the combined sample of spectroscopically confirms quasars at \(z>5\) compiled from two catalogs: ‘‘The Million Quasars catalog, v7.0a’’ (Flesch 2021) and the complete catalog of spectroscopically confirmed quasars at \(z>5\) ‘‘VHzQ’’ (Ross and Cross 2020). This sample of 542 ‘‘optical’’ quasars at \(z\gtrsim 5\) was correlated with the X-ray counterparts taken from the list of papers given in Table 6. Information about the X-ray fluxes and luminosities of the counterparts is presented in Table 5. The coordinates of the optical counterparts and the redshifts from Flesch (2021) and the redshifts from the catalog by Ross and Cross (2020) are also given there. The observed 0.5–2 keV X-ray flux is given for most of the objects. There is no information about the X-ray fluxes in Salvestrini et al. (2019) and, therefore, no fluxes are given in Table 5 for the sources from this paper. The reference to the publication from which the X-ray flux for a given source was taken is given first in the REF(\(FX\)) column; the references to the papers where the X-ray observations of the corresponding source are mentioned are given next in this column. The luminosities are given in the 2–10 keV band in the quasar rest frame.

For the remaining optical quasars, for which no X-ray counterpart was found in the literature, the search was carried out within \(6^{\prime\prime}\) of the nearest X-ray counterpart in the following catalogs: the Chandra source catalog release 2.0 (Evans et al. 2010, 2020), the XMM-Newton  4XMM-DR10 catalog (Webb et al. 2020), and the 2SXPS Swift X-ray telescope point source catalog (Evans et al. 2020b). Then, we calculated the X-ray luminosities of these quasars in the 2–10 keV energy band (in the object rest frame) \(L_{2-10{\textrm{\, keV}}}\). We assumed a power-law spectrum with \(\Gamma=1.8\) and used the Chandra and XMM-Newton measurements in the 0.5–2 keV band and the Swift data in the 0.3–10 keV band.

As a result, we produced a catalog of 52 X-ray quasars at \(z>5\), including the quasar k SRGE J170245.3+130104 being discussed here. Table 7 gives the references to the spectroscopic redshifts for these objects.

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Khorunzhev, G.A., Meshcheryakov, A.V., Medvedev, P.S. et al. Discovery of the Most X-ray Luminous Quasar SRGE J170245.3+130104 at Redshift \(\boldsymbol{z\approx 5.5}\) . Astron. Lett. 47, 123–140 (2021). https://doi.org/10.1134/S1063773721030026

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