Astronomers Discovered The Most X-ray Luminous Quasar SRGE J170245.3+130104 at Redshift z ≈ 5.5 (Astronomy)

Khorunzhev and colleagues reported on the discovery of the most outstanding (so far) object found in the DaLeQoprogram — the brightest X-ray and Radio distant quasar SRGE J170245.3+130104 at redshift, z > 5, identified by spectral observations on the 6th BTA optical telescope. Their study recently appeared in journal Astronomy letters.

SRGE J170245.3+130104 was discovered by the eROSITA telescope aboard the SRG space observatory on March 13-15, 2020 during the first half-year scan of its all-sky X-ray survey. The optical counterpart of the X-ray source was photometrically identified as a distant quasar candidate at z ≈ 5.5. Follow-up spectroscopic observations, done in August/September 2020 with the SCORPIO-II instrument at the BTA 6-m telescope, confirmed that SRGE J170245.3+130104 is a distant quasar at redshift zspec = 5.466. The X-ray luminosity of the quasar during the first half-year scan of the eROSITA all-sky survey was 3.6 × 1046 erg/s (in the 2–10 keV energy range), whereas its X-ray spectrum could be described by a power law with a slope of Γ = 1.8. Six months later (September 13–14, 2020), during the second half-year scan of the eROSITA all-sky survey, the quasar was detected again and its X-ray luminosity had decreased by a factor of 2 (at the ≈ 1.9σ confidence level).

Fig 1: X-ray images of the SRG/eROSITA region 100 × 100 in the energy range 0.3–2.2 keV, centered on the optical source coordinates SRGE J170245.3+130104 (white cross). Left panel — image from the first half-year scan of SRG/eROSITA All-Sky Survey, right panel — image from the second half-year scan. Aperture with a radius of 10 is shown by red circle. Images are smoothed with a Gaussian filter (σ = 800). © Khorunzhev et al.

Now, Khorunzhev and colleagues reported that X-ray quasar SRGE J170245.3+130104 at z ≈ 5.47 discovered by the SRG X-ray observatory and the BTA 6-m telescope, is the most X-ray luminous quasar among known objects in the early Universe (z > 5) and also one of the most powerful quasars in radio.

They also mentioned that large radio-loudness (R ∼ 10³) of the quasar indicated that it can be a blazar and to test this hypothesis, it is necessary to carry out interferometric radio observations of the object at several wavelengths. Note that, only a few blazars at z > 5 are currently known, and all of them have lower X-ray luminosities than the quasar SRGE J170245.3+130104.

Fig. 2: Spectral energy distribution of the SRGE J170245.3+130104 quasar. The yellow dots show measurements in the radio spectral range, red dots — in the near-infrared and visible ranges, black dots — X-ray data SRG/eROSITA. The gray area — 1σ uncertainty of the power law model (with Galactic absorption) of the X-ray spectrum. Dot-dashed line shows the average blazar template. Solid line shows the radio-loud quasar template, corrected at wavelengths λ < 1216 Å for neutral hydrogen intergalactic absorption; the continuation of the original template at wavelengths λ < 1216 Å without considering absorption is shown by the dashed line. The blue line in the inset panel shows the smoothed optical spectrum obtained on the BTA telescope. © Khorunzhev et al.

In addition, they suggested that spectroscopic measurements in the near-infrared range (λ ∼ 1.6 microns) could also significantly complement the physical picture. It is expected that a broad MgII emission line should appear in the nearIR, which could be used to measure the mass of the black hole.

“The quasar SRGE J170245.3+130104 shows significant X-ray variability in the first two scans of the eROSITA all-sky survey. We will continue to monitor its variability in the following scans.”

— wrote authors of the study

Featured image: The 2’×2’ image in the iPS Pan-STARRS filter. The arrow indicates the position of optical companion SRGE J170245.3+130104. The radius of the small circle corresponds to the 1σ localization region. The radius of the large circle (10 arcsec) determines the size of the region where optical companion of X-ray source was searched.© Khorunzhev et al.

Reference: G.A. Khorunzhev, A.V. Meshcheryakov, P.S. Medvedev, V.D. Borisov, R.A. Burenin, R.A. Krivonos, R.I. Uklein, E.S. Shablovinskaya, V.L. Afanasyev, S.N. Dodonov, R.A. Sunyaev, S.Yu. Sazonov, M.R. Gilfanov, “Discovery of the most X-ray luminous quasar SRGE J170245.3+130104 at redshift z≈5.5”, Letters to Astronomical Journal, Volume: 47, Number: 3 Year: 2021 Pages: 155-173. DOI:  10.31857 / S0320010821030037

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