Astronomers investigate an extremely X-ray-luminous, radio-loud quasar

Using the Spektr-RG (SRG) space observatory, astronomers from the Russian Academy of Sciences have inspected a radio-loud quasar known as SRGA J2306+1556, which is extremely luminous in the X-ray band. Results of the new study are reported in a research paper published April 18 on the arXiv preprint server.

Quasars, or quasi-stellar objects (QSOs), are active galactic nuclei (AGN) of very high luminosity powered by supermassive black holes (SMBHs), emitting electromagnetic radiation observable in radio, infrared, visible, ultraviolet and X-ray wavelengths. They are among the brightest and most distant objects in the known universe, and serve as fundamental tools for numerous studies in astrophysics as well as cosmology.

SRGA J2306+1556 is a radio-loud quasar at a redshift of approximately 0.44, discovered in 2022 with Spektr-RG. Previous observations of SRGA J2306+1556 have found that it is heavily obscured and its intrinsic X-ray luminosity exceeds 4.0 quattuordecillion erg/s.

Recently, a team of astronomers led by Grigory Uskov decided to re-investigate SRGA J2306+1556 with Spektr-RG in order to get more insights into its nature. Their study was complemented by data from NASA’s Swift spacecraft.

“To study this interesting quasar in detail, we organized its follow-up pointed X-ray observations with SRG/ART-XC and the XRT telescope aboard the Neil Gehrels Swift observatory, which revealed a strongly absorbed X-ray spectrum,” the researchers wrote in the paper.

During the new observations, the unabsorbed X-ray luminosity of SRGA J2306+1556 ranged between 1.0 and 6.0 quattuordecillion erg/s, and the source exhibited a strongly absorbed X-ray spectrum. The astronomers noted that such X-ray luminous obscured AGN are extremely rare at redshifts below 0.5.

The collected data indicate that SRGA J2306+1556 experienced an X-ray outburst in 2020–2021 and was in a “low” state in June 2023. The researchers estimate that the outburst could have lasted about one year or more.

Furthermore, it was found that SRGA J2306+1556 showcases a complex morphology in the radio band with a core and two extended radio lobes. The findings suggest that the radio counterpart of SRGA J2306+1556 is a giant radio galaxy (GRG) of the FR II type with a radio power at a level of 286 YW/Hz, which is equivalent to 0.004 quattuordecillion erg/s.

The study also found that SRGA J2306+1556 has a bolometric luminosity of around 60 quattuordecillion erg/s and that its central black hole has a mass of about 1.4 billion solar masses.

Summing up the results, the authors of the paper underlined that SRGA J2306+1556 is one of the most luminous obscured quasars in the observable universe and accretes mass at a high rate.

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