Grigory Uskov, Sergey Sazonov, Igor Lapshov, Alexander Mikhailov, Ekaterina Filippova, Alexander Lutovinov, Ilya Mereminskiy, Maria Mochalina, Andrey Semena, Alexey Tkachenko

We report on a detailed study of a luminous, heavily obscured ($N_{\rm H} \sim 2 \times 10^{23}$ cm$^{-2}$), radio-loud quasar SRGAJ230631.0+155633, discovered in the 4--12 keV energy band by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory during the first two years of its all-sky X-ray survey in 2020--2021. The object is located at $z=0.4389$ and is a type 2 AGN according to optical spectroscopy (SDSS, confirmed by DESI). Our analysis combines data from the radio to the X-ray energy range, including quasi-simultaneous pointed observations with the SRG/ART-XC and Swift/XRT telescopes, conducted in June 2023. During these follow-up observations, the source was found in a significantly fainter but still very luminous state ($L_{\rm X}=1.0^{+0.8}_{-0.3} \times 10^{45}$ erg s$^{-1}$, absorption corrected, 2--10 keV) compared to its discovery during the all-sky survey ($L_{\rm X}=6^{+6}_{-3}\times10^{45}$ erg s$^{-1}$), which indicates significant intrinsic variability on a rest-frame time scale of $\sim 1$ year. The radio data show a complex morphology with a core and two extended radio lobes, indicating a giant FRII radio galaxy. Using multiwavelength photometry and the black hole--bulge mass scaling relation, we estimate the bolometric luminosity of the quasar at $\sim 6\times 10^{46}$ erg s$^{-1}$ and the mass of its central black hole at $\sim 1.4 \times 10^9$ $M_\odot$. This implies that the black hole is accreting at $\sim 30$ of the Eddington limit. Overall, SRGAJ230631.0+155633 proves to be one of the most luminous obscured quasars in the observable Universe out to $z=0.5$ (i.e. over the last 5 billion years of cosmic time). As such, it can serve as a valuable testbed for in-depth exploration of the physics of such objects, which were much more abundant in the younger Universe.