A three-point shot from half X-ray court: Lensed quasars, flux upper limits, and SMBH binaries with SRG/eROSITA-De

eROSITA, the soft X-rays instrument on board the Spectrum-Roentgen-Gamma (SRG) observatory, has successfully completed four all-sky surveys, detecting millions of X-ray sources. Most of these sources are detected for the first time due to the sensitivity of the instrument and its large sky coverage. The nature of most of these newly detected X-ray sources is fascinating and yet to be confirmed. Using the SRG/eROSITA all-sky surveys and massive multiwavelength campaigns, I have exploited the eROSITA data to search for two of the rarest and peculiar phenomena related to actively accreting supermassive black holes (SMBH). Additionally, I also provide a sensitivity estimator for non-detected sources. In this thesis, I focus on the discovery of a bright active SMBH that is gravitationally lensed by a foreground massive galaxy, the production of eROSITA flux upper limits for non-detections, and the search and characterization of SMBHs that, due to galaxy merger events, were driven to the center of the colliding galaxies, orbiting in a rare and elusive SMBH binary system. During the first project, I report the discovery and spectroscopic identification of one of the X-ray brightest doubly lensed quasars, eRASS1 J050129.5−073309. I confirm the lensed nature of the source using long-slit spectroscopy from the NTT/EFOSC2 instrument at the ESO La Silla observatory. The lensed quasar is located at a redshift of z = 2.47 and its images are separated by 2.′′ 7. The archival optical light curve of the source shows evidence of intrinsic and extrinsic variability with a clear time delay between the arrival time of the photons from the different lensed images, where the fainter image lags the brighter by ∼ 100 days. The brightness of the fainter image has also decreased by about one magnitude since 2019. This dimming was still obvious during the spectroscopic observations and is probably caused by a microlensing event where a star from the lensing galaxy is, in this case, de-magnifying the lensed image. Legacy Survey DR10 imaging and image modeling of the lensed system reveal evidence for the lensing galaxy and a tentative lensed arc image of the quasar host galaxy. Despite the indubitable legacy of the detected, discovered, and cataloged X-ray sources of the all-sky eROSITA surveys, many more sources may be hidden in the noise of these observations due to their faint X-ray nature. Other objects could be variable sources and were in a quiescent state when the eROSITA observations took place, and therefore, were not included in the catalogs. In order to characterize these hidden sources regardless of their detection and as a contribution of the eROSITA-De collaboration to the X-ray community, I implement the first half-sky SRG/eROSITA upper limit server. I provide photometric data and flux upper limit of every pixel belonging to the German consortium to study the X-ray properties of variables, transients, or non-detected sources in the eROSITA all-sky survey data. The upper limits for the first eROSITA data release (DR1) are stored in a database that offers a fast query option where users can retrieve SRG/eROSITA upper limits for a set of several input positions. This database is publicly available after the eROSITA DR1 for the worldwide community that wants to obtain X-ray information about their sources of interest in an accessible, fast, and comprehensive way. I also led one of the first systematic searches for quasi-periodic X-ray light curves using the multi-epoch all-sky surveys capabilities of eROSITA. These quasi-periodic X-ray light curves are expected to be characteristic features of supermassive black hole binaries (SMBHBs) separated by (sub)-pc scales. These SMBHBs represent one of the latest stages of hierarchical galaxy assembly, connecting the formation of massive galaxies with the gravitational wave physics. I search for these SMBHB candidates based on their eROSITA quasi-periodic light curves and monitoring follow-up campaigns using other X-ray missions. The most promising candidate, eRASSt J0530-4125, shows X-ray quasi-periodic variability with a typical time scale of one year in the observed frame. Based on the selected sources, I also estimate an optimistic upper limit on the fraction of galaxies that could host a SMBHB of ∼ 0.05 binaries per galaxy. I emphasize that this is an optimistic estimation, and the fraction will decrease once further observational evidence is obtained to confirm or rule out the SMBHB nature of our sources. I finish by providing conclusions, a summary of the extensive multiwavelength data obtained during the PhD, the main challenges that the projects face, and the future perspective for the individual Chapters described in this work. I also discuss the need and prospects for missions such as SRG/eROSITA, that are designed to perform regular all-sky X-ray monitoring observations.