Searching for supermassive black hole binaries within SRG/eROSITA-De: I. Properties of the X-ray selected candidates

Context. Supermassive black hole binaries (SMBHBs) with separations on (sub)pc scales represent one of the latest stages of hierarchical galaxy assembly. However, many of these objects are hidden behind large columns of gas and dust at the centers of galaxies and are difficult to detect. In these systems, accretion is expected to take the form of two individual accretion disks around the individual black holes, fed by a larger circumbinary disk. The X-ray and UV emission in these systems are predicted to vary regularly on timescales that are comparable to that of the orbital period of the binary. Aims. This is the first of a series of papers where we continue to search for and characterize SMBHB candidates based on quasiperiodic light curves from the soft X-ray instrument eROSITA on board the Spectrum-Roentgen-Gamma (SRG) observatory and extensive X-ray follow-up. Methods. We searched the multi-epoch SRG/eROSITA all-sky surveys for extragalactic sources that show an up-down-up-downa or down-up-down-upa profile (from scan to scan) in their 0.2 2.3 keV flux light curves. We selected sources where the upa and downa flux levels vary by at least 3. The downa states were also allowed to correspond to nondetections. We excluded stellar objects, blazars, and radio-loud active galactic nuclei (AGNs) via Gaia DR3 parallaxes and proper motions. We also carried out a visual inspection of the images drawn from the Legacy Survey DR10 and the SIMBAD database. Results. We compiled a sample of 16 sources that are suitable for X-ray follow-up campaigns given their brightness and significant variability between bright and faint SRG/eROSITA flux levels. We triggered extensive Swift-XRT and NICER monitoring campaigns on the best SMBHB candidates to confirm or discard their tentative periodicities. Optical spectroscopic observations confirmed the nuclear and extragalactic nature of 15/16 objects and enabled single-epoch SMBH mass measurements and BPT classifications of the dominant ionization in the host galaxy. Our most promising candidate, eRASSt J0530-4125, exhibits X-ray quasi-periodic variability with a typical time scale of one year in the observed frame. By stacking the X-ray observations of each source in our sample, we found that 14 out of the 15 sources can be modeled by a power law with a photon index ranging from I 2.8. Based on our selection, we estimate an optimistic upper limit on the fraction of SMBHB candidates to be 0.05 per galaxy. We emphasize that further observational evidence is needed to confirm the SMBHB nature of our sources.