Complex MgII broad line time evolution in a quasar: Clues for a subparsec binary supermassive black hole?

Supermassive binary black hole (SMBBH) systems are believed to form during the mergers of galaxies. While their existence is strongly predicted, there are still no confirmed cases due to observational challenges, particularly at close separations. However, indirect evidence – such as periodic variability in quasar photometry and complexity in their spectral lines – supports their presence. A search for long-period variability (100 < P [days] < 600) in the SDSS Stripe 82 region, potentially missed in previous studies, was conducted. We used precisely calibrated (1%-2%) photometry in the SDSS gri bands collected over roughly six years. Lomb-Scargle periodograms identified the most likely candidates for such periodic variability, which were then cross-matched with other surveys across the electromagnetic spectrum (both photometry and spectroscopy) to confirm their variability and classification. Additional time series data from Pan-STARRS and ZTF extended the observational baseline to over 20 years. All identified candidates were quasars, with the top-ranked one (P = 278 days) marked as a variable source in the Chandra X-ray catalog. Possible explanations for the quasars’ periodic behavior include radio jet precession, tilted or warped accretion disks, tidal disruption events, and other accretion-related phenomena. The to-ranked quasar was further analyzed using new MgII line observations and archived SDSS spectra, providing three epochs of MgII data. A key finding was that the MgII line displayed a double-peaked profile that evolved over time. By comparing synthetic magnitudes with photometric data and applying the PoSKI model, which examines emission lines and light curves of SMBBH candidates, a correlation was found that suggests the presence of a closely orbiting SMBBH. These sources are of significant scientific value, as their eventual merger would release a tremendous amount of energy as gravitational waves.