Special issues No. 2,
Pages: 39-40,
https://doi.org/10.69646/15scslsa22
XV Serbian Conference on Spectral Line Shapes in Astrophysics
Published by: Astronomical Observatory Belgrade

Supermassive binary black holes (SMBBH) are expected to form during galaxy mergers, but direct confirmations remain challenging, especially at sub-parsec separations. Indirect signatures, such as periodic variability in quasar light curves and evolving spectral line profiles, offer promising alternatives for identifying such candidates. We carried out a search for long-period (100–600 days) variability in quasars using high-precision (1–2%) gri photometry from the SDSS Stripe 82 survey, spanning six years. Periodic candidates were identified through Lomb-Scargle analysis and cross-matched with ZTF and Pan-STARRS data, extending the light curve baseline to over 20 years. All five final candidates were confirmed quasars, with the most promising one (P = 278 days) also flagged as variable in the Chandra X-ray catalog. To investigate the nature of the top candidate, we analyzed three epochs of Mg II spectra, combining new observations with archival SDSS data. The line profile reveals a time-evolving double-peaked structure, suggesting dynamic changes in the broad-line region. To interpret this behavior, we compared synthetic magnitudes with photometric data and applied the PoSKI model. The results are consistent with the presence of a compact supermassive binary black hole system — a scenario of particular interest for understanding black hole coalescence and future gravitational wave emission.