Exploring year-timescale transient and long-term quasi-periodic oscillations in optical and gamma-ray light curves of blazars

A comprehensive analysis of quasi-periodic oscillations (QPOs) in the multi-wavelength observations of blazars has been carried out. Utilizing 15 years of Fermi-LAT observations of seven blazars in our sample, we identify both long-term and transient QPOs in the gamma-ray light curves, with timescales ranging from a few months to years. These periodicities were detected using the Lomb-Scargle periodogram (LSP) and REDFIT techniques. To robustly evaluate the statistical significance of the quasi-periodic signals observed in the LSPs, 2$\times {10}^5$ synthetic γ-ray light curves were generated for each source using a stochastic model known as the Damped Random Walk (DRW). These gamma-ray QPOs are further supported by the detection of optical QPOs exhibiting similar timescales. A cross-correlation analysis between γ-rays and optical emissions reveals a significant peak ($>3\sigma$) at or close to zero-lag. To investigate the physical origin of the observed gamma-ray QPOs with different timescales, we explore several plausible scenarios, with particular emphasis on a relativistic jet hosted by one of the black holes in a supermassive binary black hole (SMBBH) system, accretion disc model, and helical motion of magnetized plasma blob within the jet. The transient gamma-ray QPOs of month-like timescales are interpreted within the framework of the helical motion of plasma blob in jet, while the long-duration QPOs with multi-year timescales are explained using the SMBBH scenario. The gamma-ray light curves were modeled by employing a Markov Chain Monte Carlo (MCMC) approach, allowing us to constrain key physical parameters such as the jet Lorentz factor (Γ) and the viewing angle between the observer's line of sight (ψ) relative to the spin axis of SMBH.