Two Transient Quasi-periodic Oscillations in $\gamma$-Ray Emission from the Blazar S4 0954+658

Abstract

In this work, we report periodicity search analyses in the gamma-ray light curve of the blazar S4 0954+658 monitoring undertaken by the Fermi Large Area Telescope (LAT). Four analytical methods and a tool are adopted to detect any periodic flux modulation and corresponding significance level, revealing that (i) a 66 d quasi-periodic oscillation (QPO) with the significance level of $> 5\sigma$ spanning over 600 d from 2015 to 2016 (MJD 57145--57745), resulting in continuous observation of nine cycles, which is one of the highest cycles discerned in blazar gamma-ray light curve; (ii) a possible QPO of 210 d at a moderate significance of $\sim3.5\sigma$ lasted for over 880 d from 2020 to 2022 (MJD 59035--59915), which lasted for four cycles. In addition, we discuss several physical models to explain the origin of the two transient QPOs and conclude that a geometrical scenario involving a plasma blob moving helically inside the jet can explain the time scale of the QPO.