Detection of quasi-periodic oscillations in the γ-ray light curve of 4FGL J0309.9–6058

ABSTRACT In this work, we report, for the first time, a quasi-periodic oscillation (QPO) in the $\gamma$-ray band of 4FGL J0309.9–6058, also known as PKS 0308–611. We employed three analytical methods (the Lomb–Scargle periodogram, REDFIT, and the weighted wavelet Z-transform) to analyse the QPO signal using Fermi $\gamma$-ray light curve data. The analysis reveals a potential QPO during MJD 57983–60503, with a period of approximately 550 d and a maximum local significance of 3.72$\sigma$ and global significance of 2.72$\sigma$ derived from the WWZ analysis. To validate this result, we applied Gaussian Process (GP) to the same light curve, which independently confirms the presence of QPO signal consistent with our Fourier-based results. We further extended the analysis to the full duration of the Fermi observations, and the results consistently support and strengthen the presence of this QPO signal. Additionally, a time lag between the optical and $\gamma$-ray bands indicates separate emission regions for these two bands. Given the year-like time-scale of the QPO signal and the fact that a QPO signal with local significance over 3$\sigma$ for full Fermi-LAT observed time, we suggest that the QPO is most likely caused by a precessing jet.