Detection of Quasi-periodic Oscillations in the $γ$-Ray Light Curve of 4FGL J0309.9-6058

TL;DR

The paper reports the first gamma-ray quasi-periodic oscillation (QPO) in the blazar 4FGL J0309.9-6058, using ~16 years of Fermi-LAT data. Three Fourier-based techniques (Lomb-Scargle, REDFIT, WWZ) and Gaussian Process modeling consistently reveal a QPO with a characteristic period near ~550 days, supported by significance assessments that account for red noise and data sampling. Extended analyses over the full LAT dataset and a cross-band optical lag of about 200–230 days strengthen the robustness and suggest separated emission regions between optical and gamma-ray bands. The authors favor jet precession as the most plausible mechanism, supported by a jet-precession model that fits the data and yields reasonable jet parameters; this has implications for understanding long-term blazar variability and jet dynamics.

Abstract

In this work, we report, for the first time, a quasi-periodic oscillation (QPO) in the $γ$-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 analyze the QPO signal using \textit{Fermi} $γ$-ray light curve data. The analysis reveals a potential QPO during MJD 57983$-$60503, with a period of approximately 550 days and a maximum local significance of 3.72$σ$ and global significance of 2.72$σ$ 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 \textit{Fermi} observations, and the results consistently support and strengthen the presence of this QPO signal. Additionally, a time lag between the optical and $γ$-ray bands indicates separate emission regions for these two bands. Given the year-like timescale of the QPO signal and the fact that a QPO signal with local significance over 3$σ$ for full \textit{Fermi}-LAT observed time, we suggest that the QPO is most likely caused by a precessing jet.

Figures (9)

• Figure 1: Panel (a): The Fermi-LAT light curve of 4FGL J0309.9-6056 for $\sim$ 16 years (MJD 54683$-$60503). The gray inverted triangles represent the 95% significance level upper limits. Panel (b): The shaded section (MJD 57983$-$60503) from the left panel is enlarged. The blue dotted line represents the mean flux, and the green solid line shows a sine function with a mean period of approximately 550 days derived from LSP, REDFIT, and WWZ methods.
• Figure 2: Power-law slope distribution as obtained using the PSRESP method. The best-fitting value of $\beta_{\rm opt}$ is obtained from the peak of a Gaussian function fit, and its associated uncertainty is taken from the FWHM of the Gaussian.
• Figure 3: Histogram of $\gamma$-ray light curves for 4FGL J0309.9-6056. The red dashed line represents the log-normal fit; the green dashed one is the Gaussian fit.
• Figure 4: Gaussian Process modeling of the 0.1–300 GeV $\gamma$-ray light curve for the period MJD 57983$-$60503, obtained using the SHO$\times$2 model. Top panel: Observed data and the best-fit profile including the 1$\sigma$ confidence interval. Middle panel: Standardized residuals of the Gaussian Process fit as a function of time. Bottom panels: Autocorrelation function (ACF) of the residuals (left) and squared residuals (right).
• Figure 5: Power spectral density of the Gaussian Process modeling obtained using the SHO$\times$2 model. The shaded regions correspond to 1$\sigma$ confidence interval.