Singular Spectrum Analysis of Fermi-LAT Blazar Light Curves: A Systematic Search for Periodicity and Trends in the Time Domain

Abstract

A majority of blazars exhibit variable emission across the entire electromagnetic spectrum, observed over various time scales. In particular, discernible periodic patterns are detected in the $γ$-ray light curves of a few blazars, such as PG 1553+113, S5 1044+71, and PKS 0426-380. The presence of trends, flares, and noise complicates periodicity detection, requiring careful analysis to determine whether these patterns arise from emission mechanisms or occur by chance. We employ Singular Spectrum Analysis (SSA) for the first time on data from the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope to systematically search for periodicity using 28-day binned light curves. We aim to isolate any potential periodic nature of the emission from trends and noise, reducing uncertainties in revealing periodicity. Additionally, we characterize long-term trends and develop a forecasting algorithm based on SSA to predict future emission behavior. We apply SSA to analyze 494 sources detected by Fermi-LAT, focusing on isolating oscillatory components from trends and noise in their $γ$-ray light curves. We compute the Lomb-Scargle Periodogram for oscillatory components extracted by SSA to determine the most significant periods, assessing their local and global significance. Our analysis identifies 46 blazars as potential candidates for quasi-periodic $γ$-ray emissions, each with a local significance $\geq 2σ$. Notably, 33 exhibit a local significance of $\geq 4σ$ (corresponding to a global significance of $\geq 2.2σ$). Our findings introduce 25 new $γ$-ray candidates, doubling the number of potentially periodic sources. This study provides a foundation for future investigations by identifying promising candidates and their significance in blazar variability.