Broadband Variability Analysis of the Flat-spectrum Radio Quasar PKS 0402−362 with Indications of Quasi-periodic Modulation

Abstract We present a comprehensive temporal and spectral study of the flat-spectrum radio quasar (FSRQ) PKS 0402−362 using Fermi Large Area Telescope/Swift X-Ray Telescope/UVOT observations spanning from MJD 54686 to 60321. The γ -ray light curve exhibits multiple phases of enhanced activity, with the fractional variability parameter ( F var ) showing larger amplitudes at longer timescales, consistent with variability trends observed in other FSRQs. Statistical analysis of the flux and spectral index distributions using the Anderson–Darling test and histogram fitting reveals that both distributions deviate from a single log-normal form and are better represented by a double log-normal profile, indicating two distinct flux states. A search for quasi-periodic oscillations in the γ -ray emission using the Lomb–Scargle periodogram identified a significant periodic signal at ∼413 days with a confidence level exceeding 3 σ . However, the proximity of the timescale to one year and the limited number of observed cycles prevents a definitive interpretation. Broadband spectral energy distributions for six flux states were modeled using a one-zone leptonic framework incorporating synchrotron, synchrotron self-Compton, and external Compton components. The spectral energy distributions are well reproduced with physically reasonable parameters: high-flux states exhibit harder electron spectra and lower magnetic field strengths ( B ∼ 0.2–0.6 G), while low-flux states show softer spectra and stronger magnetic fields ( B ∼ 1.3 G). The fitted break energy decreases during high-flux states, suggesting enhanced radiative cooling and a transition toward a particle- or kinetic-energy–dominated jet. These trends are consistent with the “harder-when-brighter” behavior commonly observed in blazars.