2026ApJ...998..227N

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 3σ. 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.